which includes the #bjh original illustrations4 ,see #ecejh-;h4htm or #ecejh-;h4zip3 7https3_/_/www4gutenberg4org_/cache_/epu b_/#ecejh_/pg#ecejh-;images4html7 or 7https3_/_/www4gutenberg4org_/files_/#ec ejh_/ecejh-;h4zip7 ,images of the original pages are available through ,internet ,archive4 ,see https3_/_/archive4org_/details_/airplane photogra#jj;ivesuoft ,transcriber's note3 ,text enclosed by underscores is in italics 7italics74 ,text enclosed by equal signs is in bold face 7bold74 ,a carat character is used to denote superscription4 ,a single character following the carat is superscripted 7example3 #ajj1jjjb74 ,an underscore followed by one or more characters enclosed by curly brackets indicates that the the character7s7 within the curly brackets is_/are subcripted 7example3 ,a#a74 ,,airplane ,,photography ,illustration ,,airplane ,,photography by ,,herbert ,e4 ,,ives ,major1 ,aviation ,section1 ,signal ,officers ,reserve ,corps1 ,united ,states ,army2 ,lately ,officer in ,charge of ,experimental ,department1 ,photographic ,branch1 ,air ,service #bjh ,,illustrations ,illustration ,philadelphia and ,london ,j4 ,b4 ,lippincott ,company ,copyright1 #aibj1 by ,j4 ,b4 ,lippincott ,company ,printed by ,j4 ,b4 ,lippincott ,company at the ,washington ,square ,press ,philadelphia1 ,u4 ,s4 ,a4 ,,to ,,my ,,wife ,a ,,helpful ,,critic1 ,,even ,,though ,,she ,,neither ,,photographs ,,nor ,,flies ,,preface ,airplane photography had its birth1 and passed through a period of feverish development1 in the ,great ,war4 ,probably to many minds it figures as a purely military activity4 ,such need not be the case1 for the application of aerial photography to mapping and other peace-time problems promises soon to quite overshadow its military origin4 ,it has therefore been the writer's endeavor to treat the subject as far as possible as a problem of scientific photography1 emphasizing those general principles which will apply no matter what may be the purpose of making photographs from the air4 ,it is of course inevitable that whoever at the present time attempts a treatise on this newest kind of photography must draw much of his material from war-time experience4 ,if1 for this reason1 the problems and illustrations of this book are predominantly military1 it may be remembered that the demands of war are far more severe than those of peace2 and hence the presumption is that an account of how photography has been made successful in the military plane will serve as an excellent guide to meeting the peace-time problems of the near future4 ,it is assumed that the reader is already fairly conversant with ordinary photography4 ,considerable space has indeed been devoted to a discussion of the fundamentals of photography1 and to scientific methods of study1 test1 and specification4 ,this has been done because aerial photography strains to the utmost the capacity of the photographic process1 and it is necessary that the most advanced methods be understood by those who would secure the best results or contribute to future progress4 ,no pretence is made that the book is an aerial photographic encyclopdia2 it is not a manual of instructions2 nor is its appeal so popular as it would be were the majority of the illustrations striking aerial photographs of war subjects4 ,it is hoped that the middle course steered has produced a volume which will be informative and inspirational to those who are seriously interested either in the practice of aerial photography or in its development4 ,the writer is deeply in debt to many people1 whose assistance of one sort or another has made this book possible4 ,first of all should be mentioned those officers of the ,english1 ,french and ,italian armies through whose courtesy it is that he can speak at first hand of the photographic practices in these armies at the front4 ,it is due to ,lieutenant ,colonel ,r4 ,a4 ,millikan that the experimental work of which the writer has had charge was initiated in the ,united ,states ,air ,service4 ,to him and to ,major ,c4 ,e4 ,mendenhall1 under whom the work was organized in the ,science and ,research ,division of the ,signal ,corps1 are owing the writer's thanks for the opportunities and support given by them4 ,a similar acknowledgment is made to ,lieutenant ,colonel ,j4 ,s4 ,sullivan1 ,chief of the ,photographic ,branch of the ,army ,air ,service1 for his interest and encouragement in the compilation of this work1 and for the permission accorded to use the air service photographs and drawings which form the majority of the illustrations4 ,the greatest debt of all1 however1 is to those officers who have formed the staff of the ,experimental ,department4 ,to mention them by name3 ,captain ,c4 ,a4 ,proctor1 who was charged with our foreign liaison1 and who acted as deputy chief during the writer's absence overseas2 ,captain ,a4 ,k4 ,chapman1 in charge of the work on optical parts1 and later chief of our ,rochester ,branch2 ,captain ,e4 ,f4 ,kingsbury1 who had immediate charge of camera development2 ,lieutenant ,j4 ,b4 ,brinsmade and ,mr4 ,r4 ,p4 ,wentworth1 who handled the experimental work on camera mountings and installation2 ,lieutenant ,a4 ,h4 ,nietz1 in charge of the ,langley ,field ,laboratory of the ,experimental ,department2 ,mr4 ,r4 ,b4 ,wilsey and ,lieutenant ,j4 ,m4 ,hammond1 who1 with ,lieutenant ,nietz1 carried on the experimental work on sensitized materials4 ,a large part of what is new and what is ascribed in the following chapters to 8,the ,american ,air ,service0 is the work of this group of men4 ,were individual references made1 in place of this general and inclusive one1 their names would thickly sprinkle these pages4 ,it has been a rare privilege to have associates so able1 enthusiastic1 and loyal4 ,,the ,,author ,,november1 #aiai ,,contents ,i4 ,,introductory ,,chapter ,,page #a4 ,,general ,,survey #ae #b4 ,,the ,,airplane ,,considered ,,as ,a ,,camera ,,platform #bj ,,ii4 ,,the ,,airplane ,,camera #c4 ,,the ,,camera--,,general ,,considerations #ci #d4 ,,lenses ,,for ,,aerial ,,photography #dd #e4 ,,the ,,shutter #fh #f4 ,,plate-,,holders ,,and ,,magazines #hg #g4 ,,hand-,,held ,,cameras ,,for ,,aerial ,,work #ie #h4 ,,non-,,automatic ,,aerial ,,plate ,,cameras #ajb #i4 ,,semi-,,automatic ,,aerial ,,plate ,,cameras #aaf #aj4 ,,automatic ,,aerial ,,plate ,,cameras #abd #aa4 ,,aerial ,,film ,,cameras #acj #ab4 ,,motive ,,power ,,for ,,aerial ,,cameras #ade #ac4 ,,camera ,,auxiliaries #afc ,,iii4 ,,the ,,suspension ,,and ,,installation ,,of ,,airplane ,,cameras #ad4 ,,theory ,,and ,,experimental ,,study ,,of ,,methods ,,of ,,camera ,,suspension #agi #ae4 ,,practical ,,camera ,,mountings #aic #af4 ,,installation ,,of ,,cameras ,,and ,,mountings ,,in ,,planes #bjh ,,iv4 ,,sensitized ,,materials ,,and ,,chemicals #ag4 ,,the ,,distribution ,,of ,,light1 ,,shade ,,and ,,color ,,in ,,the ,,aerial ,,view #bba #ah4 ,,characteristics ,,of ,,photographic ,,emulsions #bbg #ai4 ,,filters #bci #bj4 ,,exposure ,,of ,,aerial ,,negatives #bdg #ba4 ,,printing ,,media #beb #bb4 ,,photographic ,,chemicals #beg ,v4 ,,methods ,,of ,,handling ,,plates1 ,,films ,,and ,,papers #bc4 ,,the ,,developing ,,and ,,drying ,,of ,,plates ,,and ,,films #bfg #bd4 ,,printing ,,and ,,enlarging #bgi ,,vi4 ,,practical ,,problems ,,and ,,data #be4 ,,spotting #bia #bf4 ,,map ,,making #cjd #bg4 ,,oblique ,,aerial ,,photography #cbj #bh4 ,,stereoscopic ,,aerial ,,photography #cbi #bi4 ,,the ,,interpretation ,,of ,,aerial ,,photographs #cea #cj4 ,,naval ,,aerial ,,photography #cfh ,,vii4 ,,the ,,future ,,of ,,aerial ,,photography #ca4 ,,future ,,developments ,,in ,,apparatus ,,and ,,methods #chc #cb4 ,,technical ,,and ,,pictorial ,,uses #chh #cc4 ,,exploration ,,and ,,mapping #dja ,i ,,introductory ,,airplane ,,photography ,,chapter ,i ,,general ,,survey ,aerial ,photography from ,balloons and ,kites4--,photography from the air had been developed and used to a limited extent before the ,great ,war1 but with very few exceptions the work was done from kites1 from balloons1 and from dirigibles4 ,aerial photographs of ,european cities had figured to a small extent in the illustration of guidebooks1 and some aerial photographic maps of cities had been made1 notably by the ,italian dirigible balloon service4 ,kites had been employed with success to carry cameras for photographing such objects as active volcanoes1 whose phenomena could be observed with unique advantage from the air1 and whose location was usually far from balloon or dirigible facilities4 ,as a result of this pre-war work we had achieved some knowledge of real scientific value as to photographic conditions from the air4 ,notable among these discoveries was the existence of a veil of haze over the landscape when seen from high altitudes1 and the consequent need for sensitive emulsions of considerable contrast1 and for color-sensitive plates to be used with color filters4 ,the development of aerial photography would probably however have advanced but little had it depended merely on the balloon or the kite4 ,as camera carriers their limitations are serious4 ,the kite and the captive balloon cannot navigate from place to place in such a way as to permit the rapid or continuous photography of extended areas4 ,the kite suffers because the camera it supports must be manipulated either from the ground or else by some elaborate mechanism1 both for pointing and for handling the exposing and plate changing devices4 ,the free balloon is at the mercy of the winds both as to its direction and its speed of travel4 ,the dirigible balloon1 as it now exists after its development during the war1 is1 it is true1 not subject to the shortcomings just mentioned4 ,indeed1 in many ways it is perhaps superior to the airplane for photographic purposes1 since it affords more space for camera and accessories1 and is freer from vibration4 ,it is capable also of much slower motion1 and can travel with less danger over forests and inaccessible areas where engine failure would force a plane down to probable disaster4 ,but the smaller types as at present built are not designed to fly so high as the airplane1 and the dirigibles1 both large and small1 are far more expensive in space and maintenance than the plane4 ,for this one reason especially they are not likely to be the most used camera carriers of the aerial photographer of the future4 ,inasmuch as the photographic problems of the plane are more difficult than those of the dirigible and at the same time broader1 the subject matter of this book applies with equal force to photographic procedure for dirigibles4 ,development of ,airplane ,photography in the ,great ,war4--,the airplane has totally changed the nature of warfare4 ,it has almost eliminated the element of surprise1 by rendering impossible that secrecy which formerly protected the accumulation of stores1 or the gathering of forces for the attack1 a flanking movement or a 8strategic retreat40 ,to the side having command of the air the plans and activities of the enemy are an open book4 ,it is true that more is heard of combats between planes than of the routine task of collecting information1 and the public mind is more apt to be impressed by the fighting and bombing aspects of aerial warfare4 ,nevertheless1 the fact remains that the chief use of the airplane in war is reconnaissance4 ,the airplane is 8the eye of the army40 ,in the early days of the war1 observation was visual4 ,it was the task of the observer in the plane to sketch the outlines of trenches1 to count the vehicles in a transport train1 to estimate the numbers of marching men1 to record the guns in an artillery emplacement and to form an idea of their size4 ,but the capacity of the eye for including and studying all the objects in a large area1 particularly when moving at high speed1 was soon found to be quite too small to properly utilize the time and opportunities available in the air4 ,moreover1 the constant watching of the sky for the 8,hun in the sun0 distracted the observer time and time again from attention to the earth below4 ,very early in the war1 therefore1 men's minds turned to photography4 ,the all-seeing and recording eye of the camera took the place of the observer in every kind of work except artillery fire control and similar problems which require immediate communication between plane and earth4 ,the volume of work done by the photographic sections of the military air service steadily increased until toward the end of the war it became truly enormous4 ,the aerial negatives made per month in the ,british service alone mounted into the scores of thousands1 and the prints distributed in the same period numbered in the neighborhood of a million4 ,the task of interpreting aerial photographs became a highly specialized study4 ,an entirely new activity--that of making photographic mosaic maps and of maintaining them correct from day to day--usurped first place among topographic problems4 ,by the close of the war scarcely a single military operation was undertaken without the preliminary of aerial photographic information4 ,photography was depended on to discover the objectives for artillery and bombing1 and to record the results of the subsequent 8shoots0 and bomb explosions4 ,the exact configurations of front1 second1 third line and communicating trenches1 the machine gun and mortar positions1 the 8pill boxes10 the organized shell holes1 the listening posts1 and the barbed wire1 were all revealed1 studied and attacked entirely on the evidence of the airplane camera4 ,toward the end of the war important troop movements were possible only under the cover of darkness1 while the development of high intensity flashlights threatened to expose even these to pitiless publicity4 ,limitations to ,airplane ,photography ,set by ,war ,conditions4--,the ability of the pilot to take the modern high-powered plane over any chosen point at any desired altitude in almost any condition of wind or weather gives to the plane an essential advantage over the photographic kites and balloons of pre-war days4 ,there are1 however1 certain disadvantages in the use of the plane which must be overcome in the design of the photographic apparatus and in the method of its use4 ,some few of these disadvantages are inherent in the plane itself2 for instance1 the necessity for high speed in order to remain in the air1 and the vibration due to the constantly running engine4 ,others are peculiar to war conditions1 and their elimination in planes for peace-time photography will give great opportunities for the development of aerial photography as a science4 ,chief among the war-time limitations is that of economy of space and weight4 ,a war plane must carry a certain equipment of guns1 radio-telegraphic apparatus and other instruments1 all of which must be readily accessible4 ,many planes have duplicate controls in the rear cockpit to enable the observer to bring the plane to earth in case of accident to the pilot4 ,armament and controls demand space which must be subtracted from quarters already cramped1 so that in most designs of planes the photographic outfit must be accommodated in locations and spaces wretchedly inadequate for it4 ,economy in weight is pushed to the last extreme1 for every ounce saved means increased ceiling and radius of action1 a greater bombing load1 more ammunition1 or fuel for a longer flight4 ,hence comes the constant pressure to limit the weight of photographic and other apparatus1 even though the tasks required of the camera constantly call for larger rather than smaller equipment4 ,to another military necessity is due in great measure the forced development of aerial photographic apparatus in the direction of automatic operation4 ,the practice of entrusting the actual taking of the pictures to observers with no photographic knowledge1 whose function was merely to 8press the button0 at the proper time1 necessitated cameras as simple in operation as possible4 ,the multiplicity of tasks assigned to the observer1 and in particular the ever vital one of watching for enemy aircraft1 made the development of largely or wholly automatic cameras the war-time ideal of all aerial photographic services4 ,whether the freeing of the observer from other tasks will relegate the necessarily complex and expensive automatic camera to strictly military use remains to be seen4 ,,chapter ,,ii ,,the ,,airplane ,,considered ,,as ,a ,,camera ,,platform ,an essential part of the equipment of either the aerial photographer or the designer of aerial photographic apparatus is a working knowledge of the principles and construction of the airplane1 and considerable actual experience in the air4 ,conditions and requirements in the flying plane are far different from those of the shop bench or photographic studio4 ,as a preliminary to undertaking any work on airplane instruments a good text-book on the principles of flight should be studied4 ,such general ideas as are necessary for understanding the purely photographic problems are1 however1 outlined in the next paragraphs4 ,illustration3 ,,fig4 #a4--,the elements of the plane4 ,construction of the ,airplane4--,the modern airplane 7,fig4 #a7 consists of one or more planes1 much longer across than in the direction of flight 7aspect ratio74 ,these are inclined slightly upward toward the direction of travel1 and their rapid motion through the air1 due to the pull of the propeller driven by the motor1 causes them to rise from the earth1 carrying the fuselage or body of the airplane4 ,in the fuselage are carried the pilot1 observer1 and any other load4 ,wheels below the fuselage forming the under-carriage or landing gear serve to support the body when running along the ground in taking off or landing4 ,the pilot1 sitting in one of the cockpits1 has in front of him the controls1 by means of which the motion of the plane is guided 7,figs4 #b and #c74 ,these consist of the engine controls--the contacts for the ignition1 the throttle1 the oil and gasoline supply1 air pressure1 etc41 and the steering controls--the rudder bar1 the stick and the stabilizer control4 ,the rudder bar1 operated by the feet1 controls both the rudder of the plane1 which turns the plane to right or left in the air1 and the tail skid1 for steering on the ground4 ,the stick is a vertical column in front of the pilot which1 when pushed forward or back1 depresses or raises the elevator and makes the machine dive or climb4 ,thrown to either side it operates the ailerons or wing tips1 which cause the plane to roll about its fore and aft axis4 ,the stabilizer control is usually a wheel at the side of the cockpit1 whose turning varies the angle of incidence of the small stabilizing plane in front of the elevator1 to correct the balance of the plane under different conditions of loading4 ,illustration3 ,,fig4 #b4--;,forward cockpit of ,de,haviland #d1 showing instrument board4 ,illustration3 ,,fig4 #c4--,rear cockpit of ,de,haviland #d1 showing rear 8stick0 and rudder bar4 ,the fuselage consists usually of a light hollow framework of spruce or ash1 divided into a series of bays or compartments by upright members1 connecting the longerons1 which are the four corner members1 running fore and aft1 of the plane4 ,diagonally across the sides and faces of these bays are stretched taut piano wires1 and the whole structure is covered with canvas or linen fabric4 ,cross-wires and fabric are omitted from the top of one or more bays to permit their being used as cockpits for pilot and observer4 ,in later designs of planes the wire and fabric construction has been superseded by ply-wood veneer1 thereby strengthening the fuselage so that many of the diagonal bracing wires on the inside are dispensed with4 ,this greatly increases the accessibility of the spaces in which cameras and other apparatus must be carried4 ,illustration3 ,,fig4 #d4--;,biplane in flight4 ,the fuselage differs greatly in cross-section shape and in roominess according to the type of engine4 ,in the majority of ,english and ,american planes1 with their vertical cylinder or ,v type engines1 the fuselage is narrow and rectangular in cross-section4 ,in many ,french planes1 radial or rotary engines are used and the fuselage is correspondingly almost circular1 and so is much more spacious than the ,english and ,american planes of similar power4 ,the shape and size of the plane body has an important bearing on the question of camera installation4 ,illustration3 ,,fig4 #e4--;,a single-seater4 ,types of ,planes4--,the most common type of plane is the biplane 7,fig4 #d71 with its two planes1 connected by struts and wires1 set not directly over each other1 but staggered1 usually with the upper plane leading4 ,monoplanes were in favor in the early days of aviation1 and triplanes have been used to some extent4 ,according to the position of the propeller planes are classified as tractors or pushers1 tractors being at present the more common form4 ,planes are further classified as single-seaters 7,fig4 #e71 two-seaters1 and three-seaters4 ,these motor and passenger methods of classification are now proving inadequate with the rapid development of planes carrying two1 three1 and even more motors1 divided between pusher and tractor operation1 and carrying increasingly large numbers of passengers4 ,aside from structure1 planes may be further classified according to their uses1 as scout1 combat1 reconnaissance1 bombing1 etc4 ,planes equipped with floats or pontoons for alighting on the water are called seaplanes 7,fig4 #ahb71 and those in which the fuselage is boat-shaped1 to permit of floating directly on the water1 are flying boats 7,fig4 #ahc74 ,the ,plane in the ,air4--,the first flight of the photographic observer or of the instrument expert who is to work upon airplane instruments is very profitably made as a 8joy ride10 to familiarize him with conditions in the air4 ,his experience will be somewhat as follows3 ,the plane is brought out of the hangar1 carefully gone over by the mechanics1 and the engine 8warmed up40 ,the pilot minutely inspects all parts of the 8ship10 then climbs up into the front cockpit4 ,he wears helmet and goggles1 and if the weather is cold or if he expects to fly high1 a heavy wool-lined coat or suit1 with thick gloves and moccasins1 or an electrically heated suit4 ,the passenger1 likewise attired1 climbs into the rear cockpit and straps himself into the seat4 ,he finds himself sitting rather low down1 with the sides of the cockpit nearly on a level with his eyes4 ,to either side of his knees and feet are taut wires leading from the controls to the elevator1 stabilizer1 tail skid and rudder4 ,if the machine is dual control1 the stick is between his knees1 the rudder bar before his feet4 ,none of these must he let his body touch1 so in the ordinary two-seater his quarters are badly cramped4 ,at the word 8contact0 the mechanics swing the propeller1 and1 sometimes only after several trials1 the motor starts1 with a roar and a rush of wind in the passenger's face4 ,after a moment's slow running it is speeded up1 the intermittent roar becomes a continuous note1 the plane shakes and strains1 while the mechanics hold down the tail to prevent a premature take-off4 ,when the engine is properly warmed up it is throttled to a low speed1 the chocks under the wheels are removed1 the mechanics hold one end of the lower wing so that the plane swings around toward the field4 ,it then 8taxis0 out to a favorable position facing into the wind with a clear stretch of field before it4 ,after a careful look around to see that no other planes are landing1 taking off1 or in the air near by1 the pilot opens out the engine1 the roar increases its pitch1 the plane moves slowly along the ground1 gathers speed and rises smoothly into the air4 ,near the ground the air is apt to be 8bumpy10 the plane may drop or rise abruptly1 or tilt to either side4 ,the pilot instantly corrects these deviations1 and the plane continues to climb until steadier air is reached4 ,at first the passenger's chief impressions are apt to be the deafening noise of the motor1 the heavy vibration1 the terrific wind in his face4 ,if he raises his hand above the edge of the cockpit he realizes the magnitude of wind resistance at the speed of the plane1 and hence the importance of the stream-line section of all struts and projecting parts4 ,when he reaches the desired altitude the pilot levels off the plane and ceases to climb4 ,now his task is to maintain the plane on an even keel by means of the controls1 correcting as soon as he notes it1 any tendency to 8pitch10 to 8roll0 or to 8yaw0 off the course4 ,the resultant path is one which approximates to level straight flying to a degree conditioned by the steadiness of the air and the skill of the pilot4 ,if he is not skilful or quick in his reactions he may keep the plane on its level course only by alternately climbing and gliding1 by flying with first one wing down and then the other1 by pointing to the right and then to the left4 ,the skilled photographic pilot will hold a plane level in both directions to within a few degrees1 but he will do this easily only if the plane is properly balanced4 ,if the load on the plane is such as to move the center of gravity too far forward with respect to the center of lift the plane will be nose-heavy1 if the load is too far back it will be tail-heavy4 ,either of these conditions can be corrected1 at some cost in efficiency1 by changing the inclination of the stabilizer4 ,when the plane reaches high altitudes in rare air1 where it can go no further1 it is said to have reached its ceiling4 ,it here travels level only by pointing its wings upward in the climbing position1 so that the fuselage is no longer parallel to the direction of flight4 ,an understanding of these peculiarities of the plane in flight is of prime importance in photographic map making1 where the camera should be accurately vertical at all times4 ,the direction and velocity of the wind must be carefully considered by the pilot in making any predetermined course or objective4 ,the progress of the plane due to the pull of the propeller is primarily with reference to the air4 ,if this is in motion the plane's ground speed and direction will be altered accordingly4 ,in flying with or against the wind the ground speed is the sum or difference1 respectively1 of the plane's air speed 7determined by an air speed indicator7 and the speed of the wind4 ,if the predetermined course lies more or less across the wind the plane must be pointed into the wind1 in which case its travel1 with respect to the earth1 is not in the line of its fore and aft axis4 ,the effect of 8crabbing10 as it is called1 on photographic calculations is discussed later 7,figs4 #acf and #ach74 ,when the plane has reached the end of its straight course and starts to turn1 its level position is for the moment entirely given up in the operation of banking 7,fig4 #f74 ,just as the tracks on the curve of a railroad are raised on the outer side to oppose the tendency of the train to slip outward1 so the plane must be tilted1 by means of the ailerons1 toward the inside of the turn4 ,a point to be clearly kept in mind about a bank is that if correctly made a plumb line inside the fuselage will continue to hang vertical with respect to the floor of the plane1 and not with respect to the earth1 for the force acting on it is the combination of gravity and the acceleration outward due to the turn4 ,only some form of gyroscopically controlled pointer1 keeping its direction in space1 will indicate the inclination of the plane with respect to the true vertical4 ,if the banking is insufficient the plane will side slip outward or skid2 if too great1 it will side slip inward4 ,illustration3 ,,fig4 #f4--;,banking4 ,as part of the 8joy ride0 the pilot may do a few 8stunts10 such as a 8stall10 a 8loop10 a 8tail spin10 or an 8,immelman40 ,from the photographic standpoint these are of interest in so far as they bear on the question of holding the camera in place in the plane4 ,the thing to be noted here is that 7particularly in the loop71 if these maneuvers are properly performed1 there is little tendency toward relative motion between plane and apparatus4 ,in a perfect loop it would1 for instance1 be unnecessary1 due to the centrifugal force outward1 for the observer to strap himself in4 ,it is1 however1 unwise to place implicit confidence in the perfection of the pilot's aerial gymnastics4 ,no apparatus should be left entirely free1 although1 for the reason given1 comparatively light fastenings are usually sufficient4 ,when nearing the landing field the pilot will throttle down the engine and commence to glide4 ,if he is at a considerable altitude he may come down a large part of the distance in a rapid spiral4 ,as the earth is approached the air pressure increases rapidly1 and the passenger1 if correctly instructed1 will open his mouth and swallow frequently to equalize the air pressure on his ear drums4 ,just before the ground is reached the plane is leveled off1 it loses speed1 and1 if the landing is perfect1 touches and runs along the ground without bouncing or bumping4 ,frequently1 however1 the impact of the tail is sufficiently hard to cause it to bump badly1 with a consequent considerable danger to apparatus of any weight or delicacy4 ,this is especially apt to occur in hastily chosen and poorly leveled fields such as must often be utilized in war or in cross-country flying4 ,appearance of the ,earth from the ,plane4--,the view from the ordinary two-seater is greatly restricted by the engine in front and by the planes to either side and below 7,figs4 #g1 #h1 and #i74 ,by craning his neck over the side1 or by looking down through an opening in the floor1 the passenger has an opportunity to learn the general appearance of the subject he is later to devote his attention to photographing4 ,perhaps the most striking impression he receives will be that of the flatness of the earth1 both in the sense of absence of relief and in the sense of absence of extremes of light and shade4 ,the absence of relief is due to the fact that at ordinary flying heights the elevations of natural objects are too small for the natural separation of the eyes to give any stereoscopic effect4 ,the absence of extremes of light and shade is in part due to the fact that the natural surfaces of earth1 grass and forest present no great range of brightness2 in part to the small relative areas of the parts in shadow2 in considerable part to the layer of atmospheric haze which lies as an illuminated veil between the observer and the earth at altitudes of #bjjj meters and over 7,figs4 #aj and #aa74 ,due to the combination of these factors the earth below presents the appearance of a delicate pastel4 ,as the gaze is directed away from the territory directly below1 the thickness of atmosphere to be pierced rapidly increases1 until toward the horizon 7which lies level with the observer here as on the ground7 all detail is apt to be obliterated to such an extent that only on very clear days can the horizon itself be definitely found or be distinguished from low lying haze or clouds 7,fig4 #d74 ,illustration3 ,,fig4 #g4--,the view ahead4 ,illustration3 ,,fig4 #h4--,the view astern4 ,airplane ,instruments4--,mounted on boards in front of the pilot and observer are various instruments to indicate the performance of engine and plane 7,fig4 #b74 ,those of interest to the photographic observer are the compass1 the altimeter1 the air speed indicator1 the inclinometers4 ,illustration3 ,,fig4 #i4--,the view between the wings4 ,illustration3 ,,fig4 #aj4--;,appearance of the earth from a low altitude--#cjjj feet or less4 ,the compass is usually a special airplane compass1 with its 8card0 immersed in a damping liquid4 ,like most of the direction indicating instruments on a plane its indications are only of significance when the plane is pursuing a steady course4 ,on turns or rapid changes of direction of any sort perturbations prevent accurate reading4 ,the altimeter is of the common aneroid barometer type4 ,on ,american instruments it is usually graduated to read in #ajj-;foot steps4 ,while somewhat sluggish1 it is quite satisfactory for all ordinary determinations of altitude in photographic work4 ,were primary map making to be undertaken1 where the scale was determinable only from the altitude and focal length of the lens1 the ordinary altimeter is hardly accurate enough4 ,illustration3 ,,fig4 #aa4--;,appearance of the earth from a high altitude--#aj1jjj feet or more4 ,the air speed indicator consists of a combination of ,venturi and ,pitot tubes1 producing a difference of pressure when in motion through the air which is measured on a scale calibrated in air speed4 ,this instrument is important for determining1 in combination with wind speed1 the ground speed of the plane1 on the basis of which is calculated the interval between exposures to secure overlapping photographs4 ,its accuracy is well above that necessary for the purpose4 ,inclinometers for showing the lateral and fore and aft angle of the plane with the horizontal1 are occasionally used1 and have also been incorporated in cameras4 ,the important point to remember about these instruments is that they are controlled not alone by gravity but as well by the acceleration of the plane in any direction4 ,they consequently indicate correctly only when the plane is flying straight4 ,on a bank the lateral indicator continues to indicate 8vertical0 if the bank is properly calculated for the turn4 ,,ii ,,the ,,airplane ,,camera ,,chapter ,,iii ,,the ,,camera--,,general ,,considerations ,chief ,uses of an ,airplane ,camera4--,the kinds of camera suitable for airplane use and the manner in which they must differ from cameras for use on the ground are determined by consideration of the nature of the work they must do4 ,four kinds of pictures constitute the ordinary demands upon the aerial photographer4 ,these are single objectives or pin points1 mosaic maps of strips of territory or large areas1 oblique views1 and stereoscopic views4 ,each of these presents its own peculiar problems influencing camera design4 ,pinpoints consist of such objects as gun emplacements1 railway stations1 ammunition dumps1 and other objects of which photographs of considerable magnification are desired for study4 ,here the instrumental requirements are sufficient focal length of lens to secure an image of adequate size2 means for pointing the camera accurately2 enough shutter speed to counterbalance the speed of the plane2 sufficiently wide lens aperture to give adequate exposure with the shutter speed required2 means of supporting the camera to protect it from the vibration of the plane4 ,mosaic maps are built up from a large number of photographs of adjacent areas4 ,in addition to the above requirements1 mosaic maps demand lenses free from distortion and covering as large a plate as possible1 in order to keep to a minimum the number of pictures needed to cover a given area2 means for keeping the camera accurately vertical1 and means for changing the plates or films and resetting the shutter rapidly enough to avoid gaps between successive pictures4 ,at low altitudes and high ground speeds the interval between exposures becomes a matter of only a few seconds4 ,oblique views are made at angles of from #ab to #ce degrees from the horizontal1 usually from comparatively low altitudes4 ,they have been found to be particularly suitable for the use of men who have no training in photographic interpretation1 being more like the pictures with which the men are familiar4 ,distributed among the infantry before an attack1 they have proved indispensable aids to the proper knowledge of the ground to be covered4 ,the additional requirement here is for high shutter speed to eliminate the effect of the relatively very rapid movement of the foreground4 ,stereoscopic views are among the most useful of all airplane pictures4 ,they are made from successive exposures1 the separation of the points of view being obtained not by two lenses at the distance of the eyes apart1 but by the motion of the plane4 ,for this purpose the views should overlap by at least #fj per cent2 this1 therefore1 requires a very short interval between exposures4 ,for stereo-oblique views this may mean that they are taken at intervals as short as one or two seconds4 ,chief ,differences between ,ground and ,air ,cameras4--,certain definite differences are thus seen to stand out between airplane cameras and the ordinary kind4 ,it is essential that the apparatus for use in the air shall have high lens and shutter speed1 means for rapid changing of plates1 and anti-vibration suspension4 ,without these features a camera is of little use for aerial work4 ,these requirements lead inevitably to greater complexity of design4 ,one simplification over ground cameras1 however1 is brought about by the fact that all exposures are made on objects beyond the practical infinity point of the lens2 consequently1 all cameras are fixed focus4 ,this fixed focus feature is a positive advantage in construction1 since it permits of the simple rigid box form1 desirable and necessary to withstand the strains due to the weight of the lens and the stresses from the plane4 ,but with the abandonment of all provision for focussing in the air must go special care that the material used in constructing the camera body is as little subject as possible to expansion and contraction with temperature1 since there is often a drop of #cj to #dj degrees ,centigrade from ground to upper air4 ,the effect of change of temperature on focus will be treated in the discussion of lenses4 ,in addition to these differences1 we must keep in mind certain requirements which are conditioned by the nature and place of aerial navigation4 ,thus all mechanical devices which will fail to function at the low temperatures and pressures met at high altitudes are entirely unsuitable4 ,experience has shown1 too1 that we must avoid all mechanism depending primarily on springs and on the action of gravity4 ,vibration1 and the motion of the plane in all three dimensions1 conspire to render mechanical motions unreliable when actuated by these agencies4 ,all plate changing1 shutter setting1 and exposing operations should be as nearly as possible positively controlled motions4 ,because of the cold of the upper air all knobs1 levers and catches must be made extra large and easy to handle with heavy gloves4 ,circular knurled heads to such parts as shutter setting movements are to be avoided in favor of bat-wing keys or levers4 ,grooves for the reception of magazines must be as large and smooth as possible1 and guides to facilitate the magazines' introduction should be provided 7,fig4 #ej74 ,no releases or adjustments which depend upon hearing or upon a delicate sense of touch are feasible in airplane apparatus4 ,wherever possible1 large visible indicators of the stage of the cycle of operations should be provided4 ,loose parts are to be shunned1 as they are invariably lost in service4 ,complete operating instructions should be placed on the apparatus wherever possible1 to minimize the confusion due to changing and uninstructed personnel4 ,the ,elements of the ,airplane ,camera4--,disregarding its means of suspension1 the airplane camera proper consists essentially of lens1 camera body1 shutter1 and plate or film holding and changing box4 ,in certain of the aerial cameras developed early in the war all of these elements were built together in a common enclosure4 ,later it was generally recognized that a unit system of interchangeable parts is preferable4 ,in the case of the lens there arose various requirements for focal length1 from #be to #abj centimeters1 according to the work to be done4 ,rather than use an entirely different camera for each different kind of work1 it is better to have lenses of various focal lengths1 mounted in tubes or cones1 all built to attach to the same camera body4 ,in the case of the shutter it is desirable to be able to repair or calibrate periodically4 ,by making the shutter a removable unit1 the provision of a few spares does away with the need for putting the whole camera out of commission4 ,similar considerations hold with reference to other parts4 ,a further material advantage that comes from making airplane cameras in sections is the greater ease with which they are inserted in the plane1 usually through the openings between diagonal cross-wires4 ,it is in fact only by virtue of this possibility of breaking up into small elements that some of the larger cameras could be inserted in the common types of reconnaissance plane4 ,illustrations of the building up of cameras from separate removable elements are given in the detailed discussion of the individual types4 ,types of ,airplane ,cameras4--,during the course of the war airplane cameras have been classified on various bases1 in different services4 ,in the ,french service1 where the de,maria type of camera was standardized early in the war1 the usual classification was based on focal length2 thus the standard cameras were spoken of as the #bf1 the #ej and the #abj 7centimeter74 ,a further distinction was then made according to the size of plate1 this being originally #ac #ah centimeters for the #bf centimeter1 and #ah #bd centimeters for the larger cameras4 ,in the ,english service the #d #e inch plate was used almost exclusively1 and their various types of cameras were known by serial letters--,c1 ,e1 ,l1 etc4 ,both these modes of classification became inadequate with the ultimate agreement to standardize on the #ah #bd centimeter size for all plates1 and to carry lenses of all focal lengths in interchangeable elements4 ,for purposes of description and discussion1 it is most convenient to classify cameras according to their method of operation and the sensitive material employed4 ,on this basis we may distinguish among cameras using plates three kinds--non-automatic cameras1 semi-automatic cameras1 and automatic cameras4 ,we may similarly discuss film cameras1 but having treated the plate cameras comprehensively1 it will be found that the discussion of all types of film camera can be handled most conveniently by studying the differences in construction and operation introduced by the characteristics of film as compared to plates4 ,,chapter ,,iv ,,lenses ,,for ,,aerial ,,photography ,general ,considerations4--,the design and selection of lenses for aerial photography present on the whole no problems not already encountered in photography of the more familiar sort4 ,indeed1 the lens problem in the airplane camera is in some particulars more simple than in the ground camera4 ,for instance1 there is no demand for depth of focus--all objects photographed are well beyond the usually assumed 8infinity focus0 of #bjjj times the lens diameter4 ,such strictly scientific problems of design as pertain to aerial photographic lenses are ones of degree rather than of kind4 ,larger aperture1 greater covering power1 smaller distortion1 more exquisite definition--these always will be in demand1 and each progressive improvement will be reflected in advances in the art of aerial photography4 ,but many lens designs perfected before the war were admirably suited1 without any change at all1 for aerial cameras4 ,of the utmost seriousness1 however1 with the ,allies1 was the problem of securing lenses of the desired types in sufficient numbers4 ,the manufacture of the many varieties of optical glass essential to modern photographic lenses was almost exclusively a ,german industry1 which had to be learned and inaugurated in ,allied countries since #aiad4 ,in consequence of this entirely practical problem of quantity production without the glasses for which lens formul were at hand1 some new lens designs were produced4 ,whether any of these possess merits which will lead them to be preferred over pre-war designs1 when the latter can again be manufactured1 remains to be seen4 ,while the glass problem was still unsolved1 aerial cameras had to be equipped with whatever lenses could be secured by requisition from pre-war importation and manufacture1 and later1 with lenses designed to utilize those glasses whose manufacture had been mastered in the allied countries4 ,it is important that the historical aspect of this matter be well understood by the student of aerial photographic methods1 for the use of these odd-lot lenses reacted on the whole design of aerial cameras and on the methods of aerial photography1 particularly in ,england and the ,united ,states4 ,almost without exception the available lenses were of short focus1 considered from the aerial photographic standpoint2 that is1 they lay between eight and twelve inches4 ,this set a limit to the size of the airplane camera1 quite irrespective of the demands made by the nature of the photographic problem4 ,lenses of these focal lengths produced images which1 for the usual heights of flying1 were generally considered too small1 and which were1 therefore1 almost always subsequently enlarged4 ,such was the ,english practice1 which was followed in the training of aerial photographers in ,america1 where exactly similar conditions held at the start with respect to available lenses4 ,french glass and lens manufacturers did succeed in supplying lenses of longer focus 7#ej centimeters71 in numbers sufficient for their own service1 although never with any certainty for their allies4 ,the ,french1 therefore1 almost from the start1 built their cameras with lenses of long focus1 and made contact prints from their negatives4 ,practices adopted under pressure of an emergency to meet temporary practical limitations often come to dominate the whole situation4 ,this is particularly true of aerial photography in the ,british and ,american services4 ,the small apparatus built around the stop-gap short focus lenses fixed the plane designer's idea of an airplane camera1 and the space it should occupy4 ,this was directly reflected in the designs of the ,english planes1 and the ,american planes copied after them4 ,meanwhile the ,american photographic service in ,france associated itself with the ,french service1 adopting its methods and apparatus1 and using ,french planes whose designs were not being followed in ,american construction4 ,the task of harmonizing the photographic practice as taught in ,america1 following ,english lines1 with ,french practice as followed in the theater of war1 and of adapting planes built on ,english designs so that they could carry ,french apparatus1 was a formidable one1 not likely to be soon forgotten by any who had a part in it4 ,photographic ,lens ,characteristics4--,whole volumes have been written on the photographic lens1 and on the optical science utilized and indeed brought into being by its problems4 ,such works should be consulted by those who intend to make a serious study of the design of lenses for aerial use4 ,no more can be attempted1 no more indeed is relevant here1 than an outline review of the chief characteristics and errors of photographic lenses1 considering them with special reference to aerial needs4 ,the modern photographic lens is1 broadly speaking1 a development of the simple convex or converging lens4 ,its function is the same3 to form a real image of objects placed before it4 ,but the difference in performance between the simple lens and the modern photographic objective is enormous4 ,the simple lens forms a clear image only close to its axis1 for light of a single color1 and as long as its aperture is kept quite small as compared to the distance at which the image is formed4 ,the photographic lens1 on the other hand1 is called upon to produce a clear image with light of a wide range of spectral composition1 sharply defined over a flat surface of large area1 and it must do this with an aperture that is large in comparison with the focal length1 whereby the amount of light falling on the image surface shall be a maximum4 ,this ideal is approximated to a really extraordinary degree by the scientific combination and arrangement of lens elements made from special kinds of glass in the best photographic lenses of the anastigmat type4 ,the result is of necessity a set of compromises1 whereby the outstanding errors are reduced to a size judged permissible in view of the work the lens is to do4 ,these errors or aberrations are briefly reviewed below1 in order that the reader may readily grasp the terms in which the performance and tolerances in aerial lenses are described4 ,illustration3 ,,fig4 #ab4--;,diagrammatic representation of spherical aberration4 ,spherical ,aberration and ,coma4--,suppose we focus on a screen1 by means of a simple convex lens the image of a distant point of light4 ,suppose for simplicity that this image is located on the axis of the lens and that light of only one color is used1 such as yellow4 ,it will be found that the smallest image that can be obtained is not a point1 but a small disc4 ,this is due to the fact that the rays of light passing through the outer portions of the lens are bent more than those passing through the lens in the region near the center4 ,this effect is shown in ,fig4 #ab by the usual mode of representing it graphically4 ,here the figures #a1 #b1 #c1 #d1 represent distances from the axis of the lens1 and the letters ,a#a1 ,a#b1 ,a#c1 ,a#d1 the points of convergence of the rays from #a1 #b1 #c1 #d1 etc4 ,these distances projected upward on to the produced lens points form a curve which shows at a glance the extent and direction of the error due to each part of the lens4 ,this information is of value where the lens is fitted with an adjustable diafram4 ,with some types of correction sharper definition may be obtained by reducing the aperture4 ,with others1 however1 diaframing impairs definition1 by destroying the balance between under and over correction which averages to make a good image4 ,in aerial lenses it is not customary to use diaframs1 as all the light possible is desired4 ,consequently the reduction of spherical aberration must be accomplished by proper choice of lens elements and their arrangement4 ,off the axis of the lens the image of a point source takes on an irregular shape1 due to oblique spherical aberration or coma4 ,chromatic ,aberration4--,because of the inherent properties of the glass of which it is made1 a simple collective lens does not behave in the same way with respect to light of different colors4 ,if one attempts1 with such a lens1 to focus upon a screen the image of a distant white light1 it will be found that the blue rays will not focus at the same point as the red rays1 but will come together nearer the lens4 ,modern photographic lenses are compounded of two or more kinds of glass in such a way as to largely eliminate this defect1 the presence of which is detrimental to good definition4 ,such lenses are called achromatic1 and the property of a lens by virtue of which this defect is eliminated is called its chromatic correction4 ,chromatic correction is never perfect1 but two colors of the spectrum can be brought to a focus in the same plane1 and to a certain extent the departure of other colors from this plane can be controlled4 ,off the axis of the lens outstanding chromatic aberration results in a difference in the size of images of different colors1 known as lateral chromatism4 ,like spherical aberration1 chromatic aberration is a contributing factor to the size of the image of a point source1 which determines the defining power of a lens4 ,it is1 however1 an error whose effect is to some extent dependent on the kind of sensitive plate used4 ,two lenses may give images of the same size 7in so far as it is governed by chromatic aberration71 if a plate of narrow spectral sensitiveness is used1 while giving images of different size on panchromatic plates of more extended color sensibility4 ,the choice of the region of the spectrum for which chromatic correction is to be made is thus governed by the color of the photographically effective light4 ,while in ordinary photography the blue of the spectrum is most important1 in aerial work where color filters are habitually used with isochromatic plates the green is most important1 and color correction centered about this region constitutes a real difference of design peculiar to aerial lenses4 ,similarly the general use of deep orange or red filters with red sensitive plates1 for heavy mist penetration1 would call for a shift of correction to that part of the spectrum4 ,astigmatism and ,covering ,power4--,suppose the lens forms at some point off its axis an image of a cross4 ,suppose one of the elements of the cross to be on a radius from the center of the field1 the other element parallel to a tangent4 ,the rays forming the images of these two elements of the cross are subject to somewhat different treatment in their passage through the lens4 ,the curvature of the lens surfaces is on the whole greater with respect to the rays from the radial element than to those from the tangential element4 ,they are therefore refracted more strongly and come to a focus nearer the lens4 ,the arms of the cross are consequently not all in focus at once4 ,this error1 termed astigmatism1 is rather well shown in ,fig4 #ae1 where the images of the outlying concentric circles are sharp in the radial1 but blurred in the tangential direction4 ,astigmatism can be largely compensated for1 and its character controlled4 ,the most usual correction brings the two images in focus together both at the axis1 and on a circle at some distance out4 ,this second locus of coincidence may or may not be in the same plane as the first1 depending on which disposition produces the best average correction4 ,the mean between the two foci determines the focal plane of the lens1 which is in general somewhat curved4 ,the covering power of a lens is given by the size of the field which is sufficiently flat and free from astigmatism for the purpose for which the lens is used4 ,this is largely determined by the astigmatism1 but the other aberrations are also important4 ,illumination4--,the amount of light concentrated by the lens on each elementary area of the image determines its brightness or illumination4 ,the ideal image would1 of course1 be equally bright over its whole area of good definition1 and for lenses of narrow angle this is approximately true4 ,but when it is desired to cover a wide angle the question of illumination becomes serious4 ,the relationship between angle from the axis and illumination is that illumination is proportional to the fourth power of the cosine of the angle4 ,this relationship is shown in the following table3 ,angle ,image brightness #j #ajj per cent4 #aj #id4a per cent4 #bj #gh4j per cent4 #cj #ef4b per cent4 #dj #cd4d per cent4 #ej #ag4a per cent4 ,if the field of view is #fj1 which corresponds to an #ah #bd centimeter plate with a lens of #be centimeter focus1 the brightness is only #ef per cent41 and the necessary exposure at the edge approximately #a4h times that at the center4 ,this effect is shown in ,fig4 #ae4 ,it is very noticeable if the exposure is so short as to place the outlying areas in the under-exposure period4 ,illustration3 ,,fig4 #ac4--;,barrel and pin-cushion distortion4 ,distortion4--,sometimes a lens is relatively free from all the aberrations1 mentioned above1 so that it gives sharp1 clear images on the plate1 yet these images may not be exactly similar to the objects themselves as regards their geometrical proportions2 in other words1 the image will show distortion4 ,lens distortion assumes two typical forms1 illustrated in ,fig4 #ac1 which shows the result of photographing a square net-work with lenses suffering in the one case from 8barrel0 distortion and in the other from 8pin-cushion0 distortion4 ,in the first the corners are drawn in relative to the sides2 in the latter case the sides are drawn in with respect to the corners4 ,either sort is a serious matter in precision photography1 such as aerial photographic mapping aspires to become4 ,it must be reduced to a minimum and its amount must be accurately known if negatives are to be measured for the precise location of photographed objects4 ,in general symmetrical lenses give less distortion than the unsymmetrical 7,fig4 #ad74 ,illustration3 ,,fig4 #ad4--;,arrangement of elements in two lenses suitable for aerial work3 a1 ,zeiss ,tessar2 two simple and one cemented components 7unsymmetrical72 b1 ,hawkeye ,aerial2 two positive elements of heavy barium crown1 two negative of barium flint1 uncemented 7symmetrical74 ,lens ,testing and ,tolerances for ,aerial ,work4--,simple and rapid comparative tests of lenses may be made by photographing a test chart1 consisting of a large flat surface on which are drawn various combinations of geometrical figures--lines1 squares1 circles1 etc4--calculated to show up any failures of defining power4 ,for testing aerial lenses the chart should be as large as possible1 so that it may be photographed at a distance great enough for the performance of the lens to be truly representative of its behavior on an object at infinite distance4 ,this means in practice a chart of #d or #e meters side1 to be photographed at a distance #bj to #cj times the focal length of the lens4 ,illustration3 ,,fig4 #ae4--,photograph of a lens testing chart1 showing failure in defining power outside area for which the lens is calculated4 ,a typical photograph of such a chart is shown in ,fig4 #ae4 ,it reveals at a glance the more conspicuous lens errors4 ,at the sides and corners the concentric circles show the lens's astigmatism1 by the clear definition of the lines radial to the center of the field and their blurring in the tangential direction4 ,the falling off in illumination with increasing distance from the center is also exhibited2 and the blurring of all detail outside the rectangle for which the lens was calculated shows that spherical1 chromatic1 and other aberrations have become prohibitively large4 ,but the only complete test of a lens is the quantitative measurement of errors made on an optical bench4 ,a point source of light1 which may at will be made of any color of the spectrum1 is used as the object and its image formed by the lens in a position where it can be accurately measured for location1 size1 and shape by a microscope4 ,a chart giving the results of such a test is shown in ,fig4 #af4 ,in the upper left-hand corner is shown the position of the focus for the different colors of the spectrum4 ,below this is recorded the lateral chromatism at #ba degrees1 in terms of the difference in focus for a red and a blue ray4 ,below this again comes the distortion1 or shift of the image from its proper position1 for various angles 7plotted at the extreme right7 from the lens axis4 ,to the right of this is the image size1 at each angle1 and finally1 to the right of the diagram1 are plotted the distances of the two astigmatic foci from the focal plane1 together with the mean of the two foci1 which practically determines the shape of the field4 ,an important point to notice is that these data are uniformly plotted in terms of a lens of #ajj millimeters focal length irrespective of the actual focal length of the lens measured4 ,thus this particular chart is for a #ej centimeter lens but would be plotted on the same scale for a #be or a #ajj centimeter lens4 ,underlying this practice is the assumption that all the characteristics of lenses of the same design and aperture are directly proportional to their focal length4 ,if this were so1 then a #ej centimeter lens would give double the size of image that a #be centimeter does1 and so on4 ,as a matter of fact1 test shows that the size of the image does not increase so rapidly as the focal length2 so that while the image size for a #be centimeter lens would be1 say1 4#je millimeters per #ajj millimeters focal length1 it will be only 4#jc or 4#jd millimeters per #ajj millimeters focal length for a #ej centimeter lens4 ,the actual size of a point image will therefore be greater1 though not proportionately greater4 ,illustration3 ,,fig4 #af4--;,chart recording measurements of lens characteristics4 ,the chart presents tests on a good quality lens1 and so gives a good idea of the permissible magnitude of the various errors4 ,in many ways the most important figure is that for image size1 including as it does the result of all the aberrations4 ,in the example given1 this varies from 4#jge to 4#ae mm4 actual size4 ,for the same type of lens of #be centimeters focus this range will be from 4#je to 4#aj mm4 ,since these are commonly used focal lengths1 a good average figure for image size1 commonly used in aerial photographic calculations1 is mm4 ,in regard to astigmatic tolerances1 the two astigmatic foci should not be separated at any point by more than #f to #g millimeters1 and the mean of these should not deviate from the true flat field by more than millimeter1 in each case the figures being based on the conventional #ajj millimeters focal length4 ,distortion should not be over 4#jh millimeter at #ah or 4#bj millimeter at #bd from the axis 7per #ajj millimeters focal length74 ,lens ,aperture4--,in the simple lens the aperture is merely the diameter4 ,in compound lenses the aperture is not the linear opening but the effective opening of an internal diafram4 ,photographically1 however1 aperture has come to have a more extensive meaning4 ,while in the telescope the actual diameter of an objective is perhaps the most important figure1 and in the microscope the focal length1 in photography the really important feature is the amount of light or illumination4 ,this is determined by lens opening and focal length together2 specifically1 by the ratio of the lens area to the focal length4 ,the common system of representing photographic lens aperture is by the ratio of focal length to lens diameter1 the lens being assumed to be circular4 ,thus ,f_/#e 7often written ,f4#e7 indicates that the diameter is one-fifth the focal length4 ,two points are to be constantly borne in mind in connection with this system of representation4 ,first1 all lenses of the same aperture 7as so represented7 give the same illumination of the plate 7except for differences due to loss of light by absorption and reflection in the lens system74 ,this follows simply from the fact that the illumination of the plate is directly proportional to the square of the lens diameter1 and inversely as the square of the focal length4 ,secondly1 the illumination of the plate is inversely as the square of the numerical part of the expression for aperture4 ,that is1 lenses of aperture ,f_/#d4e and ,f_/#f give images of relative brightness 7#f_/d4e7b #a4gh4 ,what lens aperture1 and therefore what image brightness1 is feasible1 is determined chiefly by the angular field that must be covered with any given excellence of definition4 ,the largest aperture ordinarily used for work requiring good definition and flat field free from distortion is ,f_/#d4e4 ,anastigmatic lenses of this aperture cover an angle of #af to #ah from the axis satisfactorily1 which corresponds to an #ah #bd centimeter plate with a lens of #ej centimeters focus4 ,lenses with aperture as large as ,f_/#c4e were used to some extent in ,german hand cameras of #be centimeters focal length1 with plates of #i #ab centimeters4 ,english and ,american lenses of this latter focal length were commonly of aperture ,f_/#d4e1 designed to cover a #d #e inch plate4 ,as a general rule the greater the focal length the smaller the aperture--a relationship primarily due to the difficulty of securing optical glass in large pieces4 ,thus while #ej centimeter lenses of aperture ,f_/#d4e are reasonably easy to manufacture1 the practicable aperture for quantity production is ,f_/#f1 and for #abj centimeter lenses1 ,f_/#aj4 ,this means that a very great sacrifice of illumination must be faced to secure these greater focal lengths4 ,as is to be expected from the state of the optical glass industry1 the ,german lenses were of generally larger aperture for the same focal lengths than were those of the ,allies4 ,besides the ,f_/#c4e lenses already mentioned1 their #ej centimeter lenses were commonly of aperture ,f_/#d4h1 their #abj centimeter lenses of aperture ,f_/#g1 or of about double the illuminating power of the ,french lenses of the same size4 ,demands for large covering power also result in smaller aperture4 ,the #bf centimeter lenses used on ,french hand cameras utilizing #ac #ah centimeter plates were commonly of aperture ,f_/#f or ,f_/#e4f4 ,the lens of largest covering power decided on for use in the ,american service was of #ab inch focus1 to be used with an #ah #bd centimeter plate 7extreme angle #bf72 the largest satisfactory aperture for this lens is ,f_/#e4f4 ,ordinarily the question of aperture is closely connected with that of diaframs1 whereby the lens aperture may be reduced at will4 ,diaframs have been very little used in aerial photography4 ,all the aperture that can be obtained and more is needed to secure adequate photographic action with the short exposures required under the conditions of rapid motion and vibration peculiar to the airplane4 ,any excess of light1 over the minimum necessary to secure proper photographic action1 is far better offset by increase of shutter speed or by introduction of a color filter4 ,for this reason ,american aerial lenses were made without diaframs4 ,in the ,german cameras1 however1 adjustable diaframs are provided 7,fig4 #dc71 controlled from the top of the camera by a rack and pinion4 ,in the camera most used in the ,italian service an adjustable diafram is provided1 but this is occasioned by the employment of a between-the-lens shutter of fixed speed1 so that the only way exposure can be regulated is by aperture variation1 a method which has little to recommend it4 ,the ,question of ,focal ,length4--,in aerial photography the lens is invariably used at fixed1 infinity1 focus4 ,under these conditions the simple relationship holds that the size of the image is directly proportional to the focal length and inversely proportional to the altitude4 ,if any chosen scale is desired for the picture the choice of focal length is determined by the height at which it is necessary to fly4 ,this at least would be the case were there no limitation to the practicable focal length--which means camera size--and were one limited to the original size of the picture as taken2 that is1 were the process of enlargement not available4 ,but the possibility of using the enlarging process brings in other questions3 ,is the defining power of a short focus lens as good in proportion to its focal length as that of a long focus lens8 ,if so a perfect enlargement from a negative made by a short focus lens would be identical with a contact print from a negative made with a lens of longer focus4 ,is defining power lost in the enlarging process with its necessary employment of a lens which has its own errors of definition and which must be accurately focussed8 ,certain factors which enter into comparisons of this sort in other lines of work1 such as astronomical photography1 play little part here4 ,these are1 first1 the optical resolving power of the lens1 which is conditioned by the phenomena of diffraction1 and is directly as the diameter2 and1 second1 the size of the grain of the plate emulsion4 ,the first of these does not enter directly1 because the size of a point image on the axis of the lens1 due merely to diffraction1 is very much less than that given by any photographic lens which has been calculated to give definition over a large field1 instead of the minute field of the telescope4 ,yet it may contribute toward somewhat better definition with a long focus lens because of the actually larger diameter of such lenses4 ,the second factor is not important1 because1 as will be seen later1 the resolving power of the plates suitable for aerial photography is considerably greater than that of the lens4 ,the emulsion grain is in fact only a quarter or a fifth the size of the image as given by a #be centimeter lens1 and enlargements of more than two or three times are rarely wanted4 ,a series of experiments was made for the ,u4 ,s4 ,air ,service to test out these questions1 using a number of representative lenses of all focal lengths1 both at their working apertures and at identical apertures for all4 ,with regard to lens defining power1 as shown by the size of a point image1 the answer has already been reported in a previous section4 ,lenses of long focus give a relatively smaller image than lenses of the same design of short focus4 ,in regard to the whole process of making a small negative and enlarging it1 the loss of definition is quite marked1 as compared to the pictures of the same scale made by contact printing from negatives taken with longer focus lenses4 ,this answer is clear-cut only for lenses calculated to give the same angular field4 ,thus a #aj inch lens covering a #d #e inch plate has about the same angle as a #ej centimeter lens for an #ah #bd centimeter plate4 ,when1 however1 it comes to the longer foci1 such as #abj centimeters1 the practical limitation to plate size 7#ah #bd cm47 has been passed1 and the angular field is less than half that of the #ej centimeter lens4 ,the #abj centimeter lens need only be designed for this small angle1 with consequent greater opportunities for reduction of spherical aberration4 ,it is therefore an open question whether a #ej centimeter lens designed to cover a plate of linear dimensions #ej_/abj times that used with the regular #ej centimeter lens could not be produced of such quality that it would yield enlargements equal to contacts from a #abj centimeter lens4 ,if so1 lenses of larger aperture could be used1 and a considerable saving in space requirements effected4 ,focal lengths during the ,great ,war were decided by the nature of the military detail which was to be revealed and by the altitudes to which flying was restricted in military operations4 ,in the first three years of the war the development of defences against aircraft forced planes to mount steadily higher1 so that the original three or four thousand feet were pushed to #ae1jjj1 #ah1jjj1 and even higher4 ,lenses of long focus were in demand1 leading ultimately to the use of some of as much as #abj centimeters 7,fig4 #da74 ,in the last months of the war the resumption of open fighting made minute recording of trench details of less weight1 while the preponderance of allied air strength permitted lower flying4 ,in consequence1 lenses of shorter focus and wider angle came to the fore1 suitable for quick reconnaissance of the main features of new country4 ,at the close of the war the following focal lengths were standard in the ,u4 ,s4 ,air ,service1 and may be considered as well-suited for military needs4 ,peace may develop quite different requirements4 ,focal length ,aperture ,plate size #aj inch ,f_/#d4e #d #e inch #bf cm4 ,f_/#f #ac #ah cm4 #ab inch ,f_/#e4f #ah #bd cm4 #bj inch ,f_/#f4c to ,f_/#d4e #ah #bd cm4 #dh inch ,f_/#aj to ,f_/#h #ah #bd cm4 ,the question of the use of telephoto lenses in place of lenses of long focus is frequently raised4 ,lenses of this type combine a diverging 7concave7 element with the normal converging system1 whereby the effect of a long focus is secured without an equivalent lens-to-plate distance4 ,this reduction in 8back focus0 may be from a quarter to a half4 ,were it possible to obtain the same definition with telephoto lenses as with lenses of the same equivalent focus1 they would indeed be eminently suitable for aerial work because of their economy of length4 ,but experience thus far has shown that the performance of telephoto lenses1 as to definition and freedom from distortion1 is distinctly inferior1 so that it is best to hold to the long focus lens of the ordinary type4 ,lenses ,suitable for ,aerial ,photography4--,among the very large number of modern anastigmat lenses many were found suitable for airplane cameras and were used extensively in the war4 ,a partial list follows3 ,the ,cooke ,aviar1 ,the ,carl ,zeiss ,tessar1 the ,goerz ,dogmar1 the ,hawkeye ,aerial1 the ,bausch and ,lomb ,series ,ic and ,i,ib ,tessars1 the ,aldis ,triplet1 the ,berthiot ,olor4 ,the ,question of ,plate ,size and ,shape4--,plate size is determined by a number of considerations1 scientific and practical4 ,if the type of lens is fixed by requirements as to definition1 then the dimensions of the plate are limited by the covering power4 ,from the standpoint of economy of flights and of ease of recognizing the locality represented in a negative1 by its inclusion of known points1 lenses of as wide angle as possible should be used4 ,if the focus is long1 this means large plates1 which are bulky and heavy4 ,if the finest rendering of detail is not required a smaller scale may be employed1 utilizing short focus lenses and correspondingly smaller plates4 ,thus a six inch focus lens on a #d #e inch plate would be as good from the standpoint of angular field as a #ab inch on an #h #aj inch plate4 ,this is apt to be the condition with respect to most peace-time aerial photography1 which may be expected to free itself quickly from the huge plates and cameras of war origin4 ,for work in which great freedom from distortion of any sort is imperative1 small plates will be necessary1 for two reasons4 ,one is that the characteristic lens distortions are largely confined to the outlying portions of the field4 ,the other is that a wide angle of view inevitably means that all objects of any elevation at the edge of the picture are shown partly in face as well as in plan1 which prevents satisfactory joining of successive views 7,fig4 #abh74 ,in making a mosaic map of a city1 if a wide angle lens is employed with large plates1 the buildings lying along the junctions of the prints can be matched up only for one level4 ,if this is the ground level1 as it would be to keep the scale of the map correct1 the roofs will have to be sacrificed4 ,in extreme cases a house at the edge of a junction may even show merely as a front and rear1 with no roof1 while in any case the abrupt change at these edges from seeing one side of all objects to seeing the opposite side is not pleasing4 ,the table in a preceding section gives the relation of plate size to focal length found best on the whole for military needs4 ,deviations from these proportions in both directions are met with4 ,in the ,english service the ,,lb camera1 which uses #d #e inch plates1 is equipped with lenses of various focal lengths1 up to #bj inches4 ,the ,german practice1 as well as the ,italian1 was almost uniform use of #ac #ah centimeter plates for all focal lengths4 ,toward the end of the war1 however1 some ,german cameras of #ej centimeter focal length were in use employing plates #bd #cj centimeters4 ,it will be recognized that these plate sizes are chosen from those in common use before the war4 ,a similar observation holds with even greater force on the question of plate shape4 ,current plate shapes have been chosen chiefly with reference to securing pleasing or artistic effects with the common types of pictures taken on the ground4 ,these shapes are not necessarily the best for aerial photography4 ,indeed the whole question of plate shape should be taken up from the beginning1 with direct reference to the problems of aerial photography and photographic apparatus4 ,a few illustrations will make this clear1 taking ,fig4 #ag as a basis4 ,if it is desired to do spotting 7the photography of single objectives71 the best plate shape would be circular1 for that shape utilizes the entire covering area of the lens4 ,if it is desired to make successive overlapping pictures1 either for mapping1 or for the production of stereoscopic pairs1 a rectangular shape is indicated4 ,if the process of plate changing is difficult or slow1 it is advisable1 in order to give maximum time for this operation1 to have the long side of the rectangle parallel to the line of flight 7indicated by the arrow74 ,if economy of flights is a consideration1 as in making a mosaic map of a large area1 it is advantageous to have as wide a plate as the covering power of the lens will permit4 ,reference to ,fig4 #ag shows that this means a plate of small dimensions in the direction of flight4 ,if the changing of plates or film is quick and easy1 the maximum use of the lens's covering power is made by such a rectangle whose long side approximates the dimensions of the lens field diameter4 ,this is in fact the choice made in the ,german film mapping camera 7,figs4 #fa and #fc71 whose picture is #f #bd centimeters4 ,an objection to this from the pictorial side1 lies in the many junction lines cutting up the mosaic4 ,another objection1 if the plane does not hold a steady course1 is the failure to make overlaps on a turn4 7,fig4 #fb47 ,here as everywhere the problem is to decide on the most practical compromise between all requirements4 ,illustration3 ,,fig4 #ag4--,possible choices of plate shape4 ,focussing4--,the process of focussing aerial cameras was at first deemed a mystery1 though undeservedly so4 ,a belief was long current that 8ground0 focus and 8air0 focus differ4 ,in other words1 that a camera focussed upon a distant object on the ground would not be in focus for an object the same distance below the camera when in the plane4 ,belief in this mysterious difference went so far that certain instruction books describe in detail the process of focussing a camera by trial exposures from the air4 ,careful laboratory tests performed for the ,u4 ,s4 ,air ,service showed that neither low temperature nor low pressure1 such as would be met at high altitudes1 alter the focus of any ordinary lens by a significant amount1 and that the possible contraction of the camera body was of negligible effect on the focus 7not more than #a_/bjj per cent4 per degree centigrade with a metal camera74 ,in complete harmony with these tests has been the experience that if the ground focussing is done carefully1 by accurate means1 then the air focus is correct4 ,the whole matter thus becomes one of precision focussing4 ,the best method1 applicable if the air is steady1 is to focus by parallax4 ,the ground glass focussing screen is marked in the center with a pencilled cross4 ,over this is mounted1 with ,canada balsam1 a thin microscope cover-glass4 ,the camera is directed on an object a mile or more away1 and the image formed by the lens is examined by a magnifying glass through the virtual hole formed by the affixed cover-glass4 ,with the pencil line in focus the head is moved from side to side4 ,if the image and pencil mark coincide they will move together as the head is moved4 ,if the image moves away from the pencil mark and in the same direction as the eye moves1 the image is too near the lens4 ,if the image moves away in the opposite direction to the motion of the eye1 it is too far from the lens4 ,in either case the focus is to be corrected accordingly4 ,in place of a distant object1 which may waver with the motion of the air1 we may use an image placed at infinity by optical means4 ,the collimator1 an instrument for doing this1 consists of a test object 7lines1 circles1 etc47 placed accurately at the focus of a telescope objective4 ,the camera lens is placed against this and focussed by parallax1 as with a distant object4 ,collimators are employed in camera factories1 and should be part of the equipment of base laboratories where repairing and overhauling of cameras is done4 ,lens ,mounts4--,all that is required for the mounting of an aerial camera lens is a rigid platform1 with provision for enough motion of the lens to adjust its focus accurately4 ,as already explained1 the lens works at fixed1 infinity1 focus1 and therefore needs no adjustment during use4 ,it must be held far more rigidly than would be possible by the bellows1 which is an almost invariable adjunct of focussing cameras4 ,the use of ordinary types of hand cameras on a plane is rarely successful just because of the bellows1 which is strained and rattled by the rush of wind4 ,the lens mountings thus far used have been simple affairs4 ,in the ,french cameras the lens is merely screwed into a flange which in turn is fastened by screws to a platform in the camera body4 ,adjustment for focussing is not provided2 instead1 the flange is raised on thin metal rings or washers1 cut of such thickness by trial as to bring the lens to focus1 once and for all4 ,the ,u4 ,s4 ,air ,service method of mounting is to provide the lens barrel with a long thread1 which screws into a flange that in turn is mounted on a platform in the camera cone1 by means of thumb-screws4 ,the lens is focussed by screwing in and out1 and then clamped by a screw through the side1 bearing on the thread4 ,the whole mount may be quickly removed by loosening the thumb-screws1 and once focussed in one cone1 can be transferred to another similar1 machine-made cone without change of focus4 ,fig4 #ah shows a #bj inch lens mounted in this manner4 ,the photograph shows as well the ring on the front of the lens by means of which circular color filters may be held in place4 ,this ring screws down on the filter1 and the catch is dropped into the nearest vertical groove to the tight position4 ,illustration3 ,,fig4 #ah4--ej centimeter ,f_/#f lens in ,u4 ,s4 standard mount1 showing color filter retaining ring and catch4 ,a somewhat different and better method of tightening the lens in the flange1 when focussed1 has been adopted in the ,english lens mount1 which is in general similar to the ,american4 ,the threaded part of the flange is split by a slot cut parallel to the flange base1 and a screw is run into the flange from the front1 through the split portion4 ,by tightening this screw1 which is always accessible1 the split part of the flange is squeezed together1 thus rigidly holding the lens barrel4 ,,chapter ,v ,,the ,,shutter ,permissible ,exposure in ,airplane ,photography4--,a definite limitation to the length of exposure in airplane cameras is set by the motion of the plane4 ,if we represent the speed of the plane by ,s1 the altitude of the plane by ,a1 and the focal length of the lens by ,f1 we obtain at once from the diagram 7,fig4 #ai71 that s1 the rate of movement of the image on the plate1 is given by the relation1 s ,f ------ ------ ,s ,a ,if we call the permissible movement d1 then the permissible exposure time1 t1 is given by the relation-- d ,ad t ------ -------- s ,,fs ,as a representative numerical case1 expressing all quantities in centimeters and in centimeters per second1 let ,f #ej1 ,s #bj1jjj1jjj_/cfjj 7#bjj kilometers per hour71 and ,a #cjj1jjj1 then #ej #bj1jjj1jjj s ------------------------------ 4#i centimeters #cjj1jjj #cfjj ,if we take for the permissible undetectable movement1 4#ja centimeter1 which is1 as has been shown1 a reasonable figure for lens defining power1 we have1 then1 that the longest permissible exposure is 4#jaa second--in round numbers1 one-hundredth4 ,in flying with a slow plane1 or in flying against the wind1 the exposure can sometimes be increased to as much as double this length4 ,diminishing ,f would similarly extend the allowable exposure1 but the ratio of ,f to ,a approximates to a constant in actual practice2 in other words1 a certain resolution and size of image have been found desirable4 ,if flying is forced higher1 a longer focus lens is used2 if lower flying is possible1 a lens of shorter focus4 ,this relationship has1 of course1 been derived from war-time experience4 ,probably much of the prospective peace-time mapping work will impose substantially easier requirements as to definition and will thus allow longer exposures4 ,illustration3 ,,fig4 #ai4--,relative motion of plane and photographic image4 ,for low oblique views the longest exposure is much less4 ,taking #de degrees as a representative angle for the foreground1 and #ejj meters as a representative height1 the value of t becomes #a_/fjj4 ,these figures will illustrate two important points3 they show how severe is the limitation as to exposure1 with the consequent heavy demand on lens and sensitive material speed2 and they show how important it is to secure a shutter with the maximum light-giving power for a specified length of exposure4 ,this leads to a study of the characteristics as to efficiency of the two common types of shutter1 namely1 shutters at or between the lens1 and focal-plane shutters4 ,characteristics of ,shutters ,located at the ,lens4--,of the various shutters located at the lens the most common is the type that is clumsily but descriptively termed the 8between-the-lens0 shutter4 ,this is composed of thin hard rubber or metal leaves or sectors which overlap and which are pulled open to make the exposure4 ,it may require two operations1 one for setting and one for exposing1 or it may1 as in some makes1 set and expose by a single motion4 ,clock escapements1 or some form of frictional resistance1 are depended on to control the interval between opening and closing4 ,this shutter is the one almost universally employed on small hand cameras and on all lenses up to about two inches diameter4 ,it gives speeds sometimes marked as high as #a_/cjj second1 although usually not over #a_/ajj on actual test4 ,between-the-lens shutters have been used to some extent on the shorter focus 7up to #be centimeter7 aerial cameras1 notably in the ,italian service4 ,they suffer1 however1 from two limitations4 ,in the first place we have not yet solved the mechanical problems met with in trying to make the shutter of large size 7as for #ej centimeter ,f_/#f lenses7 at the same time to give high speeds4 ,in the second place the efficiency of the type is low because a large part of the exposure time is occupied by the opening and closing of the sectors4 ,if we define the efficiency of a shutter as the ratio of the amount of light it transmits during the exposure to the amount of light it would transmit were it wide open during the whole period1 then the efficiency of the ordinary between-the-lens shutter is of the order of #fj per cent4 ,this means #a4f times the motion of the image for the same photographic action that we should have with a perfect shutter4 ,the accompanying photographic record 7,fig4 #bj7 of the opening and closing process of this type of shutter clearly illustrates its deficiencies4 ,illustration3 ,,fig4 #bj4--;,effective lens opening at equal intervals of time3 7a7 during focal plane shutter exposure2 7b7 during between-the-lens shutter exposure4 ,characteristics of the ,focal-,plane ,shutter4--,long before the days of aerial photography the problem of a high-efficiency high-speed shutter for photographing moving objects on the ground--railway trains or racing automobiles--had already led to the development of the focal-plane shutter4 ,this is a type peculiarly adapted to the problems of the airplane camera4 ,it consists essentially of a curtain1 running at high speed close to the photographic plate1 the exposure being given by a narrow rectangular slot4 ,if the focal-plane shutter is in virtual contact with the sensitive surface the efficiency1 as defined above1 is #ajj per cent41 since the whole cone of rays from the lens illuminates the plate during the whole time of exposure4 ,but if the curtain is not carried close to the plate the efficiency falls off rapidly with distance1 especially so for small apertures of the slot4 ,illustration3 ,,fig4 #ba4--;,calculation of focal plane shutter efficiency4 ,the efficiency of the focal-plane shutter may be calculated as follows3 ,let the focal length of the lens be ,f1 its diameter be ,f_/,n1 the width of the slot be a1 and the distance from plate to curtain d 7,fig4 #ba74 ,now if the curtain is moving at a uniform speed1 the time taken for the slot to traverse the whole cone of rays1 from the instant it enters till the instant it leaves1 will be directly proportional to d 7,f7 d ------ 7------7 a ------ a ,f 7,n7 ,n ,if the curtain were in contact with the plate the time taken for the same amount of light to reach the sensitive surface would be proportional to a4 ,again defining shutter efficiency as the ratio of the light transmitted to what would have been transmitted were the shutter fully open for the total time of exposure1 the efficiency1 ,e1 is given at once by the expression-- a ,e ---------------------- d_/,n a ,as an example let the lens aperture be ,f_/#f1 so that ,n #f2 let d #a1 and a #a1 then ,e #f_/g4 ,in the ,french de,maria cameras1 where d #d centimeters1 ,e #fj per cent4 for the aperture assumed1 which is representative4 ,fig4 #bb exhibits diagrammatically the chief characteristics of the focal plane shutter4 ,illustration3 ,,fig4 #bb4--;,characteristics of focal plane shutter4 ,in view of the necessity for some distance between shutter and plate it is obviously important to keep a as large as possible1 depending for the requisite shutter speed on the velocity of the curtain4 ,large aperture and high curtain speed are also found to be desirable when we consider the distortion produced by the focal-plane shutter4 ,distortions ,produced by the ,focal-plane ,shutter4--,while the time of exposure of any point on the plate can1 with the focal-plane shutter1 easily be made #a_/ajj second or less1 the whole period during which the shutter is moving is much greater than this4 ,for instance1 a #a centimeter opening which gives #a_/ajj second exposure takes second to move across a #aj centimeter plate1 or nearly second for an #ah centimeter plate4 ,with a moving airplane this means that the point of view at the end of the exposure has moved forward compared to that at the beginning1 by the amount of motion of the plane in the interval4 ,if the shutter moves in the direction of motion of the plane the image will be magnified2 if in the opposite direction1 it will be compressed along the axis of motion4 ,the amount of this distortion is calculated as follows3 ,let the velocity of the plane be ,v1 and that of the shutter be v4 ,let the focal length of the camera be ,f1 and the altitude ,a4 ,if the camera were stationary1 a plate of length l would receive on its surface an image corresponding to a distance ,a_/,f l on the ground4 ,due to the motion of the shutter the end of the exposure occurs at a time l_/v after the start4 ,in this time the plane has moved a distance ,v l_/v2 hence the point photographed at the end of the shutter travel is ,vl_/v within or beyond the original space covered by the plate1 depending on the direction of motion of the curtain4 ,the distortion1 ,d1 is given by the ratio of this distance to the length corresponding to the normal stationary field of view3 ,v_/v l ,,vf ,d ---------------------- -------- ,a_/,f l v,a ,when ,v #bjj kilometers per hour1 v #ajj centimeters per second1 ,f #ej centimeters1 ,a #cjjj meters1 we have-- #bj1jjj1jjj #ej #a ,d ---------------------------------------- approximately ------ #cfjj #ajj #cjj1jjj #ajj ,or if the actual distance error on the ground is desired1 ,vl -------- #aj4h meters v ,as a percentage error this one per cent4 is small compared with other uncertainties1 such as film shrinkage or the error of level of the camera4 ,as an absolute error in surveying1 thirty feet is1 of course1 excessive4 ,the distortion is diminished for any specified shutter speed by making the speed of travel of the curtain as large as possible and by correspondingly increasing the aperture4 ,in connection with film cameras1 another solution which has been suggested is to move the film continuously during the exposure in the direction of the plane's motion4 ,the requisite speed of the film v' to eliminate distortion is given by the relation3 v' ,f -------- ------ ,v ,a ,for the values of ,v1 ,f1 and ,a used above1 v' 4#ib centimeters per second4 ,this speed is clearly that which holds the image stationary on the film--a fact which suggests another object for such movement1 namely1 to permit of longer exposures4 ,the effect of focal plane distortion may be averaged out in the making of strip maps1 if the shutter is constructed so as to move in opposite directions on successive exposures4 ,the first picture will be magnified1 the second compressed1 and so on1 but a strip formed of accurately juxtaposed pictures will be substantially accurate in over-all length4 ,such a shutter is embodied in one of the ,german film cameras 7,fig4 #fa74 ,distortion of the kind above discussed is absent with between-the-lens shutters1 which may conceivably be improved in efficiency and in feasible size4 ,if so they would merit serious consideration for aerial mapping4 ,methods and ,apparatus for ,testing ,shutter ,performance4--,with a focal-plane shutter the desirable qualities in performance are three in number3 7#a7 ,adequate speed range1 which may be taken as from #a_/ej to #a_/ejj second for aerial work1 7#b7 good efficiency1 which has already been treated1 and 7#c7 uniformity of speed during its travel across the plate4 ,before the advent of aerial photography little attention was paid to speed uniformity1 differences of #ej per cent4 in initial and final speed being common in focal-plane shutters1 and but little noticed in ordinary landscape work because of the natural variation of brightness from sky to ground4 ,in the making of aerial mosaic maps the non-uniformity of density across the plate results in a most offensive series of abrupt changes of tone at the junction points of the successive prints 7,fig4 #adj71 an effect which must be minimized by manipulation of the printing light4 ,instruments for testing the speed and uniformity of action of focal-plane shutters are an essential part of any laboratory for developing or testing photographic apparatus and some simple device for setting and checking shutter speed should be available in the field4 ,every such speed tester must contain some form of time counting element--pendulum1 tuning fork or clock-work4 ,elaborate shutter testers1 suitable for determining all the characteristics of all types of shutter1 have been developed and used in certain of the photographic research laboratories4 ,for the study and setting of focal-plane shutters 7whose efficiency need not be measured1 as it can be simply calculated from linear dimensions71 the following simple kinds of apparatus are adequate3 ,illustration3 ,,fig4 #bc4--;,apparatus for testing focal plane shutter speed throughout the travel of the curtain4 ,clock dial type of shutter tester4 ,this consists essentially of a black clock dial carrying a white pointer which makes its complete revolution in one second or less4 ,if this dial is photographed by the camera under test1 the width of the sector traced during the exposure by the moving pointer shows the time interval4 ,if the dial is photographed at several points on the plate--beginning1 middle and end of the shutter travel--the complete characteristics of the shutter can be determined4 ,interrupted light type of shutter tester4 ,for the study of uniformity of shutter action alone the apparatus shown in ,fig4 #bc may be employed4 ,a is a high intensity light source1 such as an arc or a gas filled tungsten lamp4 ,l is a convex lens1 focussing an image of the light source on a small aperture in the screen ,e4 ,d is a sector disc which1 driven by the motor ,m1 interrupts the transmitted light with a frequency determined by the number of openings of the sector and by the speed of rotation1 which must be measured by a tachometer4 ,the light diverging from the aperture in ,e falls upon the shutter ,s1 which for this test is reduced to a narrow slit of one millimeter or less4 ,passing through the shutter opening the light falls upon the photographic plate ,p4 ,the principle is simple3 ,if the light is uninterrupted1 the plate ,p is exposed at all points2 due to the interruptions1 a series of parallel lines of photographic action result1 and their distance apart gives a measure of the speed of the shutter at any chosen point in its travel4 ,a performance curve of the ,french ,klopcic shutter is shown in ,fig4 #bd4 ,the variation in speed lies over a range of two to one4 ,so serious is this defect in these shutters that diaframs are sometimes inserted in the ,french cameras to cut off part of the light from the lens on the most exposed end of the plate4 ,this expedient produces uniformity of photographic action1 but does not overcome the movement of the image1 which is one of the chief faults of excessive exposure4 ,illustration3 ,,fig4 #bd4--,performance of ,klopcic shutter4 ,illustration3 ,,fig4 #be4--,optical system of shutter tester for ,air ,service1 ,u4 ,s4 ,army4 ,a more complete apparatus1 adapted both to absolute speed determinations and to the study of uniformity of action1 is that worked out and used in the ,united ,states ,air ,service 7,fig4 #be74 ,at ,a is a high intensity light source1 an image of which is focussed by the lens ,l#a upon a slit ,e1 in front of which stands a tuning fork ,t1 of period #ajbd or #bjdh per second4 ,the light diverging from the slit is received by a second lens1 ,l#b which is arranged either to focus the slit image upon the shutter curtain or to render the rays parallel1 so that an entire camera may be inserted4 ,in the latter case the camera lens ,l#c serves to focus the slit image on the curtain ,c4 ,after passing through the curtain aperture the light is focussed by the lens ,l#d on the rotatable drum ,d1 which carries a strip of sensitive film4 ,the operation of testing a shutter consists in focussing the slit image on the portion of the shutter whose performance is required1 striking the tuning fork to set it vibrating1 rotating the drum rapidly and setting off the shutter4 ,there is thus obtained on the sensitive film an exposed strip resembling in appearance the edge of a saw1 the number of teeth showing the time interval in vibrations of the tuning fork4 ,three exposures usually give all the points necessary for a practical knowledge of the shutter's uniformity of action4 ,a point of some importance1 learned from numerous shutter tests1 is that a focal-plane shutter should be tested in the position in which it is to be used4 ,aerial camera shutters should be tested in the horizontal position4 ,types of ,focal-plane ,shutters4--,a variety of means have been utilized for securing the necessary variation in speed in focal-plane shutters4 ,their success is to be measured by the actual speed range and by the uniformity of speed attained4 ,in aerial cameras at present in use we find variable tension of the curtain spring1 the aperture being fixed2 variable opening with fixed tension2 multiple curtain openings with fixed spring tension2 and combinations of two or all of these methods of speed control4 ,the problem of covering the aperture during the operation of winding up or setting the shutter has led to further elaborations of shutter mechanism4 ,these take the form of lens or shutter flaps1 auxiliary curtains1 and shutters of the self-capping type4 ,shutters embodying all these features are briefly described below4 ,representative ,shutters4--,the ,folmer variable tension shutter is used on the ,united ,states ,air ,service hand-held and hand-operated plate camera and on some of the film cameras4 ,it consists of a fixed aperture curtain wound on a curtain roller in which the spring can be set to various tensions1 numbered #a to #aj4 ,the range of speeds attainable is at best about three to one1 or from #a_/ajj to #a_/cjj second1 considerably shorter than the range indicated as desirable4 ,its uniformity of travel is variable with the tension1 as shown by representative performance curves in ,fig4 #cj4 ,lacking any self-capping feature the shutter is provided either with an auxiliary curtain1 or in the hand-held camera with flaps in front of the lens1 opened by the exposing lever before the curtain is released 7,fig4 #ci74 ,this shutter is made a removable unit in the #ah #bd centimeter hand-operated camera1 but is built into the hand-held and film cameras4 ,illustration3 ,,fig4 #bf4--,removable four-slit shutter of ,german 7,ica7 camera1 showing flaps4 ,the ,ica shutter used on the standard ,german aerial cameras is a good example of the multiple slit curtain 7,fig4 #bf74 ,four fixed aperture slits are provided1 with a single tension1 the openings roughly in the ratio #a1 1 1 1 which when the spring tension is properly adjusted give exposures of #a_/ij1 #a_/ahj1 #a_/cge1 #a_/gej second4 ,to pass from one exposure time to another the setting milled head is wound up to successively higher steps or else exposed one or more times without resetting1 depending on the direction it is desired to go4 ,capping during setting1 or during exposure1 in order to change the opening1 is provided for by a pair of flaps on the shutter unit1 which open into the camera body4 ,the mechanical work on these shutters is of excellent quality1 the curtain running with exceptional smoothness4 ,provision is made for adjusting the tension until the marked speeds are attained2 this is presumably done in a repair laboratory to which the shutter only need be sent1 as it is a removable unit4 ,tests made on one of these shutters wound to its highest tension are shown in ,fig4 #cj4 ,the marked speeds are not attained1 and there is considerable lack of uniformity from start to finish of the travel4 ,l camera variable-aperture shutter4 ,the shutter of the ,l type camera 7,fig4 #bg7 is representative of one of the most primitive methods of varying aperture4 ,the two jaws of the slit are held together by a long cord passing completely around the aperture1 fastened permanently at one end and attached at its other end by a sliding clasp or saddle4 ,as this saddle is forced in one direction the slit is closed1 in the opposite direction the cord becomes slack1 and after the shutter is released once or twice the slit assumes a wider opening4 ,a chronic trouble is the breaking of the cords4 ,its opening can be changed only after the plate magazine is removed4 ,illustration3 ,,fig4 #bg4--8,l0 type camera showing open negative magazines and shutter mechanism4 ,u4 ,s4 ,air ,service variable-aperture shutter4 ,this shutter is incorporated in the ,american de,ram and in other late ,american cameras 7,fig4 #bh74 ,its characteristic feature is the introduction of an idler1 whose distance from the main curtain roller can be varied4 ,tapes whereby the following curtain is attached to the spring roller pass over this idler1 and by changing its position the aperture or distance between the two curtain elements is altered over a large range4 ,tests of this shutter are shown in ,fig4 #cj4 ,a speed of #a_/ej second is provided for by a slit width of five centimeters1 and the highest speed is fixed only by the practical limit of approach of the jaws4 ,experiment shows great uniformity of rate of travel to be attainable by combining careful choice of spring length and tension with good workmanship in the mechanical features4 ,variable-aperture fixed-tension shutters have a definite advantage over the variable-tension type in that they can utilize for all speeds that tension which gives uniform action4 ,the capping feature of this shutter is provided in the ,american de,ram by flaps1 in the automatic film camera by an auxiliary curtain4 ,the shutter is removable in the de,ram1 but built into the other camera4 ,illustration3 ,,fig4 #bh4--,variable aperture curtain developed in ,u4 ,s4 ,air ,service1 and used in ,american de,ram1 and 8,k0 type automatic film cameras4 ,the ,klopcic variable-tension1 variable-aperture1 self-capping shutter is an example of an attempt to meet all shutter requirements with an entirely self-contained mechanism4 ,it is shown diagrammatically in ,fig4 #bi4 ,tapes ,g#a1 ,g#b are used to connect the following curtain ,b directly to the spring roller ,t1 at a fixed distance1 while the leading curtain1 ,a1 may be slid along the tapes by small friction buckles1 ,c#a1 ,c#b1 auxiliary springs ,r#a1 ,r#b serving to keep it taut in any position4 ,when the shutter is being set the buckles are arrested against stops while the winding-up continues for what is to be the following half of the curtain in exposing4 ,when released the curtain moves across with an aperture fixed by the point of setting of the buckle stops4 ,at the end of the travel the buckles are arrested by other stops1 while the following portion of the curtain continues its travel to the end4 ,on re-winding1 therefore1 the aperture is closed4 ,variable tension as well as variable aperture is provided1 although little used4 ,in the ,french cameras a lens flap is also inserted behind the lens1 but this is not needed if the self-capping feature functions properly4 ,on the hand cameras this flap is said to be necessary in order to prevent a curious kind of accident3 if the camera is held on the knee1 pointing upward1 an image of the sun may be formed on the curtain and burn a hole through it4 ,illustration3 ,,fig4 #bi4--,mechanism of ,klopcic variable aperture self-capping shutter4 ,the performance of the ,french shutter in respect to uniformity has already been shown in ,fig4 #bd4 ,it leaves very much to be desired4 ,besides non-uniformity of action during its travel it exhibits another common defect of variable-tension shutters1 namely1 the curtain must be released several times after a change of tension before the new speed is established 7,fig4 #cj1 tensions #e and #e74 ,illustration3 ,,fig4 #cj4--,performances of various shutters used on aerial cameras4 ,speeds expressed in reciprocals of fractional parts of one second4 ,the ,french shutter as made for the de,maria cameras is a removable unit4 ,the small size 7#ac #ah cm47 sets by the straight pull of a projecting pin1 the larger 7#ah #bd cm47 by winding up a milled head4 ,the former is the more convenient motion for an aerial camera4 ,care must be taken with either type that the motion of setting is not stopped when the first resistance is encountered2 this occurs when the tape buckles strike their stop and the slit begins to open4 ,,chapter ,,vi ,,plate-,,holders ,,and ,,magazines ,in the earlier days of airplane photography the ordinary plate-holder or double dark slide was used to some extent1 but it is ill-suited to the purpose because of the considerable time and attention required for its operation4 ,it has nevertheless the merit of adding little to the length of the camera1 and it works in any position4 ,for these reasons it has remained in occasional use for the taking of oblique views with long focus cameras in a cramped fuselage4 ,next in order of progress rank the simple box magazines1 for holding a dozen1 eighteen or twenty-four plates1 as used in the ,english ,c1 ,e1 and ,l type cameras4 ,these are little more than boxes with sliding lids which when open permit the introduction or removal of the plates4 ,figs4 #de and #df illustrate the magazine of this type as made for the ,english ,c and ,e cameras4 ,it is constructed of wood1 grooved to fit tracks on the camera1 and is furnished with a sliding door or lid hinged in the middle to fold down out of the way when open4 ,the eighteen plates are carried in metal sheaths1 both to provide opaque screens between them1 and to protect them from injury in the mechanism of the camera4 ,fig4 #bg shows the all-metal magazine made for the ,american model ,l camera4 ,this differs from the ,english in material of construction1 plate capacity 7#bd instead of #ah7 and manner of operating the slide1 which is built up of three thicknesses of phosphor bronze and draws out through metal guides bent into semicircular form4 ,a snap catch holds this slide at either end of its travel4 ,the leather strap introduced in the ,american model for carrying and handling is a distinct improvement4 ,these magazines contain no springs or other mechanism1 as the cameras with which they are used depend upon the action of gravity for emptying the upper 7feeding7 magazine1 and filling the lower 7receiving7 one4 ,illustration3 ,,fig4 #ca4--;,aerial hand camera 7,u4 ,s4 type ,a-#b74 ,next in order of complexity may be ranked the bag magazine 7,figs4 #ca and #dd74 ,in this the exposed plate is pulled out of the magazine proper by a metal slide or rod into a leather bag4 ,the rod is then pushed back1 the plate in its metal sheath is grasped through the leather bag1 lifted to the back of the magazine1 and forced in behind the other plates4 ,the number of plates exposed is indicated either by numbers on the backs of the sheaths1 visible through a red glazed opening in the back1 or else by a counter actuated by the metal slide rod4 ,usually twelve are carried in a magazine4 ,for aerial work the common design of this magazine as used for ground work must be modified by providing extra large easily grasped hooks both on the draw rod and on the dark slide1 which must be drawn before making the first exposure and replaced after the last4 ,the small rings and grips of the standard commercial magazine are almost impossible to handle through heavy gloves4 ,the next type of magazine is represented by three designs1 the ,gaumont and de,maria1 used very generally by the ,french during the war1 and the ,ernemann1 used almost universally in the ,german air service 7,figs4 #cb1 #dj and #db74 ,in all of these the operation of plate changing is the same3 the end of the magazine is pulled out and thrust back1 a more simple operation than the bag manipulation just described4 ,the internal workings are different according to size4 ,in the smaller ,french magazines 7#ac #ah cm47 the camera is first pointed upward1 all the plates are drawn out except the one to be changed1 and this1 with the aid of springs1 drops to the bottom1 after which the other plates push back over it4 ,the plates pull out in the direction of their long dimension4 ,in the larger ,french magazine 7#ah #bd cm47 only the exposed plate pulls out4 ,the pull is in the direction of the shorter dimension of the plate1 which is lifted up by heavy springs and slides back over the top of the pile4 ,in the ,ernemann magazine only six plates are carried1 which there is good reason to believe represent the maximum feasible number1 judging by the reports of jambs and breakages in the twelve-plate ,french magazines4 ,in all of these magazines laminated wood slides pull out and in at each operation1 and while satisfactory if made and operated in one climate1 experience indicates that if made in ,america and sent abroad swelling of the wood may be expected to prevent their successful operation4 ,illustration3 ,,fig4 #cb4--,various plate magazines used on aerial cameras4 ,alternative forms of magazine1 somewhat more practical from the standpoint of manufacture and export1 are several designs embodying two compartments 7,fig4 #cb74 ,in the most simple of these the plates are moved1 immediately before or after exposing1 from the unexposed to the exposed side4 ,illustrative of this type are the ,folmer designs1 in which the to-and-fro motion is imparted by a rack geared to a pinion actuated either by a lever1 in the hand camera1 or by the power drive1 in the automatic design 7,figs4 #cc and #ec74 ,another illustration is afforded by the ,piserini and ,mondini magazine1 in which the operation of changing is performed by a back-and-forth motion of a hand-grip1 which also sets the camera shutter 7,fig4 #dg74 ,illustration3 ,,fig4 #cc4--,u4 ,s4 ,air ,service hand camera1 with two-compartment magazine4 ,illustration3 ,,fig4 #cd4--;,film type hand camera4 ,in these magazines the center of gravity changes as the exposed plates are moved over1 and only half the inside space is occupied with plates4 ,these objections are overcome in the ,chassel form1 where both compartments are always full4 ,transfer of the bottom exposed plate from one compartment to the other is compensated for by the simultaneous shift of the top plate in the receiving compartment1 to the feeding side4 ,in a modification of this idea by ,ruttan the exposing position is when the plates are half-way through the shifting process1 whereby the magazine may be symmetrically mounted on the camera body4 ,illustration3 ,,fig4 #ce4--;,apparatus for straightening plate sheaths4 ,other more complicated magazines have been designed1 some of which are shown in the diagrammatic ensembles of ,figs4 #cb and #dh4 ,in the ,jacquelin1 the main body of plates is raised while the bottom 7exposed7 plate is folded against the side4 ,the main body of plates then drops back to place1 the exposed plate is carried on upward and folds down on the back of the pile4 ,the ,bellieni magazine system uses three1 a central feeding one and two below for receiving1 one on each side of the camera body4 ,these were made solely for attachment to captured ,german cameras4 ,in the ,fournieux magazine the plates are carried in an interior rotating box4 ,the plate to be exposed is dropped off the front of the pile1 down to the focal plane1 and after exposure is picked up and placed at the back of the pile1 which has turned over in the meanwhile4 ,the de,ram rotating magazine is described in connection with the camera of which it is an essential part 7,fig4 #eb74 ,illustration3 ,,fig4 #cf4--,training plane equipped for photography1 showing 8,l0 camera in floor mount and magazine rack forward of the observer4 ,for the protection of the plates during their manipulation1 and in the camera1 all plate magazines thus far developed carry them in thin metal sheaths4 ,these add greatly both to the weight and to the time necessary to handle the plates1 but no means have as yet been found for dispensing with them4 ,fig4 #ce shows a representative sheath or septum1 as used in the ,l camera4 ,on three sides the edge is bent up and turned over1 forming a ledge for the plate to press against4 ,the fourth side is left open for inserting the plate1 which is then held in by a small upward projecting lip1 and kept close against the ledges by narrow springs at the sides4 ,to insert or remove the plate the projecting lip is depressed1 either by springing the sheath by pressure from the sides or by using an appropriate tool4 ,care of sheaths4 ,unless systematically taken care of1 plate sheaths become bent or dented4 ,they are then a menace to camera operation1 catching or jamming in the plate changing process1 breaking plates and damaging camera mechanisms4 ,in order to maintain them flat and true1 steel forms are necessary on which the sheaths may be hammered to shape with a mallet 7,fig4 #ce74 ,magazine racks4 ,reconnaissance and mapping call for a number of exposures much greater than the capacity of one #ab1 #ah1 or #bd plate magazine4 ,additional magazines must therefore be carried4 ,these should be in racks convenient to the observer 7,fig4 #cf71 securely held yet capable of quick removal and insertion4 ,in the rack designed to carry two of the metal magazines for the ,american ,l ,camera1 the magazines slide into loose grooves formed by a metal lip4 ,they are prevented from slipping out by a spring catch1 past which they slide when inserted but which is instantly thrown aside by pressure of the thumb as the hand grasps the magazine handle for removal4 ,,chapter ,,vii ,,hand-,,held ,,cameras ,,for ,,aerial ,,work ,field of ,use4--,the first cameras to be used for aerial photography were hand-held ones of ordinary commercial types4 ,indeed the idea is still prevalent that to obtain aerial photographs the aviator merely leans over the side with the folding pocket camera of the department store show window and presses the button4 ,but the ,great ,war had not lasted long before the ordinary bellows focussing hand camera was replaced by the rigid-body fixed-focus form1 equipped with handles or pistol grip for better holding in the high wind made by the plane's progress through the air4 ,even this phase of aerial photography was comparatively short-lived4 ,the need for cameras of great focal length1 and the need for apparatus demanding the minimum of the pilot's or observer's attention1 both tended to relegate hand-held cameras to second place1 so that they were comparatively little used in the later periods of the war4 ,yet for certain purposes they have great value4 ,they can be used in any plane for taking oblique views1 and for taking verticals1 in any plane in which an opening for unobstructed view can be made in the floor of the observer's cockpit4 ,they can be quickly pointed in any desired direction1 thus reducing to a minimum the necessary maneuvering of the plane1 a real advantage when under attack by 8,archies0 or in working under adverse weather conditions4 ,for peace-time mapping work the hand-held camera1 when equipped with spirit-levels on top1 and when worked by a skilful operator1 possesses some advantages over anything short of an automatically stabilized camera4 ,for experimental testing of plates1 filters and various accessories1 the ready accessibility of all its parts makes the hand-held camera the easiest and most satisfactory of instruments4 ,the limitations of the hand-held camera lie in its necessary restriction to small plate sizes and short focal lengths1 and in the fact that it must occupy the entire attention of the observer while pictures are being taken--the latter a serious objection only in war-time4 ,essential ,characteristics4--,in addition to the general requirements as to lens1 shutter and magazine1 common to all aerial cameras1 the hand camera must meet the special problems introduced by holding in the hands1 especially over the top of the plane's cockpit4 ,an exceptionally good system of handles or grips must be provided whereby the camera can be pointed when pictures are taken1 and held while plates are being changed and the shutter set4 ,the weight and balance of the camera must be correct within narrow limits2 the wind resistance must be as small as possible2 the shutter release must be arranged so as to give no jerk or tilt to the camera in exposing4 ,as to the method of holding the camera1 a favorite at first among military men was the pistol grip1 with a trigger shutter release 7,fig4 #cg74 ,because of the size and weight of the camera the pistol grip alone was an inadequate means of support and additional handles on the side or bottom had to be provided for the left hand4 ,small 7#h #ab cm47 pistol grip cameras were used to some extent by the ,germans 7,fig4 #db71 and a number of #d #e inch experimental cameras of this type were built for the ,american ,air ,service 7,fig4 #cg74 ,but the grasp obtained with such a design is not so good as is obtained with handles on each side or with flat straps to go over the hands4 ,the camera balances best with the handles in the plane of the center of gravity4 ,as to weight1 no set rules are laid down1 but experience has shown that a fairly heavy camera--as heavy as is convenient to handle--will hold steadier than a light one4 ,the ,american #d #e inch cameras described below weigh with their magazines in the neighborhood of twelve pounds4 ,illustration3 ,,fig4 #cg4--,pistol-grip aerial hand camera4 ,representative ,types of ,hand-held ,cameras4--,french and ,german hand-held cameras are essentially smaller editions of their standard long-focus cameras1 and a description of them will apply to a considerable extent to the large cameras to be discussed in a later chapter4 ,the ,english and ,american hand-held cameras are generally quite different in type from the large ones1 which are used attached to the plane4 ,illustration3 ,,fig4 #ch4--;,diagram of ,french 7de,maria7 #bf cm4 focus hand camera1 using #ac #ah cm4 plates4 ,the ,french hand-held camera uses #ac #ah centimeter plates1 carried in a de,maria magazine1 and has a lens of #bf centimeters focus4 ,the shutter is the ,klopcic self-capping type already described1 and is removable4 ,the camera body1 built of sheet aluminum1 takes a pyramidal shape4 ,in ,fig4 #ch1 ,a is the shutter release and ,b the rectangular sight1 of which ,c is the rear or eye sight4 ,the complete sight may be placed either on the top or on the bottom of the camera4 ,at ,d are the handles1 sloping forward from top to bottom2 ,e is a catch for holding the magazine2 ,f is a door for reaching the back of the lens and the lens flap2 ,g is a snap clasp for holding the front door of the camera closed2 ,h is a ring for attaching a strap to go around the observer's neck2 ,i is the lever which opens the flap behind the lens and releases the focal-plane shutter2 ,j is a snap catch for holding the front door of the camera open4 ,the operations with this camera are three in number4 ,starting immediately after the exposure1 the camera is pointed lens upward and the plate changed by pulling the inner body of the magazine out and then in2 next the shutter is set2 then the camera is pointed1 and finally exposed by a gentle pull on the exposing lever4 ,the ,english hand-held camera 7,fig4 #ahf74 ,this differs from the ,french in the size of plate 7#d #e inch71 in the shape of the camera body1 which is circular1 and in the type of shutter1 which is fixed-tension variable-opening4 ,in the longer focus camera 7#aj to #ab inch7 the shutter is self-capping1 and the aperture is controlled by a thumb-screw at the side4 ,in the smaller 7#f inch7 a lens flap is provided in front of the lens and the shutter aperture is varied by a sliding saddle and cord4 ,the handles of the camera are placed vertical1 instead of sloping as in the ,french4 ,the shutter is released by a thumb-actuated lever4 ,double dark slides are used1 as the multiple plate magazine has not found favor in the ,english service4 ,the ,german hand-held camera 7,fig4 #db74 ,the ,german hand-held camera is1 like their whole series1 built of canvas-covered wood1 the body having an octagonal cross-section4 ,it is equipped with the ,ica shutter and uses the ,ernemann six plate 7#ac #ah cm47 magazine4 ,the excellent system of grips by which the camera is held and pointed is an especially commendable feature4 ,on the right-hand side is a handle similar to the ,french type1 but carefully shaped to fit the hand4 ,the left-hand grip consists of a long1 rounded block of wood running diagonally from top to bottom of the side1 with a deep groove on the forward side for the finger tips1 while over the hand is stretched a leather strap1 the whole aim being to provide an absolutely sure and comfortable hold on the camera during the plate changing and shutter setting operations4 ,illustration3 ,,fig4 #ci4--;,front view of ,u4 ,s4 aerial hand camera1 showing lens flaps partly open1 and details of tube sight4 ,united ,states ,air ,service hand cameras4 ,the hand camera developed for the ,united ,states ,air ,service and manufactured by the ,eastman ,kodak ,co4 is made in three models1 using the bag magazine1 a two-compartment magazine1 and roll film1 respectively4 ,the shutter is of the fixed 7one or two7 aperture variable tension type1 built into the camera4 ,a distinctive feature is the double lens flap1 in front of the lens actuated by the thumb pressure shutter release 7,fig4 #ci74 ,in the bag magazine camera the shutter is set1 as a separate operation1 by a wing handle1 and a similar handle controls the tension adjustment4 ,in the two-compartment type 7,fig4 #cc7 the shutter wind-up is geared to the plate changing lever1 so that but one operation is necessary to prepare the camera for exposure4 ,in the film type 7,fig4 #cd7 a single lever motion sets the shutter and winds up the film ready for the next exposure4 ,after the last exposure of all the film is wound backward on its own 7feeding7 roller before removing from the camera4 ,the film is held flat by a closely fitting metal plate behind1 and by guides at the edges in front1 an arrangement which with small sizes works fairly well although the exquisite sharpness of focus attainable with plates is not to be expected4 ,the saving in weight made possible by the use of film in place of plates in metal sheaths is about three pounds per dozen exposures4 ,in all these cameras the sight--a tube with front and back cross wires--is placed at the bottom4 ,this position has been found the most convenient for airplane work1 as it necessitates the observer raising himself but little above the cockpit1 a matter of prime importance when the tremendous drive of the wind is taken into account4 ,,chapter ,,viii ,,non-,,automatic ,,aerial ,,plate ,,cameras ,the ideal of every military photographic service has been an automatic or at least a semi-automatic camera1 in order to reduce the observer's work to a minimum4 ,yet as a matter of fact almost all the aerial photography of the ,great ,war was done with entirely hand-operated cameras4 ,the primary reason for this was that no entirely satisfactory automatic cameras were developed1 cameras at once simple to install and reliable when operated4 ,even the propeller-drive semi-automatic ,l type of the ,british ,air ,service was very commonly operated by hand1 for many of the pilots and observers regarded the propeller merely as another part to go wrong4 ,any automatic mechanism in the airplane must work well in spite of vibration1 three dimensional movements1 and great range of temperature4 ,the requirements were well recognized when the war closed1 but had not yet been met4 ,careful study of the conditions and needs by competent designers of automatic machinery may be expected to result at an early date in reliable cameras of the automatic type1 but the description below of hand-operated cameras really covers practically all the cameras found satisfactory in actual warfare4 ,general ,characteristics of ,hand-operated ,cameras4--,as distinguished from the hand-held cameras the larger hand-operated cameras are characterized by the greater focal length of their lenses1 the size of plate employed1 and the manner of holding--by some form of anti-vibration mounting attached directly to the fuselage4 ,except for the early ,english ,c and ,e type cameras which called for #aj inch lenses and #d #e inch plates1 the general practice at the close of the war by agreement between the ,french1 ,english and ,american ,air ,services1 was for the use of #ah #bd centimeter plates and for lenses with focal lengths of approximately #be1 #ej and #abj centimeters4 ,the ,english also made use of a #ad inch 7#ce centimeter7 lens1 and never made a regular practice of anything larger than #ej centimeters4 ,the ,germans and ,italians restricted themselves to the #ac #ah centimeter size of plate1 while a lens of #gj centimeters focal length was standardized with the ,germans1 in addition to the #be1 #ej1 and #abj centimeter4 ,the particular focal length was determined by the nature of the photographic mission4 ,where large areas were to be covered at low altitudes or without the demand for exquisite detail1 the shorter focus lenses suffice4 ,the most commonly used lens in the ,french ,service was the #ej centimeter1 while the #abj was employed when high flying was necessary or when minute detail was required4 ,as already mentioned1 the common practice was to keep cameras of all focal lengths available1 but the ideal at the close of the war was to have the camera nose and lens a detachable unit1 so that any focal length desired could be secured with the same camera body4 ,the standard ,french camera4 ,the hand-held form of ,french camera has already been described4 ,the cameras for larger plate sizes and longer focus lenses differ only in the addition of a ,bowden-wire distance release for the shutter and in the use of the ,gaumont magazine which operates without the necessity of pointing the exposed side of the magazine upward4 ,fig4 #dj illustrates the #ej centimeter camera1 and ,fig4 #da the #abj4 ,illustration3 ,,fig4 #dj4--ej centimeter de,maria hand operated camera on tennis ball mounting4 ,illustration3 ,,fig4 #da4--abj centimeter de,maria camera4 ,the ,german ,ica cameras4 ,these are larger editions of the light wood hand camera already described1 but with the addition of a ,bowden-wire shutter release4 ,the body of the larger cameras carries a distinctive feature in the distance control of the lens diafram1 worked by means of a lever which actuates racks1 pinions and connecting rods leading to the lens4 ,on the side of the camera body a shallow box is provided for carrying the color filter in its bayonet joint mount to fit on the lens 7,figs4 #db and #dc74 ,illustration3 ,,fig4 #db4--;,german aerial cameras4 ,illustration3 ,,fig4 #dc4--;,diagram of ,german #ej centimeter camera4 ,illustration3 ,,fig4 #dd4--,u4 ,s4 hand-operated aerial camera 7type ,m7 with #aj and #bj inch cones4 ,the hand-operated bag-magazine camera of the ,united ,states ,air ,service 7,type ,m7 is similar to the small hand-held camera1 but differs in three respects3 a removable shutter 7of the variable-tension fixed-aperture type7 embodying an auxiliary curtain for capping during the setting operation2 a ,bowden-wire shutter release2 and the employment of a set of standard interchangeable cones to hold lenses of several focal lengths4 ,the #bj inch and #aj inch cones are shown in ,fig4 #dd4 ,the operation of this camera is similar to the ,french standard cameras1 but not so simple because of the number of motions required in manipulating the bag4 ,its chief objection for war work lies in fact in the magazine1 which should be superseded by a two-compartment or other satisfactory type of plate changing chamber4 ,the camera alone1 with #bj inch cone1 weighs approximately #dj pounds2 the loaded magazine1 with its plates in metal sheaths1 #ae pounds4 ,illustration3 ,,fig4 #de4--;,english ,c type aerial camera4 ,the ,english ,c and ,e type cameras4 ,the ,c and ,e type cameras have now chiefly an historic interest4 ,they were the first used in the ,english service1 fixed to the fuselage1 and were later used in training work in ,england and in the ,united ,states4 ,they were never built for plates larger than #d #e inch nor for lenses of more than #ab inch focus1 a limitation set by the lenses available at the time of their design4 ,illustration3 ,,fig4 #df4--;,english type 8,e0 hand-operated plate camera4 ,in several respects the mode of operation of the two types is the same4 ,the unexposed plates are held in a magazine lying above the camera1 in the axis of the lens 7,fig4 #cb74 ,after exposure the bottom plate is carried to one side and allowed to fall by the action of gravity into the receiving magazine4 ,in the ,c type 7,fig4 #de7 an opaque slide is drawn between the lens and the 7variable-opening7 shutter during the setting operation4 ,during the exposure period this slide projects into a compartment on the opposite side of the camera from the receiving magazine1 thus making the camera mechanism three plates wide4 ,in the ,e type 7,fig4 #df71 a flap over the lens makes it possible to dispense with the sliding screen1 and reduces the camera to about the width of two plates4 ,in the ,c type the plates are changed by a handle on top of the camera2 in the ,e type provision is made for distance control by cords1 and for shutter release by a ,bowden wire4 ,in both cameras the operation of plate changing also sets the shutter1 a definite advance over the two preparatory motions in the ,french apparatus4 ,the ,c type was constructed of wood1 the ,e of metal4 ,illustration3 ,,fig4 #dg4--;,italian 7,piserini and ,mondini7 two compartment magazine hand-operated camera4 ,italian two-compartment magazine camera4 ,a camera designed by ,piserini and ,mondini was used to some extent by the ,italian service toward the close of the war 7,fig4 #dg74 ,this has the desirable feature just noted in the ,c and ,e cameras3 the operations of plate changing and shutter setting are performed in a single motion4 ,unlike those cameras1 however1 the plates are changed from one compartment to another of the magazine already described1 without dependence on gravity1 by an entirely positive shifting action4 ,the setting of the self-capping focal-plane shutter is accomplished by a projecting finger engaging the shutter mechanism4 ,cameras of this general type1 built for #ah by #bd centimeter plates1 with interchangeable lens cones1 removable shutters1 and preferably magazines in which the center of gravity does not shift as the plates are changed1 represent the next step in advance of the ,french practice1 and may indeed prove all that is necessary or desirable in camera complexity for peace-time photography from the air4 ,the standard ,italian camera and similar types4 ,the camera 7,lamperti7 which the ,italian ,air ,service used almost exclusively during the war exemplifies a type quite different from anything as yet described 7,figs4 #dh and #di74 ,plates to the number of twenty-four 7#ac #ah cm47 are loaded into a chamber at the top of the camera4 ,each plate is held in a septum furnished with projecting lugs at one end4 ,a lever acting through a ,bowden wire1 exposes the bottom plate1 which then swings downward about these lugs as pivots1 and is forced by a pair of fingers into a compartment at the side4 ,the between-the-lens shutter has a single speed of #a_/aej second1 and variation of exposure is achieved by altering the lens aperture4 ,illustration3 ,,fig4 #dh4--,various plate changing devices4 ,the great advantage of this camera is its simplicity1 a single motion performing all the operations4 ,its disadvantages are its dependence on gravity for operation1 its between-the-lens shutter1 the limitation set to the number of exposures1 and the necessity for removing the whole camera to take out the plates for developing4 ,in actual practice the camera has worked out well4 ,the better light found in the ,italian as contrasted with the northern theatre of war makes the between-the-lens shutter at high speed adequate1 while the limitation to the number of exposures has been met by carrying several complete cameras in each plane4 ,because of the ,bowden-wire operation these cameras need not be accessible to the observer or pilot1 so that the practice of carrying them in single-seaters was common4 ,attempts at standardization of ,allied practice through the adoption of standard lens cones were1 of course1 out of the question with this camera4 ,with its limitations of shutter efficiency and plate size it is doubtful whether it would have been satisfactory outside the service for which it was developed4 ,illustration3 ,,fig4 #di4--;,italian 7,lamperti7 single-motion plate camera1 on anti-vibration tray4 ,the limitations set by the between-the-lens shutter in this type have been overcome in an experimental camera along similar lines made by the ,premo ,works of the ,eastman ,kodak ,company1 and in the ,french ,aubry model 7,fig4 #dh74 ,these employ focal-plane shutters which swing out of the way and are set as the exposed plate swings or drops to the receiving chamber4 ,the dependence on gravity in this type could doubtless be avoided by positive finger mechanisms4 ,if so1 the resultant cameras1 set and exposed by a single motion1 would acquire a highly desirable simplicity of operation4 ,they would have peculiar merit because of the very short interval required between exposures--a characteristic needed for making low stereo-oblique views4 ,the cameras just mentioned have1 however1 departed far in form from the lines of standardized practice and have not been followed up4 ,,chapter ,,ix ,,semi-,,automatic ,,aerial ,,plate ,,cameras ,in the hand-operated camera the limit to progress is set when the number of operations is reduced to a minimum4 ,in cameras using the larger sizes of plates a reduction in the number of operations almost inevitably results in inflicting considerable muscular labor upon the operator4 ,furthermore1 distance operation becomes difficult to arrange for1 because the common reliance--the ,bowden wire--is unfitted for heavy loads4 ,consequently1 for setting the shutter and changing the plates we must resort to some other source of power than the observer's arm4 ,air-driven turbines or propellers have been used on aerial cameras1 as well as clock-work1 and also electric power1 the latter derived either from a generator or from storage batteries4 ,the relative merits of these sources of power form the subject of a separate chapter4 ,mention only is here made of the form of drive actually employed in connection with the various cameras4 ,the term semi-automatic camera is best used to designate that type in which the observer 7or pilot7 has merely to release the shutter1 after which the mechanism performs all the operations necessary to prepare for the next exposure4 ,there has been some difference of opinion as to whether it is ever advisable to go further than this with plate cameras4 ,the ,english ,service holds that completely automatic exposing1 in addition to plate changing1 is apt to encourage the making of many more pictures than necessary1 involving carrying an excessive weight of plates4 ,the ,french ,service has rather generally favored entirely automatic cameras in theory1 although during the war practically all the work of the ,french army was done by the hand-operated cameras already described4 ,the ,english ,l ,type ,camera4--,the ,l1 a modification of the earlier ,c and ,e models1 differs from its predecessors chiefly in the addition of a mechanism which when connected with a suitable source of power can be used whenever desired for changing the plates and setting the shutter4 ,as in the ,c and ,e types1 all unexposed plates are carried in a magazine above the camera1 while the exposed plates are shifted in a horizontal direction to one side and fall thence to a receiving magazine4 ,illustration3 ,,fig4 #ej4--;,american model1 ,english 8,l0 type semi-automatic camera4 ,illustration3 ,,fig4 #ea4--,mechanism of 8,l0 camera4 ,fig4 #ej shows the ,american model1 which is a copy1 with modifications1 of the original ,english design4 ,its weight with one loaded magazine is about #ce pounds4 ,its manner of functioning may be studied from the picture of the mechanism 7,fig4 #ea74 ,the part of the mechanism to the left is inoperative during hand operation1 and the large toothed wheel is locked by the removable pin shown hanging on its chain in ,fig4 #ej4 ,to change a plate and set the shutter the projecting lever 7,fig4 #ej7 is thrown over and back4 ,this causes a sliding tray1 in which the exposed plate rests1 to travel to the right1 over the receiving magazine1 where the plate is dropped4 ,after this the tray returns to the left exposing position4 ,simultaneously the shutter is wound up4 ,exposure is made either by pressing down upon the plunger1 or better1 by using a ,bowden wire4 ,provision for both methods of exposing1 one for the pilot and one for the observer1 is shown in ,fig4 #ha4 ,the shutter is the variable-aperture type already described1 provided in addition with a tension adjustment on the back of the camera4 ,a flap behind the lens does the capping during the setting operation4 ,for power operation the camera is connected through a flexible shaft with a wind driven propeller 7,figs4 #ej1 #hc and #hd74 ,the locking pin is now moved over from the toothed wheel to the lever arm1 so that the rotation of the worm driving the large toothed wheel forces the lever through its plate changing motion4 ,to prevent repetition1 a part of the periphery of the toothed wheel is cut out1 so that it stops when its cycle is run4 ,when the ,bowden wire actuates the shutter release it forces the toothed wheel around into engagement 7aided by one spring tooth7 and so starts the cycle once more4 ,when connected with the air propeller the worm is rotated continuously4 ,other sources of power--an electric motor1 for instance--can be attached through the same kind of flexible shaft4 ,if an electric motor is employed it may be run continuously or it may be operated with an insulated sector introduced into the large toothed wheel so that the electric circuit is broken and the motor stops until the wheel is once more forced around by the exposing lever4 ,faults of the ,l camera4 ,the ,l camera was the mainstay of the ,english ,air ,service4 ,in fact for the last two years of the war it was practically the only camera the ,english used1 and they thought highly of it4 ,it is1 of course1 subject to the limitation of small plate size and short focus lens4 ,it is in many ways an inconvenient camera to handle4 ,for instance1 the upper magazine cannot be closed or removed until all the plates are passed through4 ,its dependence upon gravity for the plate changing operation is a fundamental weakness1 responsible for its frequent tendency to jam in the air4 ,experience made the ,english observers very expert in relieving these jams4 ,sometimes they would turn the propeller backward 7mounting it in an accessible position to provide for this contingency71 sometimes they would shake or thump the camera4 ,but while these makeshifts would serve to secure pictures--the chief object1 of course1 of the photographic service--they can scarcely be said to render the camera satisfactory4 ,moreover1 the propeller drive has not been universally approved1 as it furnishes an additional mechanism to make trouble4 ,since it is not feasible to change from power to hand operation while in the air1 the camera is put out of commission whenever the propeller or shaft is disabled4 ,bowden-wire controls for both plate changing lever and shutter release were common in the ,british service1 which considered the extra operation or the extra muscular exertion unimportant when compared with the greater assurance of reliable action4 ,the ,english ,,lb and ,,bm ,cameras4--,during the closing months of the war an improved ,l type camera was constructed1 the ,,lb4 ,this differs from the ,l in a number of detail changes1 dictated by experience4 ,the shutter is now made removable and self-capping4 ,pivoted lugs are provided to hold the exposed plate horizontal until the very instant it drops1 in an effort to prevent jams caused by the plates piling up at an angle in the receiving magazine4 ,the chief addition1 however1 is the provision of several interchangeable cones and cylinders1 for carrying lenses of focal lengths from #d to #bj inches4 ,fig4 #ie shows the ,,lb with #bj inch lens cylinder mounted on a bell crank support in the camera bay of an ,english plane4 ,the ,,bm camera is but a larger edition of the ,,lb1 for #ah #bd centimeter plates4 ,it also carries several interchangeable lens cones4 ,the ,american model de,ram camera4--,the rotary changing box devised by ,lieutenant de,ram of the ,french army and incorporated in his entirely automatic plate camera1 has been adapted by the ,american ,air ,service to a very successful semi-automatic camera4 ,fig4 #eb shows the principle of this changing box4 ,the pile of fifty plates1 each in its sheath1 is carried in a rectangular box open at top and bottom4 ,the lower plate next the focal-plane shutter is first exposed2 the pile then rotates about a horizontal axis through a complete turn4 ,when the exposed plate arrives in a vertical position it is allowed to drop off1 by the opening of cam actuated fingers1 and lodges against the side of the enclosing camera box proper4 ,still further along in the cycle the plate is thrown off from its lodging place into a 8scoop0 on the top of the rotating container and laid on the top of the plate pile4 ,meanwhile the curtain of the focal-plane shutter winds up1 at the same time that it is depressed out of the way of the revolving plate container4 ,although the plate changing operation depends on gravity1 it nevertheless functions satisfactorily up to #cj degrees from the vertical4 ,the shutter in this model is the variable-aperture fixed-tension type1 adjusted by pivoted idlers 7,fig4 #bh74 ,in the exposing position it runs within three millimeters of the plate surface1 and is therefore of high efficiency for all openings4 ,capping during the operation of setting is performed by flaps at the bottom of the camera body4 ,interchangeable cones are supplied for lenses of various focal lengths4 ,for hand operation the changing box is turned over by means of a handle1 which rotates four times for the complete cycle 7,fig4 #ij74 ,for semi-automatic operation an additional mechanism is provided on the side of the rectangular camera body1 copied with some necessary modifications after the ,l camera power drive4 ,from the observer's standpoint the operation of the whole camera is the same as in the ,l camera1 with the important exception that power operation in no way interferes with hand operation4 ,indeed1 the hand can help out if the power flags or fails4 ,illustration3 ,,fig4 #eb4--;,diagram of automatic plate camera movements4 ,this camera is most satisfactorily driven by a #ab volt ,,hp electric motor working through a flexible shaft attached to a swivel connection at the front of the semi-automatic drive box4 ,a change once every four to five seconds is possible1 but greater speed is apt to throw the changing plate too violently for safety4 ,the chief practical objection to this camera is its bulk4 ,its great height makes it impractical for many planes4 ,its weight of nearly a hundred pounds is a formidable load for a plane to carry1 but this is no more and probably less than that of any other camera when taken up with the same number of plates in magazines4 ,the price paid for economizing in magazine weight is that the whole camera body1 excluding the lens cone1 must be carried to and from the plane for both loading and unloading4 ,,chapter ,x ,,automatic ,,aerial ,,plate ,,cameras ,general ,characteristics4--,the ideal in the automatic plate camera is to provide a mechanism which will not only change the plates and set the shutter1 as does the semi-automatic1 but make the exposures as well1 at regular intervals under the control of the operator4 ,such a wholly automatic camera would leave the observer entirely free for other activities than photography and it is to meet this tactically desirable aim that the war-time striving for automatic cameras was due4 ,it is obvious that the one essential difference between the automatic and semi-automatic types lies in the self-contained exposing mechanism with its device for the timing of the exposures4 ,there is no difficulty in arranging for the driving power to trip the shutter1 but it is no easy matter to design apparatus which will space the exposures equally1 and at the same time permit of a variation of the interval4 ,it is indeed the crux of the problem of automatic camera design to provide for the easy and certain variation of the interval from the two or three seconds demanded for low stereoscopic views to the minute or more that high altitude wide angle mapping may permit4 ,this problem is one intimately bound up with the question of means of power drive and its regulation1 and will be treated in part in that connection4 ,it is to be noted1 however1 that there are in general two modes of exposure interval regulation4 ,one is by variation in the speed at which the whole camera mechanism is driven4 ,the other is by the mere addition to a semi-automatic camera of a time controlled release which affects in no way the speed of the plate changing operation4 ,in many respects the latter is the best way to make an automatic camera4 ,while the advantages of automatic cameras are great it must not be overlooked that a camera which can only be operated automatically is of limited usefulness4 ,it is not suited for 8spotting0 at any definite instant1 as1 for illustration1 at the moment of explosion of a bomb4 ,it should1 therefore1 be the aim of the automatic camera designer to so build the apparatus that it can1 at will1 be used semi-automatically4 ,in addition1 to meet the contingency of any break-down in the source of power1 the camera should be capable of hand operation1 as in the case of the ,american semi-automatic de,ram4 ,in short1 the automatic camera should not be a separate and different type2 it should merely have an additional method of operation4 ,certain desirable mechanical features of all aerial cameras have already been enumerated4 ,some of these may be repeated here with the addition of others peculiar to automatic cameras4 ,as a general caution1 mechanical motions depending on gravity or on springs should be avoided4 ,movements adversely affected by low temperatures 7#bj to #cj degrees below zero1 ,centigrade71 are unsuitable4 ,all adjustments called for in the air must be operable by distance controls whose parts are large1 rugged1 and not dependent on sound or delicate touch for their correct setting4 ,the center of gravity of the camera should not change during operation 7important in connection with the problem of suspension74 ,the camera should work in the oblique as well as in the vertical position4 ,the power required for operation must not exceed that available on the plane4 ,electrical apparatus1 for instance1 should not demand more than #ajj watts4 ,any devices which diminish the weight of the camera are particularly desirable in automatic plate cameras1 because of the large number of exposures which such cameras encourage4 ,for instance1 if the plates could be handled without placing them in metal sheaths we should gain a substantial reduction in weight 7the sheaths weigh nearly as much as the plates7 as well as in the time necessary for handling4 ,the ,brock ,automatic ,plate ,camera4--,this camera is somewhat similar to the same designer's film camera1 both in shape1 in size1 and in its employment of a heavy spring motor for the driving power4 ,it uses #d #e inch plates1 and carries a #aj to #ab inch lens4 ,the plate-changing operation is unique4 ,as shown diagrammatically in ,fig4 #eb1 the unexposed plates are carried in a magazine on top of the camera1 the exposed ones in a magazine inserted in the body of the camera1 directly below the unexposed magazine4 ,the bottom plate of the exposed pile drops into a sliding frame and is carried along the top of the camera to the exposing position4 ,after exposure1 the plate is carried back and drops into the receiving magazine4 ,in order for the plate to fall only the proper distance at each stage of the cycle1 special plate sheaths are necessary4 ,these are cut away to form edge patterns which clear or engage control fingers so as to ride or fall through the sliding frame as required4 ,the camera is entirely automatic in operation4 ,regulation of the exposure interval is by a special form of variable length escapement controlled through a ,bowden wire1 in a manner parallel to that in the ,brock film camera1 described elsewhere4 ,these plate cameras were never produced in quantity4 ,folmer #ac #ah ,centimeter ,automatic ,camera4--,this camera1 also never manufactured in quantity1 is shown in ,fig4 #ec1 and a sketch of its manner of operation is included in the ensemble of automatic camera diagrams 7,fig4 #eb74 ,its most distinctive feature is perhaps the use of a two compartment magazine4 ,this is similar in form to the one already described in connection with the hand-held cameras1 but larger1 to hold eighteen #ac #ah centimeter plates4 ,the unexposed plates are placed in one compartment1 and after exposure are shifted to the other4 ,the transfer is effected by the motion of a rack1 which is part of the magazine and which is driven by a toothed pinion1 also part of the magazine1 which in turn engages in a toothed wheel projecting upward from the camera body4 ,this toothed wheel is turned first in one direction and then in the other by an arrangement of gears and levers driven by the source of power1 which as shown in ,fig4 #ec is a wind turbine connected through a flexible shaft4 ,operation is either automatic or semi-automatic as desired1 and the camera can be put through its cycle by hand if necessary4 ,illustration3 ,,fig4 #ec4--;,folmer #ac #ah centimeter automatic and semi-automatic plate camera4 ,illustration3 ,,fig4 #ed4--;,french model de,ram automatic plate camera4 ,as with several other designs1 the completion of the working model of this camera occurred after agreements had been reached by the ,allies1 as to plate size1 standard lens cones1 and other features1 not easily incorporated in it1 thus making manufacture inadvisable4 ,the validity of the design for peace-time work is1 of course1 not affected by this fact4 ,the de,ram ,camera4--,the only completely automatic plate camera actually produced commercially before the end of hostilities was the ,french model de,ram 7,fig4 #ed74 ,its plate-changing action has already been described in connection with the ,american semi-automatic model 7,figs4 #eb1 #ij and #ia74 ,it differs from the ,american model in the shutter1 which is of the self-capping variety1 carried on a rising and falling frame2 and in the exposing mechanism4 ,the latter embodies a clutch whose point of attachment to a uniformly rotating disc in the camera is governed through a ,bowden wire1 whereby the interval between the plate-changing operation and the shutter release is varied4 ,the intervals are indicated by figures on the dial to which the observer's end of the ,bowden wire is attached4 ,the source of power for the camera is a constant speed propeller4 ,complete semi-automatic operation is not possible1 as an interval of #a to #b seconds elapses between the time a single exposure is called for and its occurrence4 ,no arrangement is provided for hand operation4 ,it will be noted that while this camera is a true automatic apparatus it does not meet even a majority of the requirements listed above as found desirable by experience4 ,there exists a great opportunity for designing and developing an entirely satisfactory automatic plate camera--provided it is agreed that anything more than semi-automatic operation is ever advisable for plates4 ,,chapter ,,xi ,,aerial ,,film ,,cameras ,the weight of the glass and the sheaths in the plate camera forms its most serious drawback4 ,this weight must be reckoned at least three quarters of a pound for each #ah #bd centimeter plate4 ,consequently1 with the use of these large plates1 and with the demands for ever increasing numbers of pictures to be taken on long reconnaissance flights1 a serious conflict arises between the weight of the photographic equipment and the carrying capacity of the plane4 ,among plate cameras probably the most economical in weight is the de,ram4 ,it carries fifty #ah #bd centimeter plates1 and has a total weight of approximately #ajj pounds4 ,an advance to #ajj or #bjj plates--not feasible in the de,ram construction--even if we assume the lightest possible magazines1 would bring the weight of camera and plates to #aej or #bjj pounds1 which would be detrimental to the balance and would seriously infringe on the fuel carrying capacity and ceiling of any ordinary reconnaissance plane4 ,early and persistent attention was therefore paid to the possibilities of celluloid film in rolls1 as used so widely in hand cameras and in moving picture work4 ,the two great advantages of film would be its practically negligible weight 7approximately one-tenth that of plates1 not including sheaths7 and its small bulk1 which would permit the greatest freedom in the development of entirely automatic cameras to make exposures by the hundreds instead of by the dozens4 ,certain disadvantages were foreseen at the outset3 the difficulty of holding the film flat and immune from vibration in the larger sizes2 the difficulty of quickly developing and drying large rolls2 the question whether as good speed or color sensitiveness could be obtained in sensitive emulsions when flowed on a celluloid base as on glass4 ,early trials revealed a further problem to solve3 how to eliminate the discharge of static electricity occurring at high altitudes1 especially when the weather is cold4 ,as far as camera construction is concerned the chief problems are to hold the film flat1 and to eliminate static4 ,methods of ,holding ,film ,flat4--,several means have been proposed and used for holding the film flat4 ,disregarding mere pressure guides at the side1 which are suitable only for small area films 7up to #d #e inch71 the successful means have taken three forms3 pressure of a glass plate1 pressure of the shutter curtain1 and suction4 ,a glass pressure plate can be used in either of two ways2 the film may be in continuous contact with it or may be pressed against its surface only at the moment of exposure4 ,the advantage of this first method lies solely in its mechanical simplicity2 its disadvantage in the likelihood of scratches or pressure markings on the film4 ,where a glass plate is used there is always the chance of a dust or dirt film accumulating1 or of the condensation of moisture1 to impair the quality of the negative4 ,there is1 moreover1 an inevitable loss of light 7about #aj71 together with some slight distortion1 due to the bending of the marginal oblique rays through the thickness of the glass4 ,in cases where a filter would normally be employed1 the loss of light is minimized by using yellow glass for the plate1 so that it serves for filter and film holder as well4 ,pressure of the shutter curtain is utilized in the ,duchatellier film camera by furnishing the edges of the curtain aperture with heavy velvet strips1 whose light and gentle pressure during the passage of the shutter holds the film against a metal back4 ,in many ways this is the simplest film-holding device2 it occasions no loss of light1 and needs no mechanical movements or external accessories1 such as are called for in the suction devices next described4 ,there is always danger of markings on the film1 if the velvet is not of the right thickness and softness1 and the operation and speed control of the shutter are necessarily complicated by the additional frictional load4 ,suction of the film against a perforated back plate has been found a very successful means of securing flatness4 ,suction at the moment of exposure may be produced by the action of a bellows1 which has been compressed beforehand by the camera-driving mechanism4 ,continuous suction can be produced either by a continuously driven pump1 or by a ,venturi tube placed outside the fuselage4 ,the ,venturi tube 7,fig4 #ee7 consists of a pipe built up of two cones1 placed vertex to vertex1 to form a constriction4 ,when air is forced through this at high velocity suction is produced in a small diameter tube taken off at the constriction4 ,a suction of two centimeters of mercury1 acting through holes about one centimeter equidistant from each other in the back plate1 has been found adequate to hold flat a film #ah #bd centimeters4 ,one merit of suction applied only at the moment of exposure is that the film-driving mechanism does not have to work against the drag of the suction4 ,continuous suction1 on the other hand1 gives a longer opportunity for flattening out kinks in the celluloid1 and easily permits movement of the film during the exposure1 either for the purpose of permitting a longer exposure or for the purpose of preventing distortion due to the focal-plane shutter4 ,a disadvantage of continuous suction is the production of minute scratches on the celluloid surface as it drags over the suction plate4 ,these are ordinarily too small to cause trouble1 but may show up when printing is done in an enlarging camera4 ,illustration3 ,,fig4 #ee4--,venturi tube on side of plane4 ,static discharges are produced by the friction of the celluloid against the pressure back or other surfaces with which it comes into contact4 ,they show in the developed film as branching tree-like streaks 7,fig4 #ef7 and in cold dry weather may be numerous enough to ruin a picture4 ,the discharges are noticeably less frequent with film coated on the back with gelatine 78,n4,c4071 but the extra gelatine surface is extremely undesirable4 ,when handled by developing machines1 as large rolls must be1 this back gelatine surface becomes scratched and bruised in a serious manner4 ,plain unbacked film is much to be preferred if the static can be obviated4 ,to avoid static1 it is necessary to provide for the immediate dissipation of all acquired electrical charges4 ,experiments made by the ,united ,states ,air ,service have shown that nothing is so good as rather rough cloth1 thoroughly impregnated with graphite1 held in close contact with the celluloid during as great a portion of its travel as possible4 ,in the ,united ,states ,air ,service film camera which uses suction through a perforated back plate1 the plate has been covered with thin graphited cloth1 and similar cloth sheets are pressed against the film rolls by sheets of spring metal 7,fig4 #fe74 ,in cameras with this equipment no trouble has been experienced with static4 ,illustration3 ,,fig4 #ef4--,print from film camera negative1 showing static discharge1 and 7upper left-hand corner7 record of altitude and compass direction made by ,williamson film camera auxiliary lens 7,fig4 #eh74 ,representative ,film ,cameras4--,the ,english ,f type 7,williamson74 ,this is one of the earliest cameras designed for film1 as is indicated by the nature of the power drive1 which presupposes that the camera is to be carried on the outside of the fuselage4 ,its essential features are shown in ,figs4 #eg and #eh4 ,it consists of a rectangular box with a cone at the front on which is mounted a propeller1 intended to be rotated by the wind made by the motion of the plane4 ,this drives1 through a governor controlled friction clutch1 a train of gears which draws the 7#e #d inch7 film across the focal plane1 sets and exposes the shutter at regular intervals4 ,illustration3 ,,fig4 #eg4--;,english type 8,f0 7,williamson7 automatic film camera4 ,above the camera1 supported on a tripod1 are a compass and altimeter1 both recording on a single dial1 illuminated from below by the light reflected from a circular white disc painted on top of the camera4 ,an image of the dial is thrown on a corner of the film by a lens1 whose shutter is actuated in synchronism with the main focal-plane shutter4 ,no special means are provided for holding the film flat4 ,special film with perforated edges is used4 ,the camera was designed for mapping work on the ,mesopotamian and other fronts where no maps at all existed4 ,the ,duchatellier camera is essentially a film magazine to fit on the standard ,french de,maria camera bodies1 of the #ah #bd centimeter size4 ,in its simplest form it embodies a shutter 7the regular focal-plane shutter of the camera being removed7 and a film-moving mechanism1 both actuated by a single motion of the hand4 ,automatic and semi-automatic operation are accomplished by an auxiliary mechanism to which ,bowden wires from the hand lever are attached4 ,the motive power is an air propeller4 ,variation of speed is obtained by changing the point of contact of a roller on a friction disc1 the disc being directly connected to the propeller shaft1 the roller to the camera drive shaft4 ,illustration3 ,,fig4 #eh4--;,interior of type 8,f0 camera1 showing lens for photographing compass and altitude readings4 ,the most distinctive features of the ,duchatellier camera is its use of the focal-plane shutter to hold the film flat during the exposure4 ,as already explained1 this is accomplished by pressure1 velvet strips on the shutter edges keeping the film close against the back plate4 ,the return of the shutter curtain to the 8set0 position is accomplished by locking it to the film by perforating points1 so that it is pulled across as the film is wound4 ,this introduces between each pair of pictures a strip of tremendous over exposure1 as wide as the curtain opening4 ,a fixed-aperture variable-tension shutter is used4 ,the magazine carries a roll of film long enough for #bjj exposures1 feeding the long way of the picture4 ,when film needs to be changed in the air1 this is done by changing the entire magazine1 including its shutter4 ,illustration3 ,,fig4 #ei4--;,g4 ,e4 ,m4 automatic film camera4 ,the ,g4 ,e4 ,m4 camera 7,fig4 #ei7 is a very light self-contained clock-work-drive camera taking #cf pictures six inches square4 ,the film is unrolled from a small-diameter feeding roller on to a large-diameter receiving roller to which the driving mechanism is attached4 ,by this means approximately equal spacing of pictures on the film is assured4 ,the film is held flat by continuous contact with a glass plate1 which is made of yellow glass1 so that it serves at the same time as a color filter4 ,the lens--of #h to #ab inch focus--is equipped with a single speed between-the-lens shutter4 ,the operation of the camera is entirely automatic4 ,the interval between pictures is controlled by varying the clock-work speed1 through a lever on the outside of the camera box4 ,protection of the camera from vibration is sought by supporting it on four spring cushions mounted on a solid frame1 to which the camera is held by spiral springs attached to its sides4 ,illustration3 ,,fig4 #fj4--;,brock automatic film camera4 ,the ,brock ,film camera 7,fig4 #fj7 is an entirely automatic1 very compact self-contained camera1 taking one hundred #d #e inch pictures4 ,the motive power is clock-work1 regulated in speed by an escapement controlled by a flexible shaft carried to a dial which may be fastened to the instrument board or to some other convenient part of the plane4 ,the lens is #f1 #ab1 or #ah inch focus4 ,the shutter is of the fixed-aperture variable-tension type1 of long travel1 and with a flap behind the lens for covering during the setting period4 ,none of the special means above described for holding the film flat are provided4 ,a metal plate resting on the back1 and a flat metal frame in front with a #d #e inch aperture1 are considered sufficient check on the excursions of the small-sized film4 ,a ball bearing double pivoted frame serves to support the camera in a pendulous manner1 permitting it to assume a vertical position after tilting4 ,damping of oscillations and vibration is arranged for by two pneumatic dash pots4 ,the ,german film mapping camera1 shown in ,fig4 #fa1 is distinguished by a number of special features4 ,the size of the pictures1 #f #bd centimeters1 is unusual4 ,it has its advantages1 however4 ,since the short dimension is in the line of flight1 the maximum width of field covered by the lens is utilized 7,fig4 #ag74 ,this of course necessitates a larger number of exposures to complete a strip1 which is perhaps an added advantage1 since the narrower the individual pictures the better the junctions will be1 especially if large overlaps are made4 ,this proved to be the case with captured ,german mosaics4 ,difficulty is experienced in making overlaps on a turn 7,fig4 #fb71 but this is not a vital objection4 ,the shutter has a fixed aperture1 narrower at the center than at the ends1 to compensate for the falling off in illumination away from the center of the lens4 ,no safety flap is needed because the curtain moves in opposite directions on successive exposures1 thereby also compensating for shutter distortion1 as has already been discussed4 ,shutter speed is controlled by varying the tension of the actuating spring4 ,illustration3 ,,fig4 #fa4--;,german automatic film camera4 ,the camera is driven by an electric motor1 connected to a set of gears1 whose shifting provides for speed variation4 ,the film is moved by rubber rollers which are cut away for part of the circumference1 allowing the film to stand still until they bite again4 ,a yellow glass pressure plate holds the film during the exposure and serves as color filter also 7,fig4 #fc74 ,an electric heater is provided near the shutter1 as in all the later ,german cameras4 ,illustration3 ,,fig4 #fb4--,method of joining and printing film from ,german camera4 ,united ,states ,air ,service automatic film camera--,type ,k 7,figs4 #fd1 #fe1 #ib1 #ic1 #ih1 #ii74 ,this is an entirely automatic camera1 manufactured by the ,folmer and ,schwing ,division of the ,eastman ,kodak ,co41 taking #ajj pictures of #ah #bd centimeter size at one loading4 ,as with all the ,american cameras of this size1 it uses the standard lens cones of any desired focal length4 ,the camera proper consists of a compact chamber in which the film rollers are carried at each end forward of the focal plane1 the shutter lying between4 ,in consequence of this arrangement the vertical depth of the camera is the absolute minimum--short of decreasing the length of the optical path by mirror arrangements--making it possible to suspend the camera diagonally in the ,american and ,british planes1 for taking oblique pictures4 ,flatness of the film is secured by a suction plate covered with graphited cloth and connected with a ,venturi tube4 ,the top cover is removed for re-loading4 ,the shutters on the first cameras of this type are of the variable-tension fixed-aperture design1 though later ones have the variable-aperture curtain controlled by an idler1 as used in the ,american de,ram4 ,an auxiliary curtain shutter serves to cap the true shutter during setting4 ,the operation of the film driving mechanism is comparatively simple4 ,it consists of a train of gears1 driven by a flexible revolving shaft attached to some separate source of power capable of speed variation4 ,the action of the gears is to move the film1 set the shutter and then expose it2 in the earlier cameras with the film continuously moving4 ,in the first cameras constructed the space between the pictures varies as the film rolls up1 due to the increasing diameter of the roll4 ,in later cameras the film roller is disengaged from the gears just before the shutter is tripped1 so that the film stands still during the exposure1 and is then re-engaged at a new point on a ratchet wheel governed by the diameter of the receiving roll1 whereby the pictures are equally spaced4 ,in all the cameras1 punch marks made at the time of exposure enable the limits of the picture to be detected in the dark room by touch4 ,illustration3 ,,fig4 #fc4--;,film winding and exposing mechanism in ,german film camera4 ,variable speed is arranged for in any one of several ways4 ,for peace-time uses a turbine attached to the side of the plane is simple and positive1 and1 provided it is made of sufficient size--which is not the case with the one shown in the ,figure--will give adequate speed regulation upon varying the aperture through which the air enters4 ,the ,venturi tube may be carried upon the same mount1 or a small rotary pump can be attached on the same shaft4 ,where the high wind resistance of the turbine is an objection the camera is driven electrically1 by a motor acting through the intermediary of a variable speed control described in the next chapter 7,fig4 #fh74 ,the camera weighs complete about forty pounds1 and the film rolls about four pounds4 ,the latter can be changed in the air without great difficulty provided the camera is mounted accessibly and so that the top may be opened4 ,,chapter ,,xii ,,motive ,,power ,,for ,,aerial ,,cameras ,as long as circumstances permit1 hand operation still remains the most reliable and satisfactory method of driving a camera4 ,it is always available1 can be applied to just the amount desired1 and at the time and place needed4 ,for instance1 in a magazine of the ,gaumont type 7,fig4 #dj71 what is needed is the periodic application of a very considerable force rather quickly1 and while this can be done quite simply by hand1 no mechanism has even been attempted to go through this same operation automatically4 ,instead1 the fundamental design of automatic magazines has been made along other lines calculated to utilize smaller forces more steadily applied4 ,it must be granted1 however1 that for war planes1 and particularly for single seaters1 cameras should be available which are capable of operating semi-automatically or automatically4 ,this necessarily means the employment of artificial power1 whose generation1 transmission to the camera and control as to speed present a mechanical problem of no small difficulty4 ,available ,sources of ,power4--,the sources from which power may be drawn on the plane are four1 although the various combinations of these present a large number of alternative approaches to the problem4 ,these sources are3 #a4 ,the engine of the plane4 #b4 ,wind motors4 #c4 ,spring motors4 #d4 ,electric motors4 ,these may first be considered largely from the descriptive standpoint1 leaving questions of performance and efficiency for separate treatment4 ,power may be derived directly from the engine through a flexible shaft1 similar to that used for the revolution counter4 ,this source of power is inherently the most direct and efficient1 since the engine is the seat of all the lifting and driving energy of the plane4 ,there is no loss through transformation into other forms of energy1 such as electrical2 or by the use of more or less inefficient intermediary apparatus1 such as wind propellers4 ,against the direct drive of the camera from the engine may1 however1 be urged that the usual distance between engine and camera is too great for reliable mechanical connection1 as by flexible shafting4 ,objections also arise from the standpoint of speed4 ,this cannot be controlled by the camera operator2 and varies over too wide a range1 as the engine changes from idling to full speed1 to fit it for automatic camera operation4 ,the first objection may be met by that combination of methods of power drive which consists in transmitting the power electrically2 that is1 by letting the engine operate a generator from which cables run to a motor close to the camera4 ,this method1 of course1 sacrifices efficiency1 and it breaks down when the engine speed drops below the speed necessary to generate the requisite voltage4 ,this defect may in turn be met by floating in storage batteries1 which brings up the whole question of electrical drive1 to be treated presently4 ,while use of the engine for direct drive or for generating electric current has not been adopted in the ,american service1 it is known that some ,german planes were supplied with electric current in this way4 ,coming next to the wind motors1 these possess one very great merit3 they utilize a motive power that is always present as long as the plane is in motion through the air4 ,on the other hand1 the process of using the main propeller of the plane to pull another smaller propeller through the air appears a roundabout way to utilize the driving power of the airplane engine4 ,yet on the whole it is probable that some form of propeller or wind turbine is the simplest and most convenient device we have for the operation of airplane auxiliaries4 ,as long as the amount of power required is small1 such inefficiency as is inherent in its use is offset by its convenience and reliability4 ,an advantage of the propeller is that its speed is almost directly proportional to that of the plane through the air1 a desirable feature in automatic cameras provided the proportionality is under control4 ,yet it is just in this matter of varying the speed at will that the propeller presents difficulties1 to be met only by additional mechanisms for gearing down or governing4 ,propellers have the practical disadvantage that they present an easily bent or broken projection to the body of the plane 7,figs4 #hc and #hd74 ,the strength of small propellers for operating auxiliaries is never so much in question with reference to their resistance to whirling and thrust of air as it is to their ability to withstand the inevitable knocks and careless handling that will fall to their lot4 ,the propeller bracket is just what the pilot is looking for to scrape the mud off his boots before climbing in4 ,the wind turbine has the advantage over the propeller that its speed can be varied rather simply by exposing more or less of its face to the wind4 ,a turbine fitted with an adjustable aperture for admitting the wind is shown in ,fig4 #fd1 in connection with the type ,k automatic film camera4 ,the turbine has the advantage of being compact and lying close against the body of the plane4 ,in the form figured1 altogether too much head resistance is offered--just as much for low as for high speeds--but with proper design this need not be the case4 ,it is1 moreover1 quite too small to give the needed speed regulation1 as it only begins to operate near its full opening4 ,illustration3 ,,fig4 #fd4--,u4 ,s4 ,type 8,k0 7,folmer7 automatic film camera1 with wind turbine and ,venturi tube4 ,illustration3 ,,fig4 #fe4--,type 8,k0 camera1 open1 showing suction plate4 ,spring motors have the very real advantage that by their use the camera can be made entirely self contained4 ,the simplest application of the spring motor would be to the semi-automatic camera1 where no close regulation of speed is required4 ,in such a camera the operation of exposing the shutter would release the spring1 which would then change the plate or film and re-set the shutter1 repeating this operation as long as the spring retained sufficient tension4 ,small film hand-cameras of this type1 using self-setting between-the-lens shutters1 have been designed1 though not for aerial work4 ,the possibilities of using springs as motive power in semi-automatic cameras have not apparently been seriously considered4 ,when a spring motor is used for automatic camera operation it at once becomes necessary to add to the motor an elaborate clock mechanism for controlling and regulating its speed of action4 ,springs are much better fitted for giving power by quick release of their tension than by slow release1 and the necessary clock mechanisms for their regulation become very heavy1 as well as complicated and delicate1 when they are made large enough to do any real work4 ,for their repair they require the services of clock makers rather than the usual more available kind of mechanic4 ,coming next to electric motors1 we meet with a source of power of very great flexibility both in its derivation and in its application4 ,if a source of electric current is already provided for heating and lighting as it is on the fully equipped military plane1 and if it has sufficient capacity to handle the camera1 its use is rather clearly indicated1 irrespective of how efficiently or by what method it is produced4 ,especially is this the case1 from the standpoint of economy and simplicity1 if a propeller-driven generator is the source of current1 and the alternative power drive is an additional propeller for the camera4 ,if1 on the other hand1 the camera must have its own source of electric power1 the advantages and disadvantages must be closely scrutinized4 ,in this case either a generator must be provided1 or resort be made to storage batteries1 or a combination of the two4 ,ruling out a special propeller-driven generator1 we are left with either the generator driven from the engine or the storage battery4 ,inasmuch as storage batteries are practically indispensable with generators1 in order to maintain the voltage constant at all speeds1 it is on the whole advisable to rely upon batteries alone4 ,an advantage of their use is that the power plant is entirely within the plane3 ,all projections such as propellers are avoided4 ,another merit is that the power is drawn upon only as needed4 ,against storage batteries is their weight1 the need for frequent charging1 and their loss of efficiency at low temperatures--a loss so serious with those of the ,edison form as to preclude their use4 ,when once the source of electrical energy is decided upon1 its method of application needs to be considered4 ,here we meet at once the peculiar merit of electrical energy1 namely1 the ease and convenience with which it may be transmitted4 ,all we need is a pair of wires1 led to any part of the plane by any convenient route and connected by simple binding posts4 ,it may with equal ease be turned on or off by merely making or breaking a contact with a switch4 ,for operating semi-automatic cameras this feature may be utilized in the interest of economy1 if the power is automatically turned off as soon as the plate-changing operation is finished4 ,exceptionally reliable make and break contacts are necessary to insure the success of this latter scheme4 ,two methods of transforming the supply of electrical energy into mechanical motion are available4 ,the first is by the use of a solenoid and plunger4 ,this is a device practically restricted to semi-automatic cameras1 in which the operation consists of a straight to-and-fro motion1 initiated at the will of the operator4 ,it has been used little if at all4 ,the second motion is the continuous rotary one secured by the use of an electric motor4 ,this motion is the most practical one for the continuous operation of any mechanism1 but on the other hand requires that the imposed load be reasonably uniform at all times through the cycle of operations4 ,assuming that the camera mechanism is of this character1 the motor may be attached directly to the camera1 or if it must be so large as to cause danger by vibration1 it may be connected through a flexible shaft4 ,this use of an electric motor is very practical for semi-automatic cameras such as the 8,l0 or the ,american de,ram1 in planes supplied with a suitable source of current4 ,when it comes to entirely automatic cameras1 where uniform and regulatable speed is required1 as in making overlapping pictures for mapping1 the electrical drive is not so convenient4 ,the shunt-wound motor runs at nearly constant speed1 while the series-wound motor in which the speed can be regulated by the interposition of resistance1 has nothing like a sufficient range of variation for the purpose 7at least five to one is imperative7 before it fails to carry the load4 ,hence we must either incorporate in the camera some mechanism for varying the interval between exposures while the speed of the motor remains constant1 or introduce an auxiliary device to effect the required transformation in speed4 ,if we do use an auxiliary device the train of apparatus1 consisting of battery 7or generator71 motor1 speed control and camera1 is altogether too long2 it is apt to cause annoying delays in connecting up in an emergency1 and it offers an excessive number of chances for break-down4 ,performance and ,efficiency ,data4--,the first step in deciding upon methods of power drive1 and indeed in deciding whether power drive is feasible at all1 is to assemble definite data as to the power required to drive representative cameras4 ,approximate figures for some of the cameras described in previous chapters are3 ,l camera1 #bf watts1 de,ram1 #fj watts1 8,k0 film1 #cj watts4 ,these requirements--not exceeding horse power--are insignificant in comparison with the total of #ajj to #djj horse power available for all purposes from the plane's engine4 ,propeller characteristics4 ,data on the performance of small propellers are somewhat meagre4 ,however1 the results of the rather extensive researches on large ones1 suitable for driving planes1 may be applied1 with proper reservations1 to give a fair guide to the study of the application of small propellers for driving plane auxiliaries4 ,the first factor to be considered is the thrust or head resistance offered by a propeller to motion through the air4 ,this varies as the square of the velocity1 as the density of the medium1 and as the area of the body projected normally to the wind1 the formula being ,t cda,v#b where ,t thrust1 d density1 a area1 ,v velocity4 ,data on the ,l camera propeller are shown in ,fig4 #ff1 where its thrust both when free and when loaded with the camera is given1 as well as that of a solid disc of the same diameter as the propeller4 ,for this propeller1 which is double-bladed1 and six inches in diameter1 cda 4#jjjbge with the load on4 ,the total thrust amounts to only about three pounds when the plane velocity is #ajj miles per hour4 ,the head resistance of the whole plane is a matter of hundreds of pounds1 so that the propeller resistance is quite negligible4 ,illustration3 ,,fig4 #ff4--,wind propeller data4 ,the next factor is the speed of revolution of the propeller1 expressed in revolutions per minute4 ,this varies with the design--the number of blades1 their area1 and pitch4 ,for a given design the speed of revolution is directly proportional to the speed of motion through the air1 and to the density of the air4 ,representative data for the ,l camera propeller are shown in ,fig4 #fg4 ,it will be noted that the speed goes up to #hjjj for #abj miles per hour air speed4 ,this illustrates the necessity for great strength to withstand centrifugal force4 ,propellers should be constructed of tough material1 and subjected to whirling tests up to speeds considerably in excess of any the plane will attain in any maneuver4 ,at low speeds the linear relationship fails1 as a critical velocity is reached--about #cejj r4 p4 m4 for this propeller--where it refuses to turn4 ,illustration3 ,,fig4 #fg4--,relation between air speed and propeller revolutions4 ,the fact that the speed of the propeller depends on the density of the air has an interesting corollary1 which is that a propeller adequate at low altitudes will fail at high ones4 ,the density of the air varies with altitude according to the following figures3 ,at #cjjj meters1 #gb per cent4 of sea level #ejjj meters1 #ei per cent4 of sea level #fjjj meters1 #eb per cent4 of sea level ,if we take the r4 p4 m4 at #ij miles per hour at sea level as #fjjj1 then at the above altitudes the speeds will be #dcjj1 #cejj1 and #cjjj1 respectively4 ,the last figure is below that for which this size of propeller stalls with its normal load1 as noted in the last paragraph4 ,consequently1 if flying is to be done at these altitudes a larger propeller must be carried1 which will still deliver enough power at the lower density4 ,the next factor to be considered is the power furnished by the propeller4 ,as a representative figure may be quoted the performance of the ,l propeller4 ,this gives #bg watts at #cfjj revolutions per minute 7#ef miles per hour74 ,from this figure the performance of other propellers may be deduced from the basic laws1 which are3 that the power varies as the density of the medium and as the cube of the velocity 7assuming constant efficiency74 ,since the power delivered by the six inch diameter ,l propeller is already adequate at #fj miles per hour1 the necessary dimension to function satisfactorily at #ajj miles per hour would need to be only a little more than three inches1 except for the desirability of a safety factor for high altitudes and low air densities4 ,the efficiency of the propeller is defined by the relation-- power delivered by the propeller ,efficiency ---------------------------------------- ------------------------ power supplied to the propeller ,the denominator of this fraction is the thrust times the velocity1 for which the curves of ,fig4 #ff supply us data for the ,l propeller4 ,using the figures #cfjj r4 p4 m41 #ef miles per hour1 and #bg watts1 we find the efficiency to be about #ej per cent4 ,this increases with the velocity1 with a possible upper limit of #gj to #hj per cent4 ,since the main propeller of the plane is not over #hj per cent4 efficient we have at most an efficiency of #fd per cent4 in using a propeller drive1 as compared with taking the power directly off the engine4 ,in considering the use of spring and clock-work motors we meet at once with the problem of comparing the effect on the performance of a plane of a carried weight1 as against a head resistance4 ,the efficiency of a spring motor is measured in terms of its weight1 that of a propeller in terms of its head resistance4 ,the general answer to this question is given by the relation that a pound of dead weight is equivalent to pound head resistance4 ,in order to apply this relation to the study of spring motors for driving cameras1 data are necessary on the power delivery per pound weight of such mechanisms4 ,such data are not easily accessible1 largely because clock-work has not generally been seriously considered as a motive power for large apparatus4 ,to arrive at an approximate figure we may take the fact that in an #h #aj inch film camera designed by one of the manufacturers who have utilized clock-work1 the motor weighed #cj pounds4 ,this is equivalent to six pounds head resistance4 ,now the type ,k1 #ah #bd centimeter film camera is operated1 even with the addition of a friction drive speed control1 by means of the ,l camera propeller4 ,as shown in ,fig4 #ff1 at #ajj miles per hour the head resistance of this propeller is still less than three pounds4 ,consequently1 it appears that from the efficiency standpoint the clock mechanism is quite outclassed by the wind propeller4 ,coming next to the electric motors1 the ,l camera and the ,k are both operated satisfactorily with a #a_/bj horse power motor1 weighing #f pounds4 ,for the de,ram a horse power motor has been adopted4 ,taking up efficiency considerations1 we have1 if the current is supplied by a generator from the engine1 a transformation factor of #gj to #hj per cent4 from mechanical to electrical energy and a similar factor in using a motor for the camera4 ,when batteries are employed the matter of weight versus head resistance again arises4 ,the batteries found most satisfactory for operating the ,k and de,ram cameras are of the six-cell #ab volt lead type4 ,their capacity is #dj ampre hours at three ampres or #cf at five ampres--more than is necessary for a single reconnaissance1 but a practical figure when economy of charging and replacement are considered4 ,the weight of this unit is #bg pounds4 ,to this must be added the weight of the motor--#f lbs4--making a total of #cc pounds1 equivalent to a head resistance of nearly #g pounds4 ,this is more than twice the propeller head resistance invoked to do the same work4 ,these considerations of efficiency have been gone into because they are usual in studying any engineering problem and because of the insistent demand from the plane designer that every ounce of weight and head resistance be saved4 ,actually1 as already stated1 the load imposed by any method of power drive is trivial in comparison with the whole load of the plane4 ,there is1 however1 an important reservation to be made1 which applies against clock-work and batteries3 ,this is1 that while the equivalent head resistance of any camera motive power carried as dead weight is small1 its effect on balance may not be so4 ,while the use of a propeller need not disturb the plane's balance1 the weight of the camera alone1 without any driving apparatus1 is already seriously objected to on this score4 ,the merely mechanical superiority of the propeller as a source of motive power is on the whole rather marked4 ,control of ,camera ,speed4--,in the semi-automatic camera the only control required on the speed of the operating motor is at the upper and lower limits4 ,it must not go so fast as to anticipate the completion of any steps in the cycle of camera operation1 such as the fall of plates or pawls into position1 which would jam the camera4 ,on the other hand1 it must not be so slow that pictures cannot be obtained with the requisite overlap for maps or stereoscopic views4 ,in the ,american de,ram camera the cycle of operations cannot safely be put through in less than four seconds1 a short enough interval for most purposes4 ,it is also highly desirable in the semi-automatic camera to have the motive power capable of stopping completely4 ,this saves wear and tear on both motor and camera mechanism4 ,in the automatic camera an extreme range of speed is called for by the several problems of mapping1 oblique photography1 and the making of stereoscopic views4 ,for mapping alone1 the shortest likely interval may be taken as that required for work at approximately #ajjj meters altitude1 for a plane speed of #aej kilometers per hour1 which demands an interval of six seconds with a ten inch lens on a #d #e inch plate4 ,for vertical stereos at the same altitude and speed this interval is divided by three1 and low oblique stereos need even quicker operation4 ,hence a range of from #a to #cj pictures per minute should be provided for4 ,this requirement is difficult to meet with any simple mechanism4 ,from the standpoint of simplicity in speed regulation the wind turbine of adequate vane surface has much to recommend it4 ,it is only necessary to present more or less of its vane area to the wind in order to secure a considerable range of speed4 ,the method of doing this by a shutter interposed in front is uneconomical1 but it is probable that the design can be so altered that more or less of the turbine is exposed beyond the side of the plane1 possibly by varying the angle1 to secure the same result without introducing useless head resistance4 ,a serious practical objection to the turbine lies in the large vane surface necessary to give adequate power combined with proper speed variation4 ,in the automatic film camera 7,type ,k7 this area should be as much as #dj to #ej square inches4 ,the wind propeller does not lend itself at all well to speed variation4 ,it cannot be partially covered from the air stream1 as can the turbine1 because of the resulting strain on its mount4 ,a possible form of variable speed propeller1 one which1 however1 has not yet been practically developed1 is a propeller with controllable variable pitch4 ,if this could be made mechanically sound it would be well-suited for camera operation4 ,that such a propeller could be worked out is indicated by the good performance of a constant speed propeller developed for radio generators and used on the ,french de,ram camera 7,fig4 #ed74 ,parenthetically1 it maybe questioned whether a constant speed propeller is really desirable with an airplane camera4 ,what is required is not exposures at a definite time interval--although most of the data are in that form--but exposures at definite intervals with respect to the motion of the plane1 which practically means with reference to its air speed4 ,rather than build a camera calculated to give exposures at intervals of so many seconds when it is attached to a constant speed propeller1 we would do better to use a propeller which responds to the speed of the plane1 in conjunction with some form of tachometer to show the rate at which exposures are being made4 ,this in turn should be cordinated with the indications of a proper camera-field indicating sight4 ,one solution of the problem of speed control with a propeller of practically fixed speed1 is to use a governor and slip clutch as in the ,english ,type ,f film camera 7,fig4 #eg74 ,here the propeller shaft and the camera driving axle are connected by two friction discs4 ,that on the camera mechanism is forced against the other by a spiral spring1 whose tension is controlled by a ball governor4 ,if the camera speed becomes too high the governor reduces the tension on the spiral spring and the discs slip over each other4 ,the point where this slipping occurs is determined by the position of the governor as a whole1 and this is controlled by a lever on top of the camera4 ,another speed control device1 perhaps more positive but certainly more complicated and wasteful of power1 consists of a large flat disc1 driven by the propeller or electric motor1 and from which the camera is driven by a shaft from a smaller friction disc which may be pressed against any point from the center to the periphery of the larger disc4 ,the speed range attainable in this way is limited only by the size of the large disc4 ,an application of this idea is shown in the speed control 7,fig4 #fh71 designed for the ,american ,type ,k camera when operated on an electric motor or on a simple propeller4 ,the same idea is utilized in the ,duchatellier film camera1 in connection with the constant speed propeller already described4 ,illustration3 ,,fig4 #fh4--;,friction disc speed control4 ,on the whole it is eminently desirable from the standpoint of power operation that the automatic camera should embody its own means for altering the interval between exposures1 so that all the external attachment needed is a single connection to a source of power either of constant speed1 as an electric motor1 or of speed proportional to that of the plane1 as with a simple wind propeller4 ,this makes the camera largely independent of the nature of the power supply1 whereas a camera designed for a special variable speed device is of little use on a plane where this is not available4 ,transmission of ,power to the ,camera--,it has already been pointed out that the ease of transmission of electrical energy makes it particularly convenient for use in a plane4 ,all other sources of power1 except clock-work incorporated in the camera1 require flexible shafting1 so that the question of bearings and connections becomes a serious one1 especially when the shaft runs continuously for long periods at very high speeds4 ,the shafting found most suitable is the spirally wound form commonly known as dental shafting4 ,this must be encased in a smoothly fitting sheath1 flexible enough to permit of easy bends4 ,the ends of the shaft should be equipped with square or rectangular pins to fit into corresponding slots in the motor and camera shafts4 ,the ends of the shaft casing may be fitted either to attach by bayonet joints or by smoothly fitting screw collars4 ,at the point of attachment to the camera it is desirable to have some form of junction adjustable as to the direction from which the shaft may be connected1 so that it need be bent as little as possible4 ,a right angle bevel gear offers one means of doing this4 ,bearings1 such as those of the propeller1 should be of the ball variety1 while heavy lubrication1 such as vaseline1 should be freely used1 both in the bearings and in the shaft casing4 ,an important feature of any power drive system should be a safety device1 so that the power will race in case of any jam or stoppage in the camera4 ,this will often prevent serious damage through the breakage of some relatively weak part of the camera mechanism on which the whole force of the driving apparatus is suddenly thrown4 ,the 8,l0 camera propeller is fitted with a spring friction clutch with the idea that if the camera refuses to operate the propeller will slip instead of wrenching the shaft to pieces4 ,,chapter ,,xiii ,,camera ,,auxiliaries ,distance ,controls and ,indicators4--,all operations connected with the exposing and changing of plates 7except the changing of whole magazines7 should be arranged for accomplishment at a distance4 ,other operations1 such as changing the shutter speed or the interval between exposures in an automatic camera1 which are usually done on the ground1 may sometimes be satisfactorily left for performance at the camera4 ,conditions of extreme inaccessibility may1 however1 make it necessary to carry even these controls to a distance4 ,indicators of the number of exposures already made1 and of the readiness of the camera for the next exposure1 may be attached to the camera1 but often are more profitably placed at a distance4 ,distance control and indication are especially necessary if the pilot makes the exposures--a common ,english practice in two seaters1 and the only recourse in single seaters4 ,when electric power is available1 electrical distance control devices are perhaps the simplest kind1 as they transmit motive power without displacing or jarring the camera4 ,solenoids suffice for the simple pressing of releases or for counting mechanisms1 while small service motors may be utilized for operations involving more work4 ,a standing practical objection to electrical control lies in the necessity for using contacts1 which are apt to be uncertain under conditions that involve vibration4 ,the ,bowden wire--a wire cable carried inside a heavy non-extensible but flexible sheath--constitutes the most satisfactory mechanical means for transmitting straight pulls4 ,by means of 8the ,bowden0 a pull may be transmitted so as to be made entirely relative to two parts of the same body1 calling forth no tendency of the body as a whole to move4 ,thus in the ,l camera shutter release 7,fig4 #ej71 the releasing lever with its attached counter is several feet distant from the camera4 ,if the plate bearing the lever and sheath end is rigidly fastened down1 the pressure exerted on moving the lever acts between the lever and the end of the sheath4 ,this pressure passes immediately to the other end of the sheath1 while the pull on the wire is transmitted to its farther end on the camera4 ,in this way the conditions at the lever are reproduced1 but with the advantage that1 due to the flexible cable and sheath1 any vibration of the lever support is damped out4 ,due to its stretching1 there is a pretty definite limitation to the feasible length of the ,bowden wire4 ,this length is about four feet4 ,where according to ,english practice the pilot makes the exposure1 a considerably longer wire and sheath are called for4 ,in this case the effective length of the release is increased by giving the pilot a second releasing lever1 connected to the first by a rigid rod 7,fig4 #fi74 ,the releasing lever1 wire1 and all mechanical parts of the ,bowden release should be made much stronger than would be indicated by bench tests of the camera4 ,in the air it is impossible to decide either by sound or by delicacy of touch whether the mechanism has acted1 so that the observer is apt to pull much harder than necessary and to strain or break the release if it is weak4 ,the ,bowden wire is used in the ,american service only for shutter release4 ,in the ,english service it has been used for plate changing with the ,l camera4 ,illustration3 ,,fig4 #fi4--;,bowden wire release in rear cockpit1 with rod connected to similar release for the pilot4 ,illustration3 ,,fig4 #gj4--;,bowden wire release with stop watch attached1 for use in timing for overlaps4 ,sights4--,in airplane photography the need for a finder or sight is fully as great as in everyday work4 ,a new condition1 however1 prevails1 for except with hand-held cameras1 and even to some extent with them1 the operation of pointing the camera involves pointing the whole vehicle that carries the camera4 ,the pointing of airplane cameras is therefore akin to the sighting of great guns4 ,while the observer may perform the actual operation of taking the picture1 the responsibility for covering the objective rests with the pilot4 ,teamwork counts equally with tools4 ,airplane camera sights may accordingly be divided into two classes3 sights attached to the camera1 for use principally with hand-held apparatus1 and sights attached to the plane1 for the use of pilot1 of observer1 or of both4 ,sights for ,hand-held ,cameras4--,the simplest form of sight attached directly to the camera is modeled on the gun sight1 consisting of a forward point or bead and a rear ,v4 ,this sight of course serves merely to place the objective in the center of the plate and gives no indication of the size of field covered4 ,another simple sight of rather better type is the tube sight--a metal tube of approximately one inch diameter and three inches length1 carrying at each end pairs of wires crossed at right angles4 ,the camera is in alignment when the front and back cross wires both exactly match on the object to be photographed4 ,the best way to mount the cross-wires is with one pair turned through #de degrees with respect to the other1 so that it is at once apparent which is the front and which the rear pair 7,figs4 #ca and #ci74 ,sights to indicate the size of the field are usually less needed on hand cameras than on fixed vertical cameras4 ,yet certain circumstances make them most desirable1 for instance in naval work where a complete convoy must be included on the plate4 ,a sight of this kind can be made up of two wire or stamped metal rectangles1 a large one in front and a smaller one behind1 of such relative sizes and separations that the true camera field is outlined when the eye is placed in position to see the two rectangles just cover each other4 ,the dimensions should be so chosen that the correct position of the eye is approximately its natural location with respect to the camera when this is held in the hands in the plane4 ,it is usual to provide the rectangular sights with cross-wires to indicate the center of the field4 ,alternative rear sights are simple beads or peep-holes--the use of the bead assuming that the camera is held at about the right distance from the eye for the rectangle to indicate the field4 ,the peep-sight is not a desirable form1 as it is hard to hold the camera as near the face as is necessary4 ,these various types of rectangle sights are well illustrated in the cameras shown in ,figs4 #ch1 #dj and #ahf4 ,they are all made so as to fold down flat on the camera and to snap quickly open when needed4 ,the springs to support the sights must be fairly strong1 and the surface presented to the wind as small as possible4 ,wire frames give very little from the pressure of the wind1 but flat metal frames are apt to be bent back4 ,the position of the sight on the camera is important4 ,if the observer can stand1 or if he sits up well above the edge of the cockpit1 the conventional position of the sight on a pistol1 namely1 on top1 is unobjectionable4 ,but if the observer sits very low1 as he usually does1 then the sight should be on the bottom of the camera1 thereby avoiding any need for the observer to raise his head unduly into the slip stream4 ,similarly1 if the camera is used over the side for verticals1 as it is in flying boats1 a sight on the top is impractical1 since it requires the observer to lean out dangerously far 7,fig4 #ahe74 ,sights ,attached to the ,plane4--,any of the sights just described can be attached to cameras fixed in the plane1 but they would be useless in the positions ordinarily occupied by the camera4 ,it has therefore become common practice to attach the camera sight to some accessible part of the plane4 ,the most primitive method of sighting is merely to look downward over the side--a method in general use to the very end of the ,great ,war4 ,one step in advance of this is to mark a large inverted ,v on the side1 with its vertex at a point where the observer can place his eye and so see the fore and aft extension of the field of view covered by the camera4 ,this kind of sight was common on the ,french 8photo0 planes4 ,on some of the ,english planes the tube sight was carried on the outside of the cockpit4 ,any of the sights described can be carried on the inside of the fuselage1 provided a hole is cut in the floor4 ,for satisfactory sighting a hole in the floor is really necessary1 as it enables the terrain on both sides of the vertical to be seen4 ,one drawback to the simple hole1 however1 is that it cannot be made large enough to show the whole field from the ordinary height of the observer's eye1 thus forcing him to bring his head down near the floor4 ,this difficulty is gotten over in a very beautiful way by the use of the negative lens sight shown diagrammatically in ,fig4 #ga4 ,illustration3 ,,fig4 #ga4--;,diagram of negative lens sight4 ,let ,f#a be the distance at which the edge of the hole 7or a rectangle marked on the lens7 appears the size of the camera field 7if the hole is the size of the plate1 ,f#a is the focal length of the camera lens74 ,let ,f#b be the distance from the floor to the observer's eye4 ,what is desired is a concave lens which will diverge the rays from their normal meeting point at ,f#a to a new meeting point1 ,f#b4 ,the focal length of lens required is given at once by the simple lens formula-- #a #a #a -------------- - -------------- ------ ,f#a ,f#b ,f ,thus if ,f#a is #ab inches1 and ,f#b is #cf inches1 ,f will be #ah inches4 ,the lens is to be marked with a rectangle showing the shape and size of the camera field1 and a central mark such as a cross4 ,an upper rectangle1 or a bead1 or a pair of cross wires a few inches below the lens1 may be used for the other sight4 ,for precision work the sight above or below the lens should be adjustable in position1 especially where the camera suspension permits the camera to be adjusted for the angle of incidence of the plane4 ,a negative lens sight should be placed in the observer's cockpit1 if he takes the pictures1 and also in the forward cockpit1 so that the pilot may be accurately guided in his part of the task4 ,in addition1 it is advisable to place a negative lens well forward in the pilot's cockpit1 to enable him to see the country some distance ahead4 ,the lenses should be planoconcave with the flat side upward2 otherwise1 all the loose dirt in the airplane settles in the middle of the concave depression4 ,a negative lens sight in a metal frame forming a completely self-contained unit ready for mounting in the plane is shown in ,figs4 #gb and #gc4 ,devices for ,recording ,data on ,plates4--,numbering devices4 ,the number of the camera is impressed on negatives taken with the ,american ,l camera through the agency of a small transparent corner of celluloid4 ,it would be entirely possible to incorporate a rotating disc which should turn by the operation of plate changing and carry a series of numbers1 so that each exposure could be numbered serially4 ,numbering of individual plates is more commonly done by holes1 notches1 or even numerals1 in the turned over portion of the sheaths1 which are then recorded photographically when a picture is taken 7,fig4 #ge74 ,the chief objection to this method is the difficulty of keeping the sheaths together in sets1 especially as individual ones become damaged or lost4 ,in practice there is also danger of the sheaths being carelessly loaded in wrong order4 ,illustration3 ,,fig4 #gb4--,negative lens and mount1 viewed from above4 ,the more ambitious idea of recording on the plate all the information given by the instrument board of the plane occurs independently and spontaneously to all aerial photographic map makers4 ,these ideas vary from attempts to photograph the actual instrument board on every plate--a difficult task indeed with the instruments and camera placed as they are in the ordinary plane--to the incorporation of compass1 altimeter1 and inclinometer in the camera itself4 ,figure #eh shows the plan adopted in the ,english ,f type film mapping camera already described1 for photographing a compass and an altimeter on the film4 ,here the combined compass and altimeter dial is above the camera1 and is mounted in a cell with a glass bottom4 ,below it is a lens focussing the needles and compass points on the plane of the film4 ,the light for photography is furnished by a diffusely reflecting white surface on top of the camera1 illuminated by the sky4 7,the camera was carried outboard47 ,in ,fig4 #ef is shown a picture with the compass image impressed upon it4 ,illustration3 ,,fig4 #gc4--,negative lens and mount1 side view4 ,figure #gd shows a type of inclination indicator found in some captured ,german cameras4 ,it consists essentially of two small pendulums or plumb-bobs2 one to indicate lateral1 the other longitudinal inclination1 arranged to be photographed in silhouette on the plate1 as shown in the lower part of the diagram and in the print from a captured negative 7,fig4 #ge74 ,illustration3 ,,fig4 #gd4--;,diagram of inclinometer used in some ,german cameras4 ,illustration3 ,,fig4 #ge4--,photograph made with ,german camera1 showing inclinometer record1 four points for locating diameters and center of plate1 and 7upper right-hand corner7 number of the plate sheath4 ,both these devices suffer from the deficiencies of the instruments they photograph4 ,the compass and the inclinometer1 as already mentioned in the discussion of airplane instruments1 only behave normally in straight-away flying1 failing to indicate correctly when the plane is subject to accelerations in any direction4 ,in general all attempts to record directional data in the camera are of little promise1 unless either the instruments or the camera are automatically held level by some gyroscopic device4 ,if the instruments are so controlled1 rather elaborate means for photographing them are necessary4 ,if the camera is stabilized1 the inclinometers are unnecessary1 and the compass behaves rationally4 ,another scheme for indicating inclinations1 which is not subject to the above objections1 is to photograph the horizon either on a separate film or on the same sensitive surface1 simultaneously with the principal exposure4 ,the difficulty here is the practical one that it is only feasible in localities of great atmospheric clearness4 ,ordinarily1 especially anywhere near the sea-coast1 the horizon is too rarely seen to be a reliable mark 7,fig4 #d74 ,it is possible1 however1 that this objection could be overcome by the use of specially red sensitive plates and suitable color filters1 as discussed in the chapter on 8,filters40 ,the method would in any case be useless in mountainous country4 ,the difficulties discussed with reference to direction indicating instruments of course do not hold with the altimeter4 ,ordinarily1 though1 the altitude changes slowly enough to permit of sufficiently accurate records being made by pencil and pad4 ,for high precision map making a photographic record of altimeter readings has a legitimate claim4 ,as we have seen1 a small altimeter is incorporated in the ,english ,f camera1 but the bulk which a really precision altimeter would assume would be a bar to its use in this way4 ,a time or serial number record on the plate or film1 synchronized with a similar record on the film of an auxiliary camera which photographs the altimeter and other instruments1 may be the simplest way to preserve the majority of the desired data4 ,devices for ,heating the ,camera4--,parts of the camera mechanism which depend on the uniformity of action of springs or upon adequate lubrication are susceptible to change with variation of temperature4 ,at high altitudes low temperatures are met which may freeze ordinary machine oils or may cause springs to seriously alter their tension1 even to break4 ,to meet this difficulty1 and probably also to dispel the occasional condensation of moisture on the optical parts1 the ,german cameras are equipped with an electrical heating coil placed just below the shutter1 and arranged to connect with the general heating and lighting current of the plane4 ,two contacts are ordinarily provided1 for offsetting the effects of temperatures of -#ae and -#cj degrees centigrade4 ,an additional function of this heating coil is perhaps to maintain the sensitiveness of the plates or film4 ,,iii ,,the ,,suspension ,,and ,,installation ,,of ,,airplane ,,cameras ,,chapter ,,xiv ,,theory ,,and ,,experimental ,,study ,,of ,,methods ,,of ,,camera ,,suspension ,general ,theory4--,in addition to the limitation of exposure set by the ground speed of the plane another limitation is set by the vibration of the camera4 ,this may be caused either by the motor1 or by the elastic reactions of the plane members to the strains of flight4 ,unlike the movement of the image due to the simple motion of the plane1 movements due to vibration may be eliminated by proper anti-vibrational mounting of the camera4 ,the effect of vibration may show as an indistinctness of the whole image--this is its only effect with a between-the-lens shutter--or as a band or bands of indistinctness parallel to the curtain opening 7,fig4 #gf74 ,these are due to shocks or short period vibrations during the passage of the focal-plane shutter4 ,the obvious remedy for vibration troubles is to mount the camera on some elastic1 heavily damping support1 like sponge rubber or metal springs4 ,such a mounting should1 however1 be designed on sound principles derived from a proper analysis of the nature and effect of the possible motions of the camera4 ,otherwise1 the vibrational disturbances may be increased rather than diminished by the camera mount4 ,such an analysis1 based merely on general mechanical principles1 shows that all motions of the camera are resolvable into six4 ,these are three translational motions1 namely1 two at right angles in one plane such as the horizontal1 and one in the plane at right angles to this 7vertical72 and three rotational motions1 one about each of the above directions of translational motion as an axis 7,fig4 #gg74 ,illustration3 ,,fig4 #gf4--;,captured ,german photograph1 showing zones of poor definition due to vibration during passage of focal plane shutter aperture4 ,brief consideration will show that only the latter--the rotational motions--are of any importance when the small displacements due to vibration are in question4 ,to illustrate the negligible effect of vibrations which merely move the camera parallel to itself in any direction it is only necessary to imagine the camera moved parallel to the ground through a large distance1 such as #aj centimeters4 ,now #aj centimeters motion of the camera at #cjjj meters elevation means1 with a #be centimeter camera lens1 4#be #a -------- #aj -------- centimeter #cjjj #abjj motion on the plate1 which would be only a tenth the distance separable by a good lens4 ,if we reduce this motion to the small fraction of a centimeter which vibration would actually produce1 it is evident that such vibration is of absolutely no importance4 ,similarly1 if we imagine the camera1 under the same conditions1 moved vertically with reference to the ground by ten centimeters1 the scale of the picture would merely be changed by #a_/abjjj or by #a_/ajjj centimeter on a #ab centimeter plate1 again quite negligible4 ,when we consider motions of rotation1 however1 the case is quite different4 ,if the camera is mounted so that the effect of any vibration is to rotate it around a horizontal axis1 this is exactly equivalent to rotating the beam of light from the lens so that it sweeps across the plate4 ,thus a millimeter displacement of the lens of the camera with the plate remaining fixed gives approximately a millimeter motion of the image4 ,consequently1 a rotation producing only a fraction of a millimeter's relative motion of lens and plate during the period the curtain aperture is over a given point would cause fatal blurring--and the visible vibration of plane longerons and cross members is easily of half millimeter amplitude or more4 ,reduced to angular units it is easily shown that a rotation of one degree per second--which is quite slow as plane oscillations go--is beyond the limits of toleration4 ,translational motions of large amplitude may be allowed1 but the mounting of the camera must not permit these translations to be at all different for different parts of the camera4 ,illustration3 ,,fig4 #gg4--;,diagram showing possible motions of the airplane camera3 three of translation and three of rotation1 and their combinations4 ,the proper way to eliminate vibrational effects is to devise a mounting that will transmit only the translational shocks or that will transform the rotational ones into translations4 ,platforms pivoted and cross-linked so as to be free to move only parallel to themselves 7described in the next chapter7 represent one attempt to reach this result4 ,quite the simplest and most scientific form of mounting to achieve this end is to support the camera solely in the plane of the center of gravity4 ,the principle here involved is easily grasped if we note that when we jar a camera supported above or below its center of gravity1 the effect is to start the camera vibrating with the center of gravity oscillating pendulum-like about the point of support4 ,the closer the center of gravity to the center of support1 the smaller the moment of the body about the latter point4 ,experimental ,study of ,methods of ,camera ,support4--,conclusive evidence as to the merits of any system of camera mounting can be obtained only under conditions that eliminate the effect of other variables which may be equally efficacious in diminishing the effects of vibration1 but which have only limited application4 ,very brief exposures--#a_/ejj second and less--will1 for instance1 result in good pictures with almost any condition of vibration4 ,hence a sharp picture offers no proof of the merits of a camera mounting unless it is known that the exposure was no shorter than the limit set by the ground speed of the plane4 ,in fact it may be said that the chief object of studying methods of camera suspension is to increase the allowable exposure to a maximum1 thus lengthening the working hours and multiplying the useful working days for aerial photography4 ,the most satisfactory method of test yet developed is to fly over a light or a group of lights on the ground with the camera shutter open4 ,in the first use of this method1 which originated in the ,english ,service1 such flights were made at night1 but later it was found that good results could be got by placing the lights in a forest and making the tests when the sun was fairly low4 ,one of the group of lights must be periodically interrupted1 at a known rate1 to furnish the time intervals4 ,illustration3 ,,fig4 #gh4--,tests of camera mounting1 made by flying over a bright light against a dark background4 7a7 ,rigid fastening on side of plane2 7b7 held in the hand1 inexperienced observer2 7c7 held in the hand1 experienced observer2 7d7 camera mounted at center of gravity on gimbals bedded in sponge rubber4 ,some characteristic 8trails0 obtained by this method of test are shown in ,fig4 #gh4 ,the first trail is that given by a camera rigidly fastened to the fuselage4 ,the second and third show hand camera trails1 made by an inexperienced and by an experienced observer1 respectively4 ,they show by comparison with the other figures that the human body is an excellent block to vibration1 but in unskilled hands a poor check to rapid erratic 7probably rotational7 motions of the camera4 ,the fourth is the trail given by a camera supported by gimbals bedded in sponge rubber accurately in the plane of the camera's center of gravity4 ,other trails are shown in the next chapter in connection with the description of practical camera mountings4 ,clearly the best suspension is that giving the smallest amplitude of displacement during the interval of time covered by an average exposure4 ,it is1 in fact1 possible to determine from these trails the permissible exposure for any assumed permissible blurring of the image4 ,the rigid mounting trail indicates very bad conditions1 calling for literally instantaneous exposures4 ,the center of gravity trail1 at the other extreme1 shows practically no limitation of exposure in so far as vibration is concerned1 thus bearing out the theoretical conditions above discussed4 ,an interesting conclusion from these experiments is that a rapidly running motor gives less harmful vibration than a slow one1 although in the war it was a common practice to throttle the motor before exposing4 ,as might be expected1 the greater the number of cylinders1 the shorter the period and the smaller the amplitude of the vibration4 ,pendular ,camera ,supports4--,the design of the camera support may be approached from a different standpoint1 namely1 with the aim of carrying the camera so that it will tend to hang always vertical4 ,in mapping this is of fundamental importance4 ,it is1 indeed1 a question whether aerial mapping will ever be worthy of ranking as a precision method unless the camera can be mounted so that its pictures are taken in the horizontal1 undistorted position4 ,the simplest way to hold the camera vertical is to mount it on gimbals1 with its center of gravity below the point of support4 ,when so mounted the camera swings as a pendulum4 ,delicacy of response to variation of level is obtained by leaving a considerable distance between the center of gravity and the center of support4 ,oscillation about the vertical position is to be prevented by some system of dash pots or other damping4 ,a suspension of this kind is furnished with the ,brock film camera 7,fig4 #fj74 ,it will be seen at once that the relation of center of gravity to center of support called for here is in direct contradiction to the requirements for eliminating vibration4 ,either one requirement or the other must be sacrificed1 or else a compromise made in which neither delicate response to inclination of the plane nor fully satisfactory freedom from vibration is attained4 ,this is a very serious objection to the pendular support4 ,but the really vital objection to the pendular support is that it performs its function only very partially4 ,it is entirely satisfactory only under conditions of steady flying1 as in a uniform climb or glide1 with the plane tail or nose heavy1 or in flying with one wing down4 ,as soon as we introduce any acceleration1 as in making a turn1 the camera follows the plane and not the earth4 ,it is true that mapping photography is done from a plane flying as level as possible1 and that except under bad air conditions it holds its course with very little turning1 if handled by a skilled pilot4 ,nevertheless1 a surprisingly small deviation from straight flying causes quite serious variations from the vertical4 ,it is of interest to calculate how large may be the horizontal accelerations that accompany swervings from a straight course which one might think insignificant4 ,for instance1 consider the horizontal acceleration due to a turn having a radius of a kilometer when the plane is moving at #ajj kilometers per hour4 ,if a is the acceleration1 v the velocity of the plane1 and r the radius1 we have from elementary dynamics that v#b a ---------- r ,substituting the values chosen1 we have-- #ajj1jjjb meters a -------------------------- 4#gg ------------ #cfjjb #ajjj sec#b ,the acceleration of gravity being #i4hj meters_/sec#b we have that the ratio of the horizontal acceleration to the vertical is 4#gg -------- 4#jgh #i4hj ,this is the tangent of the angle of deviation from the vertical1 from which the angle turns out to be about #d degrees1 a very considerable error1 rapidly multiplied as the speed of the plane is increased4 ,it is1 indeed1 open to question whether the average deviations from the vertical are not apt to be less with the camera rigidly fixed to the plane1 if guided by a skilled pilot who will hold the ship level at the expense of 8skidding0 the slight turns he must make to hold his direction4 ,gyroscopic ,mountings4--,the ideal support for the aerial camera will undoubtedly be one embodying gyroscopic control of the camera's direction4 ,by proper utilization of the principles of the gyroscope it is to be expected that not only can the camera be maintained vertical1 but it may be supported anti-vibrationally as well4 ,at the present time the problem of gyroscopic control is in the experimental stage1 so that only the elements of the problem and the possible modes of solution can be laid out4 ,the gyroscope consists essentially of a heavy ring or disc rotating at a high speed on an axis free to point in any direction 7,fig4 #gi74 ,if mounted so that the axes of the supporting gimbals pass through the center of gravity of the rotating disc1 the result is a neutral gyroscope4 ,its characteristic is that its axis maintains its direction fixed1 but this fixity is with respect to space and not with respect to the gravitational vertical4 ,consequently1 as the earth revolves the inclination of the gyroscopic axis changes with respect to the earth4 ,in latitude #de this change is approximately a degree in five minutes4 ,furthermore1 the action of friction in the supports1 which can never be entirely eliminated1 also acts to slowly alter the direction of the gyroscopic axis4 ,therefore1 unless some erector is applied even the gyroscope will not perform the task required of it4 ,illustration3 ,,fig4 #gi4--;,diagram of simple gyroscope4 ,before discussing possible forms of erectors it may be noted in general1 first1 that these must depend upon gravity2 second1 that such being the case1 they must respond to the resultant of gravity and any acceleration1 that is1 to the apparent or pseudo-gravity4 ,as already seen1 this pseudo-gravity1 during a turn1 is exactly what limits the usefulness of the pendular support1 and necessitates recourse to the gyroscope4 ,the problem thus becomes one of making an erector-gyroscope combination which will respond to true gravity and not to pseudo-gravity4 ,in general this problem would be insoluble1 since there is no difference in the nature of the acceleration of gravity and that due to centrifugal force4 ,a way out is offered1 however1 by the fact that true gravity acts continuously and at a small angle to the axis of the gyro1 while the components which cause the pseudo-gravity are of short duration1 liable to rapid changes of direction1 and1 on a turn1 act at a large angle4 ,what we require1 therefore1 is an erector which will respond slowly but surely to the average acceleration1 which is downward1 but too sluggishly to be affected by the shorter period accelerations due to turns or rolls4 ,slowness of response is a matter of the erecting forces being small and of the mass and angular velocity of the gyro disc being large4 ,the success of the compromise called for depends on the relative times taken for the gyroscope to tilt seriously from the true vertical1 due to the causes above mentioned1 and for the average turn or roll4 ,fortunately the former is a matter of minutes1 the latter of seconds or at the worst of fractions of a minute4 ,more than this1 since the roll or turn is apt to be of much greater angle than any normal deviation of the gyroscopic axis from the vertical in the same time1 we are offered the possibility of some device for filtering out the deviations which alone are to effect the erector4 ,for instance1 by shunting the restoring force whenever it is called upon to act through more than a predetermined small angle4 ,as to the method of erecting the gyroscope1 its characteristic property must be kept in mind4 ,this is that the axis does not tilt under an applied force in the direction it would if the gyro were not rotating1 but around an axis at right angles to that of the applied couple4 ,thus in ,fig4 #gi1 if a weight is attached as shown1 the disc does not incline downward toward the weight1 around the axis ,y1 ,y1 but precesses about the vertical axis ,z1 ,z4 ,some means is therefore needed to translate the pull which any gravitational control1 such as a freely swinging pendulum1 would give1 into a pull with at least a component at a finite angle to this4 ,in the ,gray stabilizer several metal balls are slowly rotated in a tray above the center of gravity of the gyroscope4 ,specially shaped grooves or compartments limit the freedom of motion of these balls so that when the gyro is inclined the balls travel at different distances from the center on the ascending and descending sides4 ,by this scheme a couple is produced about the axis through the center and the low point of the disc1 which tilts the apparatus to the gravitational vertical4 ,in an alternative form the balls are carried past the low point by their momentum and are prevented from returning by the walls of the containing compartment1 which have meanwhile been advanced by the rotation of the erector as a whole4 ,the net result is to shift the center of gravity of the system of balls in the proper direction to erect the gyro4 ,the rectifying action is purposely made quite slow so that the displacements of the balls due to pseudo-gravity will be averaged out4 ,in a design due to ,lucian1 small pendulums work through electric contacts to actuate solenoids which in turn move small weights in the appropriate directions to give the desired tilt4 ,response is made fairly quick and delicate1 and pseudo-gravity1 due to turns and rolls1 is rendered inoperative by the contacts breaking whenever the pendulums swing more than three or four degrees4 ,this can only happen if they move too quickly for the erecting forces to act1 reliance being here placed on the characteristic differences of action in respect to time of real and pseudo-gravitational forces4 ,besides the neutral gyroscope as just considered there is the pendular or top type1 in which the center of gravity is not in the plane of the supports4 ,in general this type depends on a couple resulting from the gravitational pull and the inevitable friction of the supports to slowly tilt the axis to the gravitational vertical4 ,this type is slower to respond than the designs in which a definite couple in the proper direction is provided and it reaches the true vertical only through a circuitous path4 ,illustration3 ,,fig4 #hj4--;,diagram of camera linked to gyroscopic stabilizer4 ,three methods of controlling a camera by a gyroscope are suggested4 ,one is to fasten the gyroscope rigidly to the camera and mount the whole system on gimbals4 ,a second is to mount both camera and gyro side by side on gimbals1 linking the two so that the camera is moved parallel to the gyro 7,fig4 #hj74 ,a third method is to utilize the gyro to make electric contacts to operate motors which in turn move the camera4 ,considerable weight and space are required for a gyroscope capable of stabilizing a camera4 ,the rotating disc should be about half the weight of the camera1 and with its mounting may be expected to double the room required for the camera alone4 ,motive power for maintaining the gyro in continuous rotation may be supplied by an air blast1 or the gyro may be made up as an induction motor--the latter necessitating an alternating current supply4 ,in view of the space and weight limitations in a plane it is a question still to be decided whether it is more economical to stabilize the camera or to stabilize an inclinometer and photograph its indications simultaneously with the release of the shutter which takes the aerial picture4 ,,chapter ,,xv ,,practical ,,camera ,,mountings ,general ,considerations4--,camera mountings as used during the war were far from being developed on the basis of scientific study or test4 ,at first the need for special supporting apparatus was not realized1 and the suspensions later in use were largely field-made affairs1 often dependent on adjustments made according to individual taste4 ,through lack of accurate methods of test and of conclusive evidence on the subject1 it was quite common to find extremists who1 on the one hand1 denied the efficacy of suspensions in general1 and on the other ardently supported crazily conceived supporting arrangements which accurate comparative test show to be even worse than useless4 ,in the ,french service1 despite numerous types of suspension available1 the very general practice was to lift the camera from its support and hold it between the knees4 ,or else the hand was pressed on the top of the camera during exposure1 more reliance being placed on the damping qualities of the body than on any of the rubber or spring mechanisms4 ,as is clearly shown by the experimental data described in the last chapter1 a correctly designed supporting device1 carrying the camera accurately in the plane of its center of gravity1 accomplishes practically perfect elimination of vibrational troubles4 ,so important is the use of a mount and so important is it that the mount should be correctly dimensioned and adjusted for the camera1 that an entirely different attitude should be adopted from the prevalent one which focuses attention on the camera and regards the mounting as a mere auxiliary to be left more or less to chance4 ,the mounting should be considered an integral part of the camera4 ,the man in the field should receive camera and mount together1 leaving as his only problem the attachment of the complete camera--and--mount unit to the plane4 ,this may be arranged1 by proper designing1 to be a simple matter of rigid bolting or strapping1 requiring ingenuity perhaps but not the scientific knowledge which is required for mounting design4 ,illustration3 ,,fig4 #ha4--8,l0 camera mounted outside the fuselage4 ,observer using exposure plunger1 pilot using ,bowden wire release4 ,outboard ,mountings4--,in the ,english service the camera was first attached to the plane outside the fuselage by a rigid frame1 to which the camera was strapped or bolted 7,fig4 #ha74 ,obvious objections exist to placing the camera in this position1 such as the resistance of the wind and the difficulty of changing magazines4 ,however1 in the earlier ,english planes with their fuselages of small cross section no other accessible place for the camera was to be found4 ,vibrational disturbances with the rigid outboard mounting are quite serious1 as is so clearly indicated by the trace shown in ,fig4 #gh4 ,extremely short exposures are alone possible1 and a very large proportion of the pictures are apt to be indistinct4 ,floor ,mountings4--,a step in advance of the outboard mounting is to support the camera snout in a padded conical frame on the floor of the plane 7,fig4 #hb74 ,this mounting avoids the objection on the ground of wind resistance that holds with the outboard1 and has possibilities of being worked out as an entirely satisfactory support4 ,yet to be satisfactory1 the point of support must lie in the plane of the center of gravity of the camera1 and the camera must be of a type that preserves its center of gravity unchanged in position as the plates are exposed4 ,unless these conditions are fully met the floor mounting gives results little better than does the outboard4 ,cradles or ,trays4--,floor space in the cockpit being unavailable in the battle-plane1 due to duplicate controls1 bomb sights1 etc41 the ,english service was driven to the practice of carrying the camera in the compartment or bay behind the observer4 ,here it was attached either to the structural uprights or longerons1 or to special uprights and cross-pieces built into the plane to serve photographic ends4 ,as an intermediary between the camera and the supporting cross-pieces there was introduced the camera tray or cradle4 ,this is essentially a frame carrying sponge rubber pads into which the camera is more or less deeply bedded4 ,figs4 #hc and #hd show an ,american ,l camera cradle based on the design of the ,english ,l camera tray4 ,thick sponge rubber pads support the two ends of the camera top plate1 and additional pads are provided to hold the nose of the camera4 ,careful tests show this cradle to be superior to the outboard mounting1 but still leave much to be desired4 ,its performance is better with the nose of the camera left free4 ,illustration3 ,,fig4 #hb4--8,l0 camera in floor mounting4 ,illustration3 ,,fig4 #hc4--8,l0 camera and cradle mount in skeleton ,de,haviland #d fuselage1 side view4 ,tennis-ball ,mounting4--,a very simple mount used by the ,french consists of a frame enclosing the nose of the camera1 and carrying four tennis balls1 on which the whole weight rests 7,fig4 #dj74 ,if the center of support is in the plane of the center of gravity and if the four balls are of uniform age and elasticity1 this form of support is good4 ,as provided by the camera manufacturer1 the tennis ball frame fits much too far down on the camera4 ,another application of the tennis ball idea was frequently made in the ,french service1 in which the balls were close up under the shutter housing 7,fig4 #he74 ,additional support was1 however1 given to the camera nose by flexible rubber bands1 the success of the whole being largely a matter of the adjustment of the tension on the bands4 ,illustration3 ,,fig4 #hd4--8,l0 camera and cradle mount in skeleton ,de,haviland #d fuselage1 front view4 ,parallel ,motion ,devices4--,a form of suspension favored by the ,french consists of parallel bell cranks1 rigidly linked together and held up by springs4 ,mountings of this sort are illustrated in ,figs4 #hf1 #hg1 #hh and #if4 ,the guiding principle is that any sort of shock will be transformed into a straight up-and-down or side-wise motion of the camera1 which is harmless4 ,the mounting as adapted by the ,english surrounds the camera body1 making the plane of support somewhere near the center of gravity4 ,in certain of the ,french suspensions employing this principle the whole camera is hung below the bell cranks 7,fig4 #hf71 and then the nose is restrained by heavy rubber bands4 ,the net result is largely a matter of adjustment4 ,illustration3 ,,fig4 #he4--,tennis ball suspension1 assisted by elastic bands attached to nose of camera4 ,illustration3 ,,fig4 #hf4--;,french spring and bell crank suspension4 ,illustration3 ,,fig4 #hg4--,u4 ,s4 hand-operated #ah #bd centimeter plate camera on bell crank mount with rotating turret4 ,illustration3 ,,fig4 #hh4--,same camera in plate changing position4 ,illustration3 ,,fig4 #hi4--,tests of camera mountings3 7a7 de,ram camera on bell-crank-and-spring mount1 below the center of gravity2 7b7 same1 at center of gravity2 7c7 type 8,k0 film camera on universal mounting 7,fig4 #hh74 ,tests on the ,english design made in the ,united ,states ,air ,service appear to show that the chief virtue of the mounting lies in the approximation of the point of support to the center of gravity in the ,english cameras4 ,a de,ram camera supported by its cone1 so that its center of gravity was considerably above the center of support gave rather poor results 7,fig4 #hi;a71 but when the bell cranks were attached near the center of gravity1 highly successful results were obtained 7,fig4 #hi;b74 ,the ,french de,ram camera as ordered for the ,american ,expeditionary ,force was fitted with a bell crank supported in this position4 ,figures #ij and #ia show a bell crank mounting furnished with a rotating turret4 ,this was designed to facilitate the changing of magazines in the ,english ,b ,m camera1 which is swung around through #ij degrees from the exposing position to bring the magazine near the observer4 ,the camera shown in the mounting is the ,american hand-operated model 7type ,m71 in which there is the same necessity for turning in order to manipulate the bag magazine easily4 ,the camera is shown in both exposing and plate changing positions4 ,an important detail of these mounts is a safety catch1 which must be fastened before the plane lands1 in order to prevent the shocks of landing from producing oscillations sufficient to throw the camera out of the mount4 ,center of ,gravity ,rubber ,pad ,supports4--,given a camera whose center of gravity does not change during operation1 a simple and entirely adequate anti-vibration support is furnished by a ring of sponge rubber in the plane of the center of gravity4 ,but if provision has to be made for oblique views or for adjusting the camera to the vertical1 something more elaborate is necessary4 ,illustration3 ,,fig4 #ij4--,u4 ,s4 model de,ram camera on anti-vibration mounting adjustable for the angle of incidence of the plane4 ,illustration3 ,,fig4 #ia4--,u4 ,s4 de,ram camera and mount installed in photographic ,de,haviland #d 7,fig4 #ajj74 ,viewed from above the observer's cockpit4 ,illustration3 ,,fig4 #ib4--,u4 ,s4 type 8,k0 film camera on universal mounting1 vertical position4 ,illustration3 ,,fig4 #ic4--,u4 ,s4 type 8,k0 film camera on universal mounting1 oblique position4 ,mountings for the ,american de,ram and for the ,air ,service film camera1 embodying the results of complete study of the anti-vibration problem1 are shown in ,figs4 #ij1 #ib and #ic4 ,trusses carrying the cameras on pivots rest on four pads of sponge rubber which are mounted on frames correctly spaced ready for attachment to the cross-pieces of the airplane camera supports4 ,in the de,ram 7,fig4 #ij7 the pivots1 attached to the camera body1 permit it to be leveled fore and aft1 to compensate for the inclined position of the fuselage assumed at high altitudes or in some conditions of loading4 ,this will sometimes amount to as much as #aa or #ab degrees1 which is very serious1 since one degree causes 7with an angular field of #bj degrees7 about one per cent4 difference of scale at the two sides of the plate4 ,the film camera mounting carries the camera in a conical ring1 and is pivoted not only for vertical adjustment1 but for the taking of obliques as well 7,fig4 #ic74 ,these mounts transmit practically no vibration4 ,illustration3 ,,fig4 #id4--,tests on two types of camera mount3 7a7 ,support at bottom of camera2 7b7 support above center of gravity4 ,a caution must be noted with regard to center of gravity mountings4 ,any change in the camera1 in particular the substitution of a short for a long lens cone1 must be made so as to cause no alteration of the relative positions of the center of support and the center of gravity4 ,either the short cone must be weighted1 or additional supporting pivots must be provided in the plane of the new center of gravity4 ,the ,italian and ,g4 ,e4 ,m4 ,mountings4--,these mounts 7,figs4 #di and #ei7 are similar in that the protection from vibration is furnished by an elastic support at the bottom of the camera4 ,tests show that these two cameras give very similar results1 of the unsatisfactory sort to be expected from this kind of mounting in view of the lessons of the last chapter on the proper point of support4 ,fig4 #id1 a1 shows a trace given by the ,italian mount4 ,the permissible exposure1 on the criterion adopted1 is very short with either mount1 about #a_/bjj second4 ,the ,brock ,suspension4--,this consists of a pair of frames into which the camera is fitted by ball bearing pivots1 so that it is free to move in any direction 7,fig4 #fj74 ,in order to permit gravity to control the direction of the camera1 the point of support is made considerably 7ten inches7 above the center of gravity4 ,air dash pots are provided for damping the swings4 ,as already explained1 the pendular method of support is in basic contradiction to the requirements for vibration elimination4 ,tests of the ,brock suspension1 shown in ,fig4 #id1 b1 indicate it to be of low efficiency in damping out the short period vibrations which are responsible for poor definition4 ,,chapter ,,xvi ,,the ,,installation ,,of ,,cameras ,,and ,,mountings ,,in ,,planes ,conditions to ,be ,met4--,the characteristic difficulty in installing the airplane camera is that there is no place for it4 ,after the gasoline supply1 the armament1 the wireless1 the oxygen tank1 the bombs1 and other necessities are taken care of there is neither space available nor weight allowable4 ,where space may be found it will be inaccessible1 or accessible only through a maze of tension and control wires2 or it will be in a position where any weight will endanger the balance of the plane4 ,plane design has in fact been more or less of a conflict between the aeronautical engineer1 who is designing the airplane primarily as a machine to fly1 and the armament and instrument men1 who look upon it as a platform for their apparatus4 ,lack of appreciation of the extreme importance of aerial photography resulted1 during a large part of the war1 in the camera installation being neglected until the plane was supposedly entirely designed1 and even in production4 ,at that stage the installation could be but a makeshift4 ,only in the later stages of the war1 when plane design became a matter of coperation between all concerned1 were fairly convenient and satisfactory arrangements made for the camera4 ,always1 however1 the rapid succession of new plane designs1 with various shapes of fuselage and details of structure1 made camera installation in the war plane a matter calling for the greatest ingenuity4 ,the problem was met in part by constructing both cameras and mountings in sections1 to be laboriously wormed in through inadequate apertures1 in part by later structural changes in the planes1 such as the substitution of veneer rings or frames for the tension wires4 ,in certain cases the rear cockpit controls were omitted1 thereby freeing accessible and often adequate space for the larger cameras4 ,rear controls were never used in the ,german planes1 so that their standard practice was to carry the camera forward of the observer4 ,this1 together with the general restriction to the #ac #ah centimeter size plate1 made the installation problem less difficult in the ,german aircraft than in the ,allied4 ,practical ,solutions4--,an important feature of camera installation has already been mentioned1 but may well be repeated for emphasis4 ,the camera and its anti-vibration mounting should always be considered as a unit1 and should be so designed that simple bolts or straps will suffice to fasten it in its place in the plane4 ,even should the spacing of the structural parts of the plane not correspond to that anticipated by the mounting design1 the ingenuity of the man in the field may be depended upon to make the necessary alterations or additions to the plane4 ,the design of the camera suspension itself cannot1 however1 be left to uneducated ingenuity4 ,assuming the camera and mounting supplied1 the next step--a very difficult one--is to insure uniformity in the structures to be built into the planes for the purpose of supporting the camera mountings4 ,with this uniformity must1 however1 be combined the greatest possible flexibility to provide for various designs of cameras4 ,in the ,english service the standard camera installation consists of wooden uprights with cross bars athwart the plane1 adjustable as to height 7,fig4 #ie74 ,a distance between the cross bars of #ac inches has been standardized1 and all camera cradles and mountings are notched or otherwise spaced to fit this dimension4 ,the installation adopted in the ,american planes is similar1 but with a distance of #af inches between cross bars4 ,these uprights and cross bars are ordinarily situated in the bay behind the observer1 but can be placed in any available space4 ,fig4 #hc shows1 in a model bay1 the arrangement of uprights and cross bars in the ,american ,,dh #d1 with the ,l camera in place in its cradle4 ,it is just possible to introduce camera and cradle separately from the observer's cockpit through the tension wires1 and1 by uncomfortable reaching1 magazines may be changed4 ,illustration3 ,,fig4 #ie4--8,,lb0 camera with #bj-;inch lens1 mounted on bell-crank suspension in skeleton fuselage4 ,stream-lined hood below to cover projecting end of lens cylinder4 ,propeller and ,bowden release in place4 ,a step in advance is made when the top tension wires and superstructure are replaced by a rigid frame with an opening large enough to admit the entire camera and mounting4 ,when this is done considerably larger cameras may be accommodated in the same sized bay1 as shown in ,fig4 #if4 ,a further advance1 from the standpoint of accessibility and convenience of installation1 follows when the tension wires between observer's and camera bay are replaced by a ply-wood ring1 as shown in ,fig4 #ig4 ,here the only serious limitations are those due to the vertical height of the camera1 and of course its weight4 ,openings for the lens to point through are simply provided in the fabric covered aircraft1 by cutting through the canvas and stiffening the edge of the hole by wire4 ,tension wires are often in the way4 ,they may either be disregarded1 since they merely cut off a little light1 or replaced in part by metal rings1 as shown in ,fig4 #if4 ,in veneer covered fuselages the hole must of course go through the wood4 ,this may be undesirable1 since the veneer is depended on to furnish structural strength1 a point which further emphasizes the importance of the photographic requirements being thoroughly considered while the plane is being designed4 ,single seater or scout planes do not lend themselves to the insertion of such standardized uprights and cross-pieces1 because of their small size and the common utilization of all space inside the fuselage for gasoline tanks and control wires4 ,some ,french scouts1 whose fuselages are very wide1 due to the rotary engines1 have been fitted with compartments for contemplated automatic film cameras4 ,the most commonly used camera in the single seater was1 however1 the ,italian #bd-plate single-motion apparatus 7,fig4 #di74 ,this camera and its carrying tray occupy very little lateral space and have in actual practice been carried beneath the seat or pushed up through an opening in the bottom of the fuselage under the gasoline tank4 ,whatever criticism may be made of the adequacy of the mounting1 it must be said that the camera1 as used1 is perhaps the most eminently practical of all developed in the war1 as its use on scouts testifies4 ,illustration3 ,,fig4 #if4--,u4 ,s4 type 8,k0 film camera on bell-crank mount1 in camera bay of ,de,haviland #d4 ,veneer frame at top of bay in place of usual cross-wires4 ,illustration3 ,,fig4 #ig4--,section of fuselage of veneer construction affording superior accessibility to camera4 ,special ,photographic ,planes4--,as cameras grew in size1 the difficulty of installing them in planes built without regard to photographic requirements greatly increased4 ,few planes could carry even the #ej centimeter focus camera obliquely without the necessity of poking its nose through the side where it would catch wind and oil2 while the #abj centimeter camera could be carried obliquely only in the fore and aft position4 ,even vertical installation of the latter camera was really feasible in but few planes2 sometimes the camera was carried to the exclusion of the observer--and1 in fact1 this size was never used by the ,english1 whose fuselages were small in cross-section4 ,this situation led1 late in the war1 to steps toward producing planes designed primarily for photographic reconnaissance4 ,in these the camera would be entirely accessible1 and cameras of any size could be carried in any desired position4 ,one scheme which properly belongs under this heading was the provision of a special removable photographic cockpit1 for the front or nose of a twin-motored three seater4 ,other noses1 for bombing and heavy machine guns1 were also planned1 all to be interchangeable4 ,since the regular photographic bay with uprights and cross-pieces was also provided to the rear1 this special photographic ship could on occasion do two classes of work1 such as long focus spotting and short focus mapping4 ,the most completely worked out photographic plane was probably the model designated ,p#a by the ,united ,states ,air ,service4 ,this is a modified ,de,haviland #d in which the rear controls have been removed and the cowling raised and at the same time made squarer in cross-section4 ,the space formerly occupied by the rear controls provides ample room for all types of camera4 ,these are carried on uprights at the standard distance apart1 #af inches1 with cross-pieces adjustable as to height4 ,the camera space is accessible not only from the observer's cockpit1 but from above1 upon folding back the metal cover4 ,doors at the bottom and at each side permit not only vertical but oblique exposures4 ,the latter are not interfered with by the wings1 as they would be in some designs of plane if the camera occupied the same position relative to the cockpits4 ,fig4 #ia shows the de,ram camera in place1 as seen from the rear4 ,figs4 #ih and #ii show the #ah #bd centimeter film camera1 set both for vertical and oblique views4 ,negative lenses are provided for both pilot and observer1 the one for the pilot permitting him to see from a point far ahead to directly underneath1 while the observer's is furnished with cross wires below and etched rectangles of the camera field sizes on the upper surface4 ,windows of non-breakable glass assist in sighting obliques4 ,the accompanying picture 7,fig4 #ajj7 of the plane showing an oblique camera in position gives an excellent idea of its appearance4 ,its special features are worthy of copying in peace-time photographic aircraft4 ,installation of ,auxiliaries4--,it is quite necessary that the camera lens be protected from splashing mud and often from oil spray due to the motor4 ,for this purpose an easily opened and closed door is essential1 unless the camera is carried well up in the plane4 ,an alternative1 possessing certain advantages1 is to incorporate into the camera protecting flaps operating in front of the lens1 which open only when the exposure is made4 ,if the camera projects beyond the fuselage1 stream lined hoods 7,fig4 #ie7 must be provided to protect the camera nose with the minimum of air resistance4 ,illustration3 ,,fig4 #ih4--bj-;inch focus automatic film camera mounted obliquely in photographic ,,dh-#d4 ,illustration3 ,,fig4 #ii4--bj-;inch focus automatic film camera mounted vertically in photographic ,,dh-#d4 ,illustration3 ,,fig4 #ajj4--;,de,haviland #d re-constructed as a special photographic plane1 showing #bj-;inch camera mounted for oblique photography4 ,the mounting of the regular camera auxiliaries--releases1 sights1 propellers1 speed controls1 motors--is usually a great bother1 due to lack of space and to the severe restrictions on methods of fastening4 ,screws in longerons or uprights are taboo4 ,metal straps to go around structural parts are the approved device1 but with variations in the size of these members1 the holes1 straps1 bolts and nuts provided are very apt not to fit4 ,changes of construction1 such as that from skeletons covered with fabric to veneer bodies1 also interfere with any standard means of attachment1 and leave this1 like many other problems in war-time aerial photography1 to the resourcefulness of the man in the field4 ,magazine racks must be tucked away in any available space4 ,under the seat is a position frequently utilized4 ,especially with plates is it desirable to carry the extra magazines in a position to interfere as little as possible with the balance of the plane4 ,in the ,,dh #d this means that they should be carried if possible forward of the observer1 even though he must turn completely around to get and insert each magazine4 ,,iv ,,sensitized ,,materials ,,and ,,chemicals ,,chapter ,,xvii ,,the ,,distribution ,,of ,,light1 ,,shade ,,and ,,color ,,in ,,the ,,aerial ,,view ,the general appearance of the earth as viewed from above has already been described and illustrated 7,figs4 #aj and #aa74 ,it remains to deal with the earth's appearance in a more analytic and quantitative manner1 in order to decide upon the characteristics to be sought in our photographic sensitive materials4 ,range of ,brightness4--,the absence of great contrasts so apparent in the view of the earth from a plane is confirmed by photometric observations4 ,these show that the average landscape1 as seen from the air1 rarely presents a range of brightness of more than seven to one1 even when seen without the presence of veiling haze4 ,it is to be remembered that shadows constitute no important part of the aerial landscape4 ,vertical walls in shadow1 which form a substantial part of the surfaces seen by an observer on the ground1 are invisible or greatly foreshortened from the air4 ,moreover1 they are never contrasted against the sky1 which is photographically often the brightest part of the ordinary picture4 ,to the aviator's eye shadows on the ground are only of any length at early and late daylight hours4 ,even at these times they cover but a small area1 since the number of high vertically projecting objects in a representative landscape is small4 ,lacking shadows1 the brightness range is only that between various kinds of earth1 water1 and vegetation4 ,chalk 7from freshly dug trenches71 reflected sunlight from water1 or marble buildings1 furnish almost the only extensions to the brightness scale as above given4 ,diurnal and seasonal changes4 ,during the winter months on the ,western ,front photography from the air was only possible for two or three hours around noon1 on clear days4 ,this calls attention to another factor of prime importance1 namely1 the large variation in the intensity of daylight during the course of the day and during the course of the year4 ,illustration3 ,,fig4 #aja4--,variation of average daylight intensity during the day4 ,measurements showing typical variations from morning to night are exhibited in ,fig4 #aja1 from which it appears that there is an increase in illumination of four to five times from #h o'clock--when it would be considered full daylight for purely visual observation--until noon1 while there is a corresponding decrease by four o'clock4 ,fig4 #ajb shows sets of measurements by two different authorities which give the average intensity of daylight for each month throughout the year4 ,from ,december to ,july there is an increase of approximately ten times4 ,from both sets of data it therefore appears that--neglecting the frequent occurrence of clouds which reduce the illumination to a half or a quarter or even less--a variation in illumination of forty or fifty times occurs between mid-day in summer and morning in winter4 ,in the photography of stationary objects on the ground this range of intensities is easily taken care of by selection of lens stop and shutter speed4 ,on the airplane it is quite otherwise1 because the shutter speeds called for at the lower illuminations are much slower than the motion of the plane will allow4 ,illustration3 ,,fig4 #ajb4--,variation of intensity of daylight through the year2 two different sets of measurements4 ,haze4--,at low altitudes the brightness range is substantially that which would be obtained by photometric measurements of soil and vegetation made at the earth's surface4 ,at higher altitudes1 especially above #bjjj meters1 this brightness range is materially decreased by atmospheric haze4 ,the significance of this lies in the fact that for safety from anti-aircraft guns1 war-time aerial photography must be carried out at very great elevations4 ,toward the end of the ,great ,war photographic missions traveling at from #ejjj to #gjjj meters were the rule4 ,at these heights1 even in very clear weather1 a veil of bluish-white haze reduces the already small contrasts still more4 ,some means for overcoming the effect of this haze becomes imperative1 therefore1 in order to secure in the picture even the normal contrast of the object4 ,haze is to be sharply distinguished from clouds or fog4 ,clouds and fog consist of globules of water vapor of large size1 opaque to light4 ,haze1 on the contrary1 is more opaque to some colors than to others1 or is selective in its veiling effect4 ,its scattering action on light is greatest in the violet and blue of the spectrum1 decreasing rapidly through the green1 yellow1 and red1 the exact relation being that the scattering is inversely as the fourth power of the wave-length4 ,it is1 consequently1 possible to pierce or cut haze by using yellow1 orange1 or red color screens4 ,it is this possibility which has led to the extensive use of yellow or orange goggles for shooting and for naval lookout work4 ,in aerial photography the equivalent is to be found in color filters1 used with color sensitive 7orthochromatic or panchromatic7 plates1 which have been found essential for all high altitude work4 ,color4--,visual observation from the airplane is aided in no inconsiderable degree by the differences of color that exist between various objects of nearly the same brightness4 ,this means of distinguishing differences of character fails in the photographic plate1 which is color-blind2 that is1 it reproduces all objects as grays of varying brightness4 ,it is color-blind in another sense as well1 in that it evaluates colors as to brightness differently from the way the eye does1 overrating blues and violets and underrating yellows and reds4 ,this first kind of color-blindness is a positive disadvantage1 for it leaves available for differentiating objects only their brightness differences4 ,the second kind of color-blindness may on occasion actually be an advantage4 ,for it may happen by accident1 or by design 7through the skilful use of color filters71 that objects appearing nearly the same to the eye appear different in the plate4 ,more will be said about this in connection with the use of filters for the detection of camouflage4 ,the range of hues seen in the aerial landscape is not large4 ,greens 7grass and foliage7 predominate1 followed by browns 7earth71 neither color being bright or saturated4 ,over towns or cities we find that grays 7roads7 and redder browns 7brick7 are conspicuous4 ,blues are practically never seen1 although it is to be noted that a fair share of the illumination of the ground is by blue sky light and that the haze itself is bluish4 ,consequently1 the general tone of a landscape is much bluer than one would be apt to imagine it from consideration of the general green and brown character of the constituent objects4 ,a color photograph from the air would greatly resemble a pastel in its low range of tones and the absence of bright colors4 ,the ,photographic ,requirements ,dictated by ,brightness and ,color ,considerations4--,considering only the demands made by the character of the view presented to the airplane camera1 and leaving out of account other limitations to photographic operations in the plane1 certain requirements as to sensitized materials may be outlined4 ,first of all1 the photographic process must not reduce1 but should rather be capable of exaggerating1 the range of brightness of the object4 ,preferably the seven-to-one range of the object photographed should be lengthened out to the full range of the printing paper1 which may be two to three times this4 ,with such an increase of range1 those minute differences of brightness are accentuated1 on which the detection of many objects depends4 ,next1 the plate or film must be sensitive to the portion of the spectrum transmitted by a yellow or orange filter which will cut out the effect of haze4 ,this calls for orthochromatic or panchromatic plates1 depending on the depth of filter required4 ,next1 if the objects to be photographed differ little in brightness but are different in color composition1 we may have to rely on color filters of peculiar transmissions1 capable of translating these color differences into brightness differences4 ,these will1 in general1 call for fully color sensitive1 or panchromatic plates4 ,in conclusion it may be pointed out that the endeavor in ordinary orthochromatic photography--to reproduce the visual brightness of colors in the photographic print--has no real justification in aerial work4 ,neither in respect to color values nor in respect to brightness range is it the object of aerial photography1 especially for war purposes1 to present a truthful tone reproduction4 ,its aim is rather the adequate differentiation of detail1 by whatever means necessary4 ,,chapter ,,xviii ,,characteristics ,,of ,,photographic ,,emulsions ,the purely photographic problem in aerial photography1 as distinct from the instrumental one1 is the selection of photo sensitive materials which will yield useful results under the conditions peculiar to exposure from the air4 ,after such materials have been found by extensive field tests1 it is preeminently desirable to determine their characteristics in such terms that the kind of plate or film may thereafter be specified and selected on the basis of purely laboratory tests4 ,specification must be made in terms of the ordinary sensitometric constants of the photographic emulsion--its speed1 contrast1 fog1 development factor1 its color sensitiveness1 its ability to render fine detail1 and its grosser physical properties such as hardness and shrinkage4 ,sensitometry4--,the most generally used system of sensitometry is that of ,hurter and ,driffield1 commonly referred to as the 8,h ,d40 ,by this system1 in order to determine the characteristics of a given photographic plate1 it is necessary to take a series of graduated exposures1 a standard illumination of the plate being varied in known amount by a rapidly rotating disc cut to a series of different openings1 or by some other suitable means4 ,the negative thus obtained is developed in a standard developer for a definite time1 at a fixed temperature1 and is then measured for transmission on a photometer4 ,the following terms are defined and used in plotting the results3 intensity of light transmitted ,i ,transparency ,t ---------------------------------------- -------------------- -------------- intensity of incident light ,i,o intensity of incident light ,i,o #a ,opacity ,o ---------------------------------------- -------------------- -------------- ------ intensity of transmitted light ,i ,t ,density ,d -log#aj,t log#aj,o ,hurter and ,driffield pointed out that a negative would give a true representation of the differences in the light and shade of the object if it reproduced these differences by equivalent differences in opacity4 ,this is equivalent to stating that if the densities are plotted against the logarithms of the corresponding exposures1 a straight line should be obtained at #de degrees to the axis of exposure times4 ,if the line is at another angle the opacities of the negative will be proportional to the brightness of the object photographed1 but the contrast will be different4 ,a typical ,h ,d plot is shown in ,fig4 #ajc4 ,it will be noted that two curves are shown4 ,these are obtained with different developments1 and illustrate the fact that the contrast or proportionality between exposure differences and opacity differences is a matter of time of development4 ,each of these curves exhibits certain characteristics which are common to all made in this way4 ,there is primarily a straight line portion1 where opacities are proportional to illumination4 ,this is commonly called the region of correct exposure4 ,the slope of this straight line portion--the ratio of density_/7log exposure7--is the development factor1 commonly denoted by 810 a gamma of unity denoting exact tone rendering4 ,below the region of correct exposure is a 8toe10 or region of smaller contrast1 called the region of under exposure4 ,above the correct exposure region is another where the opacity approaches constancy 7afterwards decreasing or 8reversing071 called the region of over exposure4 ,the speed of a plate on the ,h ,d scale is given by the intersection of the straight line portion of the characteristic curve when produced1 with the exposure axis4 ,this intersection point1 called the inertia1 is the same irrespective of the time of development1 as is shown in ,fig4 #ajc4 ,the numerical value of the speed is obtained by dividing #cd by the inertia1 when the exposure is plotted in candle-meter-seconds4 ,if a plate is developed until no more density and contrast can be obtained1 its development factor is then 1 7gamma infinity71 and the larger this is the more a plate can be forced in development4 ,if the plate fogs in its unexposed portions this fog is measured and recorded in density units along with the other constants4 ,the speed of development is represented by the velocity constant1 commonly symbolized by 4 ,illustration3 ,,fig4 #ajc4--,typical characteristic curves of photographic plate4 ,the length of the straight line portion determines the latitude of the plate1 or the range of permissible exposures to secure a 8perfect negative40 ,thus if we assume that an object has a range of brightness of #a to #cj1 then a plate with a straight line characteristic extending over a range of #a to #abj would have a latitude of #abj_/cj or #d4 ,that is1 the exposure could be as much as four times the necessary one1 and still give the same result on a sufficiently exposed print4 ,if the latitude of the plate is too small1 the shadows will fall in the under exposure region1 the high-lights in the over exposure portion of the characteristic curve1 with consequent poor rendering of contrasts4 ,criteria of ,speed4--,in airplane photography speed is of paramount importance1 but great care must be exercised to insure that all the factors are considered which can contribute toward yielding the desirable pictorial quality in the brief exposure which alone is possible from the moving plane4 ,a 8fast0 plate on the ,h ,d scale is not necessarily suitable for aerial work1 when we remember that accentuation of natural contrast is desirable1 particularly under hazy conditions4 ,for1 as is shown in ,fig4 #ajd1 it is a common characteristic of 8fast0 plates to have comparatively small latitude and low contrast at their maximum development4 ,it is to be noted that the ,hurter and ,driffield measure of speed is bound up with the idea of correct tone rendering and with the use of the straight line portion of the characteristic curve4 ,other criteria of speed exist4 ,for instance1 the exposure necessary to produce a just noticeable action 7threshold value72 and the exposure necessary to give a chosen useful density in the high-lights when development is pushed to the limit set by the growth of fog4 ,as has already been pointed out1 correct tone rendering is not necessary or even indicated as desirable in aerial views4 ,it is1 moreover1 a matter of experience that the majority of aerial exposures with existing plates fall in the 8under exposure0 period1 where contrasts with normal development are less than in the subject4 ,this being the case1 the problem is to select not necessarily a fast plate1 by the ,h ,d criterion1 but a plate which will develop up workable densities in the under exposure region4 ,a plate of medium speed will sometimes develop to greater densities in the short exposure region1 if development is forced1 than will a fast plate4 ,the contrast in the normal exposure region will be excessive1 but this is of no significance if no exposure falling in this region is present on the plate4 ,illustration3 ,,fig4 #ajd4--;,characteristic curves of fast and slow plates1 developed to maximum contrast4 ,in addition to its capacity for developing density1 the plate should have as low a threshold as possible1 thus meeting to some extent the requirements of both the alternative criteria of speed given above4 ,at the same time it is true that low threshold and good density for short exposures are not to be found in really slow plates4 ,consequently1 while high speed1 as ordinarily understood1 is undoubtedly the first requirement1 we may expect the complete specification for the best aerial plate to be a rather complicated thing1 describing the characteristics of a workable 8toe0 of the curve1 in terms of which several 7e4g41 contrast and speed7 are derived from another and quite different exposure region4 ,effect of ,temperature on ,plate ,speed4--,it has been found by ,abney and ,dewar that very low temperatures materially decrease the speed of photographic emulsions4 ,this decrease may amount to as much as #ej per cent4 in the temperature range from #cj degrees ,centigrade above zero to #cj degrees below zero1 which is the range over which aerial photographic operations will have to be carried on in war-time4 ,this effect has not been at all fully studied1 and it is not known whether it is general or only found in certain kinds of plates4 ,the remedy indicated is to provide means for heating the plates or films when low temperatures are encountered4 ,this is fairly easy in film cameras1 or in plate cameras like the de,ram1 where the entire load of plates is carried in the camera body4 ,plates carried in magazines present a more difficult problem4 ,the heating coil incorporated in the ,german cameras is perhaps partly for this purpose4 ,color ,sensitiveness4--,complete specifications for an aerial plate cannot be made solely on the basis of its speed1 contrast1 latitude1 threshold1 and other sensitometric values which have to do only with the intensity of the light acting on it4 ,these in general apply to photography from low altitudes1 where the illumination and natural contrast of the subject are the only factors to consider4 ,when higher altitudes are reached the interposition of haze decreases the already deficient contrast1 calling either for the development of more contrast in the plate1 or for the use of color filters to cut out the action of the blue and violet light predominant in haze4 ,along the lines discussed in the last section1 it is not surprising to find that some plates are better than others for bringing out gradations masked by haze1 even though no filters are used and though the plates are similar in color sensitiveness4 ,but the limitations to securing contrast by manipulating the characteristic curve of the plate are soon reached1 and it becomes necessary to resort to haze-piercing color filters1 used with color sensitive plates4 ,roughly1 two general types of color sensitive emulsions may be distinguished3 first1 those in which sensitiveness to green and yellow is added to the natural blue sensitiveness1 and second1 those sensitive in a useful degree to all colors of the spectrum4 ,the former are called iso- or ortho-chromatic1 the latter panchromatic emulsions4 ,spectrograms exhibiting the distribution of sensitiveness throughout the spectrum for several representative plates are shown in ,fig4 #aje4 ,orthochromatic plates are adequate for use with light yellow filters and have the slight practical working advantage that they can be handled by red light4 ,panchromatic plates are necessary for use with dark orange or red filters4 ,they must be handled in total darkness or in an exceedingly faint blue-green light1 taking advantage of the common drop in sensibility in that region of the spectrum4 ,plates can1 indeed1 be sensitized for the red alone1 leaving a gap of almost complete insensibility in the green1 as shown in the fourth spectrogram of ,fig4 #aje4 ,when used with a yellow filter these plates behave as do panchromatic plates with a red filter4 ,illustration3 ,,fig4 #aje4--,spectrograms of representative photographic plates3 a1 ordinary plate2 b1 orthochromatic plate2 c1 specially green-sensitive plate2 d1 red sensitive plate1 insensitive to green2 e1 panchromatic plate2 f1 specially red-sensitive panchromatic plate4 ,a rougher idea of color sensitiveness than is given by spectrograms is furnished by the tri-color ratio1 which is the ratio of exposure times necessary with white light to give equal photographic action through a certain set of red1 green and blue filters1 expressed in terms of the blue exposure as unity4 ,in an excellent panchromatic plate the three exposures would be equal4 ,in an orthochromatic plate the red exposure will be too large to be figured4 ,in interpreting either spectrograms or tri-color ratios care must be taken that the absolute exposures necessary are known4 ,thus a relatively high red sensitiveness may mean merely low absolute blue sensitiveness4 ,two methods are used in imparting color sensitiveness4 ,either the sensitizing dye is incorporated in the plate emulsion before it is flowed2 or the plate is bathed in a dye solution not long before using4 ,the latter method gives higher color sensitiveness but poorer keeping quality1 and is not a practical method for field operations4 ,greatly enhanced sensibility may be given by treatment with ammonia1 but this again is a method for laboratory rather than field use4 ,resolving ,power4--,a question which arises in connection with all photography of detail is the size of the grain of the photographic emulsion4 ,dependent on the size of the grain is the resolving power1 or ability to separate images of closely adjacent objects4 ,this varies with the speed1 fast plates being of coarser grain than slow ones2 with the exposure2 and with the method and time of development4 ,in general1 it may be said that the resolving power of the plate does not enter practically into aerial work1 because the resolving power of all plates so far found usable corresponds to a smaller distance than the size of a point image as limited by the performance of the camera lens and the speed of the plane4 ,remembering that mm4 is a fair value for the size of a point image as rendered by the lens1 the rle of plate-resolving power is shown by consideration of the following table4 ,resolving powers are given in terms of lines to the millimeter just separable4 ,emulsion4 ,resolving ,power4 ,seed ,graflex #be ,eastman ,aerial ,film #cg ,hammer ,ortho #dd ,cramer ,isonon #dh ,cramer ,spectrum ,process #eg ,eastman ,portrait ,film #fa ,tabulation of ,requirements for ,aerial ,emulsions4--,in terms of the sensitometric quantities just discussed the general requirements for aerial plates may be listed as follows3 #a4 ,speed4 ,the speed usually connected with the contrast and density required for the exposure times available is about #aej ,h ,d4 ,faster plates in general have too low contrast1 but the highest speed that will give the necessary contrast is desired4 #b4 ,contrast4 ,the contrast capable of development without fog should be from #a4e to #b4 ,this contrast should be produced by light of daylight quality1 and1 in orthochromatic and panchromatic plates1 with the yellow or orange filters intended to be used with them4 ,this contrast means a gamma infinity approaching #b4e4 #c4 ,speed of development4 ,a gamma of nearly #b should be developed in #b minutes at #bj degrees ,c4 in the developers recommended below4 #d4 ,fog4 ,not over 4#be for this degree of development1 and not over 4#dj for six minutes development4 #e4 ,color sensitiveness4 ,this should in general be as high as possible4 ,in terms of certain representative filters 7described in a subsequent chapter7 color sensitiveness should be such that with the white light speed above specified the relative exposures through the filters shall not be greater than as follows3 ,no filter ,aero #a ,aero #b #ba #bc;a #be ,panchromatic plate #a #c #d4e #g #i #ab ,ortho plate #a #b4e #c4e #f ,relative ,behavior of ,plates and ,films4--,the advantages of film from the standpoint of weight and bulk have been discussed in connection with aerial cameras4 ,were there no other considerations film would unquestionably be the most appropriate medium for aerial photography4 ,there is1 however1 the question of ease of handling1 to be treated in a subsequent chapter1 and the question whether the purely photographic characteristics of film are satisfactory4 ,can the same speed1 contrast1 and color sensitiveness be obtained on film as on glass8 ,is the picture so obtained as permanent or reliable as the plate image8 ,it must be confessed that up to the present emulsions on film have not proved the equal of those on glass4 ,it has been found by emulsion manufacturers that the same emulsion flowed on film and on glass gives better quality on the glass4 ,emulsions specially prepared for film fall somewhat short of the best plate emulsions4 ,it has also been found harder to color-sensitize film1 and to insure good keeping quality in the color sensitized product4 ,in addition to the question of photographic quality there arises the matter of shrinkage and distortion4 ,these are negligible with plates1 but are a more or less unknown quantity in film4 ,irregular shrinkages of as much as two per cent4 are found on experiment4 ,this defect1 of course1 would be an obstacle only in exact mapping work4 ,positype ,paper4--,the need sometimes arises in military operations to secure prints ready for examination within a few minutes after the receipt of the negatives4 ,even the #ae or #bj minutes within which a negative can be developed and a wet print taken may be considered too long4 ,while such occasions are probably more apt to occur in popular magazine stories than in actual warfare1 it is important to have available methods of producing prints with an absolute minimum of delay4 ,this need is met to some degree by a direct print process1 commercially exploited under the name of 8,positype40 ,in this process the exposure is made directly on a sensitized paper or card1 which is developed1 the image dissolved out1 the residue exposed1 and again developed2 thus furnishing a positive picture 7reversed right and left74 ,the time necessary to develop a print ready for examination need not be more than three minutes4 ,only a single print is available1 but this is all that would be called for under the extreme conditions suggested4 ,if later1 copies are desired they may be made by the same process4 ,plates and ,films ,found ,satisfactory for ,aerial ,work4--,the following plates and films have been found particularly good for aerial photography4 ,the list is not intended to be complete4 ,furthermore1 it may be expected to be soon superseded1 as the efforts of various manufacturers are directed toward developing special aerial photographic plates4 ,among orthochromatic plates3 ,the ,cramer ,commercial ,isonon1 the ,jougla ,ortho4 ,among panchromatic plates3 ,the ,ilford ,special ,panchromatic1 the ,cramer ,spectrum ,process4 ,film3 ,ansco ,speedex1 ,eastman ,aero4 ,,chapter ,,xix ,,filters ,the ,function of ,filters in ,aerial ,photography4--,the use of color screens or filters has been very common in ordinary landscape photography1 for the purpose of securing approximately correct renderings of the brightnesses of colored objects4 ,plates of the non-color-sensitive type have their maximum of sensitiveness in the blue of the spectrum 7,fig4 #aje7 and in consequence blue skies photograph as white1 while other colors are likewise reproduced on a totally wrong scale4 ,filters for correct brightness rendering are calculated for a given color sensitive plate so that the resultant reaction to the light of the spectrum copies the sensitiveness of the eye1 which is greatest in the yellow-green4 ,such filters for use with the common orthochromatic plates are of a general yellow color4 ,filters for aerial work are meant to serve quite a different purpose4 ,correct tone or color rendering is of quite secondary importance to another use of filters1 namely1 to cut or pierce aerial haze4 ,it is quite a matter of accident that the same general color of filter is called for both to give correct color rendering and to pierce aerial haze1 namely1 yellow4 ,yet on closer analysis it is found that quite different types of yellow filter are demanded1 spectroscopically considered4 ,figure #ajf 7,k#a and ,k#b7 shows the spectral transmission curves of the ,wratten ,k#a and ,k#b filters1 intended for correct color rendering with orthochromatic plates4 ,the absorption increases gradually toward the blue4 ,in the same figure is shown on an arbitrary scale the spectroscopic character of typical haze illumination1 increasing in brightness inversely as the fourth power of the wave-length1 that is1 with great rapidity in the blue and violet4 ,it is evident from this that a much more abrupt absorption than that of the ,k#a or ,k#b filter is desirable1 because in the green of the spectrum the haze light is comparatively weak1 and more will be lost by any absorption in this region through decreasing useful photographic action than will be gained by cutting out the haze4 ,this latter consideration is important4 ,the use of any filter means an increase of exposure2 the use of yellow filters multiplies it several times4 ,careful experiment has shown that no filter of depth less than ,k #a1 to use the ,wratten filters as a basis for discussion1 are of real value in haze piercing4 ,the filter ratio1 or ratio of exposures with and without filter1 is #d4g for the ,k #a with the ,cramer ,isonon plate--a figure which shows the importance of securing the necessary haze-piercing character with the minimum absorption of useful photographic light4 ,illustration3 ,,fig4 #ajf4--;,characteristics of various filters4 ,practical ,filters4--,since the character of the absorption of the 8,k0 filters is not all that could be desired1 new filters1 both of dyed gelatin and of glass1 have been produced4 ,the glass1 a ,corning product having a very sharp-cut absorption1 has not yet been produced on a commercial scale with the high transparency in green1 yellow and red that selected samples have shown4 ,the ,united ,states ,air ,service has adopted filters of a new dye1 called the ,,ek1 from the name of the company in whose laboratory it was produced4 ,these filters are standardized in two depths of staining1 called the 8,aero ,no4 #a0 and 8,aero ,no4 #b40 ,their spectral transmission curves appear in ,fig4 #ajf1 along with those of certain darker filters useful only with panchromatic plates for exceptionally heavy haze4 ,the characteristic of these ,aero filters is their great transparency through all the spectrum except the blue1 whereby the greatest haze-cutting action is attained together with a low filter factor4 ,the filter factors of the ,aero ,no4 #a and ,no4 #b with ,cramer ,isonon plates are #c and #e1 respectively4 ,effects ,secured by the ,use of ,filters4--,the efficiency of yellow filters for haze-cutting is best shown by photographs taken at high altitudes with filters and without4 ,such illustrations are given in ,figs4 #ajg and #ajh1 where the first photograph is one taken at #aj1jjj feet without a filter1 the second taken at the same altitude under the same conditions1 but with an orange filter4 ,both are on panchromatic plates1 and it will be seen that even with these plates the filter makes all the difference between a useless and a useful picture4 ,but it must be clearly understood that the difference here lies between a plate sensitive chiefly in the blue and violet1 and a plate affected only by the yellow1 orange and red4 ,the difference is not between what the eye sees and what a plate with a filter sees1 as is sometimes supposed4 ,as shown in ,fig4 #ajh1 a filter enables the plate to photograph through the haze between clouds1 but not through the clouds themselves4 ,in general1 no filter and plate combination which is feasible for aerial exposures is capable of showing more than the eye can see if yellow or orange goggles are worn4 ,to do this it would be necessary for the photographic action to take place by deep red or infra-red light1 which would demand exposures now out of the question4 ,illustration3 ,,fig4 #ajg4--;,a photograph taken at #aj1jjj feet1 without a filter4 ,filters are almost always necessary in photographing from high altitudes or in making distant obliques4 ,at times1 particularly after a heavy rain1 the air is clear enough so that filters may be dispensed with4 ,clearing weather was therefore chosen whenever possible for making obliques of the battle front4 ,illustration3 ,,fig4 #ajh4--,photograph taken at same time and over same neighborhood as ,fig4 #ajg1 but with an orange filter4 ,filters for the ,photographic ,detection of ,camouflage4--,in the photographic as in the visual detection of camouflage1 the problem is to differentiate colors which ordinarily look alike1 but which are actually of different color composition4 ,particularly important are the differences between natural foliage greens and the paints used to simulate them4 ,if these differ in their reflection spectra1 a proper choice of filter will show up the two greens as markedly different4 ,two kinds of difference may be produced2 either the two colors may be changed in relative brightness1 or they may be altered in hue4 ,thus foliage green1 due to its possessing a reflection band in the red of the spectrum1 which is absent in most pigments1 may be made to appear red while the camouflage remains green or turns black4 ,filters which cause changes of color are of course of no use for photographic detection of camouflage1 since the photographic image is colorless4 ,brightness differences are alone available4 ,those same filters which have been worked out primarily for producing brightness differences in visual detection of camouflage could be used photographically1 provided the plates employed were color sensitive1 and were as well screened to imitate the sensibility of the eye4 ,but the most useful visual filters are those causing color differences to appear2 more than this1 the visual camouflage detection filters as a class have low light transmissions1 so that their usefulness in photography is doubtful4 ,little work has actually been done with camouflage detection filters for photography4 ,yet in spite of this photography has been of real service in this form of detective work4 ,its utility for the purpose comes from the fact that the natural sensitiveness of the plate to blue1 violet and invisible ultra-violet acts to extend the range of the spectrum in which differences between identical and merely visually matched colors may be picked up4 ,consequently the plain unscreened plate has proved a very efficient camouflage detector--so efficient in fact that all camouflage materials have had to be subjected to a photographic test before acceptance4 ,fig4 #aga shows how an ordinary photograph reveals the unnatural character of the camouflage over a battery4 ,methods of ,mounting and ,using ,filters4--,the most primitive way of mounting a gelatin filter is to cut a disc from a sheet of dyed gelatin and insert it between the components of the lens4 ,for this purpose the gelatin must be perfectly flat1 which is insured by its method of preparation and test4 ,one disadvantage of this method is that the filter can be inserted and removed only upon the ground4 ,it is less satisfactory the larger the diameter of the lens1 and the wastage of filters due to insertion and removal is apt to be high4 ,the camera should be refocussed after filters of this kind are inserted4 ,glass filters1 ground optically true1 or gelatin filters1 mounted between optically flat glass plates1 are the most convenient and satisfactory4 ,they may be mounted in circular cells to screw or attach by bayonet catches to the front of the lens4 ,or they may be mounted in rectangular frames to slide into transverse grooves in the camera body4 ,fig4 #dd shows the mount of this latter form adopted in the larger ,united ,states ,air ,service cameras4 ,this is particularly convenient if it is desired to insert or change the filter while in the air--a practice not generally considered feasible in war work with the photographically inexperienced observer1 but likely to be common with the employment of skilled photographers for peace-time aerial photography4 ,german cameras are reported in which the glass filter is carried behind the lens1 on a lever which also carries a clear glass plate of the same thickness1 to be thrown in when no filter is needed1 thus maintaining the focus4 ,the performance of the lens will be impaired by this scheme1 unless it is specially calculated to offset the effect of the glass introduced in the path of the rays behind the lens--optically true glass has no effect on definition if placed in front of the lens4 ,glass filters may also be placed in close contact with the plate or film1 in which case they must be much larger1 but do not need to be of as good optical quality4 ,self-screening ,plates4--,mention must be made of a quite different mode of realizing the filter idea1 a method available where the sensitive plate is always to be used with a filter4 ,this is to incorporate a yellow dye in the gelatin of the plate itself4 ,the dye must be one which has no direct chemical effect on the plate1 but which acts simply as a coloring agent for the gelatin4 8,self-screening0 plates1 as they are called1 have been produced by the use of the dye called 8filter yellow0 and have found some use in orthochromatic photography4 ,they effect a useful saving of light through the elimination of the reflection losses at the surfaces of glass and gelatin filters4 ,the filtering action of the dye in the plate is somewhat different from its ordinary one1 since the deeper portions of the sensitive film are subject to greater action than the surface1 and this tends to diminish contrast4 ,,chapter ,,xx ,,exposure ,,of ,,aerial ,,negatives ,the principal factors governing the length of exposure in the airplane camera have already been discussed under various headings4 ,these are briefly1 the nature of the aerial landscape1 the practically attainable lens apertures1 the form of the camera support1 the speed of the plane1 and the characteristics of plates1 films and filters4 ,it is convenient however1 to re-assemble this information in one place1 in such form as to apply to the practical problem of determining the exposure to be given in any specific case4 ,limitations to ,exposure4--,in the ordinary photography of stationary objects1 exposure is a variable entirely at the operator's command4 ,plates of any speed may be selected1 so that attention may be focussed on latitude1 color sensitiveness1 and other tone rendering characteristics4 ,the exposure may be made of a length sufficient to insure all the useful photographic action lying in the 8correct exposure0 portion of the sensitometric curve4 ,the exposure ratio of any filter it is desired to use is a matter of indifference--its effect on color rendering need alone be considered4 ,airplane photography is sharply distinguished from ground 8still0 photography by its severe limitations as to the amount of the exposure4 ,the actual duration is definitely restricted by the high speed of the plane4 ,in peace work this can be offset in part by using slower planes or by flying against the wind4 ,the practical limitation to #a_/ajj second1 set by war-time requirements as to definition of fine detail1 may be increased to #a_/ej of a second1 or even more1 where mapping of grosser features is the object4 ,a common1 but entirely avoidable limitation1 is that due to vibration of the camera4 ,by proper mounting this may be entirely overcome1 leaving the ground speed of the plane the only source of exposure-limiting movement4 ,the amount of light reaching the plate constitutes a primary factor in exposure1 and this is a matter of lens aperture4 ,generally1 lens aperture is smaller the larger the plate required to be covered1 and the greater the focal length4 ,because of their larger aperture1 the short-focus lenses which will be favored for peace-time large-area mapping will permit more and longer working days than have been the rule in long-focus war photography4 ,the necessary use of filters1 particularly at the high altitudes which would be chosen in mapping1 in order to economize in the number of flights needed to cover a given area1 introduces an inevitable decrease in the amount of light available at the plate1 as compared with surface photography under the same illuminations4 ,broadly speaking1 it may be said that all the demands made in reference to aerial photographic exposure work are to decrease the amount of light reaching the plate4 ,any surplus offered1 as by the midsummer noon-day sun1 must be immediately snapped up1 either by decreasing the exposure to get greater sharpness1 or by introducing filters to get greater photographic contrast4 ,the absolute exposure of the plate tends to be kept at the irreducible minimum4 ,as already stated1 it lies1 with present photographic materials1 on the 8toe0 of the 8,h ,d0 curve1 just reaching up into the straight line portion4 ,estimation of ,exposure4--,according to the foregoing argument the problem of estimating an aerial exposure resolves itself largely into one of deciding how short this may be made4 ,or1 if the light is strong1 whether it is sufficient so that a filter may be introduced without demanding more than the #a_/ajj second or thereabouts which is dictated by the motion of the plane4 ,deciding upon exposures in the field has been largely a matter of experience and judgment4 ,a majority of the cameras in use during the war were not furnished with shutters calibrated in definite speeds4 ,consequently1 the sergeant upon whom the decision usually devolved became a storehouse of knowledge as to the slit widths and tensions appropriate to each individual camera4 ,this knowledge had to be acquired from the results of actual photographic reconnaissances1 or from special test flights1 both of them wasteful methods4 ,but the chief objection to this state of affairs lies in the fact that the knowledge thus acquired is of no use to anyone else1 nor is it applicable to other types of camera4 ,the first essential to placing exposure estimation upon a sound basis is therefore an accurate knowledge of shutter performances4 ,either the shutter speeds should be placed upon the camera by the manufacturer and periodically checked1 or a regular practice should be followed of calibrating shutters1 either at a base laboratory or even in the field4 ,assuming that the speeds of all shutters are accurately known1 the process of estimating the requisite exposure becomes less a matter of mere guesswork and more nearly a matter of precision4 ,for this purpose data on the variation of light intensity during the day and during the year 7,figs4 #aja and #ajb7 should be taken as a guide4 ,these data refer of course to visual and not to photographic light1 but since it is always necessary to use color filters1 which make the active light of approximately visual quality1 this is no valid objection4 ,the effects of clouds and mist must of course be learned largely by experience1 but with the above daylight data at hand1 anyone in possession of definite information on the correct exposure with a given plate for a known day and hour need not go far wrong in estimating exposures at any other time in definite fractions of a second4 ,illustration3 ,,fig4 #aji4--;,chart showing aerial exposures for all times of the day and year4 ,data on basis of ,f_/#e4f lens1 ,jougla orthochromatic plate1 and clear sunlight1 no filter4 ,exposures to be doubled and tripled for overcast and cloudy weather4 ,exposure data charts4 ,fig4 #aji shows a chart1 prepared in the ,french service1 indicating aerial exposures for all hours of the day throughout the year4 ,these are for clear sunlight1 for a lens of aperture ,f_/#e4f and for 8ortho0 plates without a filter4 ,they are based on what is probably an over-estimate of the actual speeds given by the ,french shutters4 ,for 8light0 clouds the exposures are to be doubled1 for 8heavy0 clouds quadrupled1 and for forests and dark ground 8lengthened40 ,charts of this form should be extremely useful1 but they were actually not of great service because of the prevalent lack of knowledge of true shutter speeds4 ,exposure meters4 ,aerial photography offers an excellent opportunity for the use of exposure meters1 particularly those of the type in which a sensitive surface is exposed to the light for a measured time sufficient to darken a predetermined amount4 ,the sensitive paper of the meter may either be exposed from the ground to the direct light of sun and sky1 or from the plane to the light reflected from the ground4 ,the first method will give figures subject to some correction for the character of the ground to be photographed--whether fields1 forests1 or snow4 ,the second method is to be preferred where the shutter speed can be adjusted in the air1 according to the indications of the meter1 or where the filter can be selected and put in place during flight4 ,trials with a commercial ,wynne exposure meter1 used in the latter manner1 give as a working figure an exposure of 4#jja second for each #d seconds taken to darken the sensitometer strip to match the darker comparison patch4 ,this relation applies to a lens of aperture ,f_/#d4e1 on ,cramer ,commercial ,isonon plates without filter4 ,,chapter ,,xxi ,,printing ,,media ,skilled photographers can examine a negative and can interpret its renderings with practically as much satisfaction as they get from a print1 whereby a considerable amount of time can be saved in an emergency4 ,the original glass negative should always be used when accurate measurements are to be made4 ,these and a few other cases constitute the only use of a negative apart from its normal one1 namely1 for producing positive prints1 usually in large numbers4 ,the commonest form of print is on paper1 although the most satisfactory print from the photographic standpoint is the transparency on glass or celluloid film4 ,transparencies4--,transparencies are made by the regular photographic processes of exposure and development1 on glass plates or films placed in contact with the negative1 or in the appropriate position in an enlarging camera4 ,the sensitometry and the terms used to describe the qualities of plate or film for this purpose are those already given in connection with the general discussion of plates and films4 ,but the kind of emulsion to be selected is quite different from the aerial negative emulsion4 ,there is here no practical limitation to the speed1 contrast or latitude4 ,consequently1 we can choose a positive emulsion on which the exposure through the aerial negative falls entirely on the straight line portion of the characteristic curve1 thus reproducing all of its tones1 and the contrast of the negative may be increased to any desired extent4 ,the possibilities of positive emulsion are indeed rather greater than the usual aerial negative can utilize4 ,a range of clearly graduated opacities of two or three hundred to one is possible1 so that not only can detail be well rendered in the high-lights1 but also equally well in dark shadows where1 indeed1 an increase of illumination is necessary for it to be made easy to examine4 ,this range is to be contrasted with the #a-to-#g range in the aerial landscape1 which may be doubled by a contrasty plate4 ,in resolving power1 the positive emulsion1 which is slow1 exceeds the negative emulsion4 ,it easily bears examination through a magnifying glass1 thus making any enlargement unnecessary in the printing process4 ,glass transparencies are of course impractical for general distribution1 on account of their fragility4 ,heavy film transparencies are not open to this objection1 and1 especially in the form of stereos1 constitute the most beautiful form of aerial photographic print4 ,paper ,prints4--,prints on paper suffer by comparison with transparencies1 in the range of tones which they exhibit4 ,this lies between the white of the paper1 which never has more than #hj per cent4 reflecting power1 and its darkest black1 which differs with the kind of paper4 ,in dull or mat papers the blacks will reflect as much as #e per cent42 in glossy papers1 ordinarily used for aerial negatives1 the reflection from the black may be as low as one per cent41 but in order to get the benefit of this the paper must be so held as not to reflect any bright object to the eyes4 ,this deficiency in the range of paper gradations is not so serious with aerial negatives as with ordinary properly exposed negatives because of the small range of brightness in the aerial view4 ,the sensitometry of papers is similar to that of plates1 with the difference that reflecting powers take the place of transparency4 ,as in the case of transparency emulsions there is in papers no dominating requirement for extreme speed1 to which other characteristics must be subordinated4 ,yet speed is of sufficient importance in handling large quantities of prints so that aerial negative printing for military purposes has been done almost entirely on the rapid enlarging papers1 rather than on the true contact printing papers1 which are slower4 ,illustration3 ,,fig4 #aaj4--;,characteristic curves of bromide paper4 ,the two principal types of rapid enlarging papers1 the bromide and the 8gas light10 exhibit certain characteristic differences which are important to bear in mind in seeking to obtain any particular quality of print4 ,bromide papers1 of which 8,nikko0 is a good example1 show sensitometric curves rather like those of plates4 ,that is1 they increase in contrast with continued development4 ,at the same time1 as is shown in ,fig4 #aaj1 they increase somewhat in speed with development2 that is1 under exposure can be compensated for to a small degree by protracted development4 ,these characteristics of bromide paper can be utilized to secure prints of a quality quite different from that of the negative4 ,thus1 if the negative has a long range of tones1 a flat print can be secured by full exposure and short development4 ,if1 as is apt to be the case with aerial negatives1 a print of greater contrast than the negative is desired1 a short exposure with long development is called for4 ,illustration3 ,,fig4 #aaa4--;,characteristic curves of gas light paper4 ,the sensitometric curves of a typical gas light paper 8,contrast ,enlarging ,cyco10 are shown in ,fig4 #aaa4 ,here the contrast is a fixed characteristic of the paper1 and the only effect of changing development is on the speed2 that is1 exposure and development are1 within limits1 interchangeable4 ,choosing a printing paper is a matter of deciding on the contrast required for the class of negative1 and selecting a paper which will give this contrast with a good range of tones from a clear white to a deep black4 ,the ideal paper would be one which was all straight line in the ,h ,d plot4 ,in such a paper there would occur no loss of contrast in the lighter tones when the high-lights were rendered by the clear white of the paper4 ,too great contrast with a short straight line portion1 results in loss of detail at the ends of the scale4 ,a negative possessing a very great range of tones cannot be correctly represented on one paper print--two printings are required1 one for high-lights and one for shadows1 but this difficulty is rarely to be faced in aerial views4 ,the greatest demand for aerial printing papers has been for those of considerable contrast1 because of the flat character of the negatives4 ,,chapter ,,xxii ,,photographic ,,chemicals ,general ,considerations4--,developing1 fixing and other chemicals for aerial work differ in no essential respect from those used in ordinary photography4 ,full discussions of these are to be found in numerous texts and articles4 ,the aerial photographic problem is to select those most suited for the under-exposed flat negatives characteristic of photographs from the air4 ,at the same time selection from among the chemicals of appropriate quality must be governed by considerations of the conditions surrounding work in aerial photographic laboratories4 ,these laboratories1 especially in war-time1 are apt to be most primitive in their facilities4 ,characteristics of ,developers for ,plates and ,films4--,from the standpoint of practicability1 aerial negative developers should have good keeping power1 be slow to exhaust1 and work well over a considerable range of temperatures4 ,from the standpoint of the photographic quality desired in the negative1 the developer should bring up the maximum amount of under-exposed detail4 ,this means that it should impart the highest possible speed to the plate1 with good contrast1 and low fog or general reduction of unexposed silver bromide4 ,there are many characteristics to study in a developer3 its effect on inertia or speed1 gamma infinity1 fog1 time of appearance1 8,watkins factor10 speed of development1 temperature coefficient1 dilution coefficient1 keeping power1 exhaustion1 length of rinsing1 stain1 color coefficient and resolving power4 ,these are defined and described as follows3 ,effect on inertia4 ,the meaning of inertia has already been given under the discussion of plate speed4 ,while this is a constant1 independent of time of development1 for any one developer1 it is altered appreciably by change of the latter4 ,time-gamma relation4 ,contrast1 symbolized by 1 has likewise been discussed under plate sensitometry4 ,viewed from the standpoint of the developer1 the point of interest is the rate at which varies with development1 and the maximum contrast which can be reached or infinity4 ,speed of development is commonly defined by the velocity constant1 symbolized by 1 which is arrived at mathematically from a consideration of the time of development to produce two different contrast values4 ,high infinity is desired for aerial negatives1 and for rapid work must also be high4 ,fog4 ,the opacity due to chemical fog is to be kept at a minimum in aerial negatives1 as it is chiefly prejudicial to under exposures4 ,time of appearance and ,watkins factor4 ,the time of appearance is measured in seconds4 ,the ,watkins factor is a practical measure of the speed of development1 and is determined by the ratio of the time of development required for a definite contrast1 to the time of appearance4 ,it is useful also as a guide to development time4 ,temperature coefficient4 ,this is the factor by which the time of development at normal temperature 7#bj ,cent47 must be increased or decreased in order to obtain the same quality negative1 for a change of seven degrees either side of normal4 ,temperature limits are the temperatures between which development can be carried out with any degree of control or without serious damage to the negative4 ,these factors are of great importance where climatic or seasonal changes have to be endured4 ,dilution coefficient4 ,this is the factor by which the development time is increased in order to maintain a given quality negative in different dilutions of the developer4 ,it is useful in tank development4 ,keeping power4 ,the keeping power of a developer1 mixed ready for use1 is determined by its ability to resist aerial oxidation4 ,a developer of poor keeping power1 which must be made up immediately before use1 causes delay and waste of time whenever emergency work has to be done1 whereas a developer of good keeping power may be left in its tank ready for instant use4 ,exhaustion of a developer is the rate at which it becomes useless for developing1 due both to aerial oxidation and to the using up of its reducing power by the work done in developing plates4 ,it is conveniently measured by the area of plate surface developable before the solution must be renewed4 ,length of rinsing4 ,the time required for rinsing between development and fixing bath plays a not unimportant part in total development time4 ,dichroic fog is caused with some developers if1 due to insufficient rinsing1 any of the caustic alkali is carried over to the fixing bath4 ,stains develop also if the fixing bath is old1 or if light falls on the unfixed plate while any developer remains in the film4 ,color coefficient4 ,the function of the sulphite1 which forms a constituent of all developing solutions1 is two-fold4 ,it acts partly as a preservative1 and partly to prevent the occurrence of a yellow color in the deposit4 ,the yellow color1 if present1 increases the photographic contrast4 ,this phenomenon has been purposely utilized1 particularly in the ,british service1 to give 8stain0 to negatives which otherwise would show insufficient printing density4 ,the color index or coefficient of a negative 7with a given printing medium7 is the ratio of photographic to visual density4 ,if we take a pyro developer containing five parts of pyro per thousand and ten parts of sodium carbonate1 and then vary the amount of sulphite from none to fifty parts per thousand1 the color index varies as follows3 ,sulphite ,color ,index ,parts per ,thousand #ej #a4af #be #a4bd #ae #a4cj #aj #a4de #e #a4hj #j #b4ge ,the color index is somewhat different with various kinds of printing media4 ,this staining effect is a variable one1 depending upon length of development1 dilution of the developer1 length of rinsing1 temperature1 the fixing bath used 7plain hypo being necessary for a maximum effect71 the length of washing after fixation and the properties of the water used4 ,standardization of these conditions in the field is difficult2 hence any developer which will give the same effective contrast without resorting to stain is to be preferred4 ,resolving power4 ,some developing processes and conditions will introduce bad grain into the negative4 ,hence the resolving power which a developer brings up must be investigated among its other characteristics4 ,practical ,developers for ,aerial ,negatives4--,in the ,english service a pyro metol developer was generally used1 producing stained negatives4 ,the ,french1 ,american and ,italian practice was to use metol-hydrochinon1 without staining4 ,a special chlor-hydrochinon developer1 worked out by the ,eastman ,research ,laboratory for the ,united ,states ,air ,service1 has probably the greatest merit of any yet tried4 ,a comparison1 given below1 between it and a pyro metol formula used on a representative plate1 illustrates the use of the various bases of study given above4 ,,pyro ,,formula ,solution ,a ,solution ,b ,pyro1 #c4ge grams ,sodium carbonate1 #ec g ,potassium metabisulphite1 #c4ge g ,metol1 #c4je g ,potassium bromide1 #a4e g ,water1 #ejj c4c4 ,water1 #ejj c4c4 ,use #a part of ,a to #a of ,b ,,chlorhydrochinon ,,formula ,solution ,a ,solution ,b ,chlorhydrochinon1 #be g ,sodium carbonate1 #cj g ,metol1 #f g ,sodium hydrate1 #aj g ,sodium bisulphite1 #b4e g ,potassium bromide1 #c g ,sodium sulphite1 #be g ,water to #fgj c4c4 ,water to #ccj c4c4 ,use #b parts of ,a to #a of ,b ,pyro ,chlorhydrochinon ,h ,d speed #aej #ahj ,gamma infinity #a4de #b4ab ,fog 7at maximum gamma7 4#cb 4#fj ,time of appearance #e seconds #e seconds ,watkins factor #be #aj ,velocity factor 80 4#cbj 4#djj ,temperature coefficient #a4dj #b4j ,temperature limits #d to #cb ,c #d to #cb ,c ,keeping power #de minutes #h days ,exhaustion 7#ajj c4c47 #cj sq4 in4 #cjj sq4 inches ,dilution coefficient #b #b ,color coefficient #a4ej #a4jj ,resolving power #dg #ec ,owing to the difficulty of securing pure chlor-hydrochinon a metol hydrochinon of very similar properties has been worked out4 ,its composition is ,metol #af grams ,hydrochinon #af grams ,sodium sulphite #fj grams ,sodium hydroxide #aj grams ,potassium bromide #aj grams ,water to #a litre ,to keep the ingredients in solution in cold weather1 #ej c4c4 of alcohol should be included in every litre of solution4 ,all things considered this is probably the most practical and satisfactory developer for aerial negatives4 ,developers for ,papers4--,the following formula has been found very satisfactory for papers3 ,metol 4#i gram ,hydrochinon #c4f gram ,sodium carbonate #bj4j gram ,sodium sulphite #ad4j gram ,potassium bromide 4#e to #a4j gram ,water to #a litre ,fixing ,baths4--,for plates the following fixing and hardening bath is recommended3 ,sodium thiosulphate 7hypo7 #cej grams ,potassium chrome alum #f grams ,sodium bisulphite #aj grams ,water to #ajjj c4c4 ,during hot weather1 the above quantities of chrome alum and bisulphite are doubled4 ,for papers the following3 ,hypo1 #ce per cent4 #ajj volumes ,acid hardener #e volumes ,the acid hardener is constituted as follows3 ,alum #ej grams ,acid acetic #bh #djj c4c4 ,sodium sulphite #ajj grams ,water to #a litre ,intensification and ,reduction4--,these processes have been little employed in air work4 ,reduction is rarely necessary1 for obvious reasons4 ,intensification would often be of value1 but the common practice1 which saves some time1 is to use printing paper of strong contrast for those negatives which are deficient in density and contrast4 ,when intensification is desirable or permissible1 either the ordinary mercury or uranium intensifier may be used4 ,water4--,in the field it is found necessary in many cases to purify the water that is to be used in mixing up chemicals4 ,water may contain suspended matter or dirt1 dissolved salts1 and slime4 ,it is important to remove the suspended matter1 as it may cause spots on the plates and papers1 while any slime would coagulate1 forming a sludge in the developer which would also tend to settle on the plates and cause marks during development4 ,the dissolved salts may or may not cause trouble4 ,two methods of purification are possible3 7a7 ,filter the water through a cloth into a barrel1 add about one gram of alum for every four litres of water1 and allow to settle over night4 ,draw off the clear liquid from a plug in the side as required4 7b7 ,boil the water and allow it to cool over night4 ,if the water contains dissolved lime1 boiling will often cause this to come out of solution4 ,v ,,methods ,,of ,,handling ,,plates1 ,,films ,,and ,,papers ,,chapter ,,xxiii ,,the ,,developing ,,and ,,drying ,,of ,,plates ,,and ,,films ,field ,requirements4--,developing1 fixing1 drying and printing in the field demand simple and convenient apparatus that may be carried about and installed with the least amount of labor4 ,on top of these requirements military needs impose others that are more difficult4 ,speed is1 on occasion1 imperative4 ,a print may be required within a few minutes after landing1 and many thousands within a few hours4 ,quantity production must be achieved under the most primitive conditions4 ,nothing1 in fact1 shows the calibre of the photographic officer better than his choice of workplaces as the army moves forward4 ,ingenuity and practical judgment are at a premium4 ,cellars1 stables1 dog kennels1 or huts hastily built from packing cases1 must be equipped and in working order over night4 ,all the facilities offered by a great city are urgently needed--water1 electric light1 power for driving fans--but must be dispensed with if the photographic section is to be convenient to the airdrome1 whose portable hangars are most apt to be pitched in the open country4 ,water must be carried1 electricity generated1 and to the photographic problem is added the military one of concealment and protection4 ,dugouts and bomb proofs must be built for supplies1 and 8funk holes0 for the men4 ,entire underground emergency extensions have sometimes been built in stations occupied for extended periods1 for airdromes are a favorite bombing target4 ,for getting the exposed plates to the photo section1 messengers1 on motorcycles if possible1 are employed4 ,in some cases1 where hangars and photographic hut are forced to be widely separated1 recourse has been had to parachutes 7,fig4 #aab71 a device also employed to distribute prints to infantry during an advance4 ,illustration3 ,,fig4 #aab4--,receiving pictures from plane by parachute4 ,for warfare of movement1 especially in sparsely settled or devastated country1 where cellars are unavailable1 the dark room must be taken along4 ,motor trucks and trailers 7,figs4 #aac1 #aad1 #aae71 the former for hauling supplies and electric light generating plant1 the latter fitted as a complete developing and printing laboratory1 form the headquarters of each photographic section in the field4 ,usually altogether too small for the amount of work required1 they were extended by tents and lean-to's1 or ingeniously used as a nucleus for the organization of the favored stable or cellar4 ,illustration3 ,,fig4 #aac4--,mobile photographic laboratory4 ,methods of ,plate ,development4--,where speed is not required the simplest and commonest mode of developing plates is in the tray1 one plate at a time4 ,common practice is to examine the plate at intervals during development1 and discontinue the operation on the basis of its appearance4 ,this is only possible if the plates used are insensitive to some light by which the eye can see4 ,deep red light is suitable for ordinary and most orthochromatic plates4 ,a faint blue-green may be used with some panchromatic plates4 ,the best practice1 however1 is to develop by time in total darkness1 whereby all chance of dark room fog is avoided4 ,development time for plates of the average exposure of the one to be developed is either known from previous experience1 or is found by trial on the first one4 ,development by time results in negatives of densities varying with the exposures1 but1 as was brought out in the discussion of sensitometry1 this difference can be compensated for by the choice of the paper used for printing1 and by its treatment4 ,illustration3 ,,fig4 #aad4--;,interior of photographic trailer1 developing room4 ,where larger quantities of plates are to be handled tank development is adopted4 ,in ordinary tank development the plates are placed in grooved tanks1 into which is poured first the developer1 next the rinsing water1 and then the hypo4 ,it has been customary in tank development as practiced for peace-time work to use dilute developer1 requiring from ten to thirty minutes1 but speed requirements in war-time aerial photography dictate the use of full-strength quick-acting developer4 ,an improvement on the simple grooved tank is provided by metal cages or racks1 each holding a dozen or more plates1 which may be introduced or removed from the tank as a unit 7,fig4 #aaf74 ,illustration3 ,,fig4 #aae4--;,interior of photographic trailer4 ,enlarging camera and printer4 ,the core rack system combines certain of the features of both tray and tank development4 ,each plate is inserted in a separate metal frame with projecting lugs to rest on the top of the tank and so suspend the plate in the solution4 ,the process of development is the same as in the tank system1 but any individual plate may be examined and removed4 ,illustration3 ,,fig4 #aaf4--,tank and rack for tank development4 ,film ,developing and ,fixing4--,the problem of quickly handling roll film of large size is one upon whose solution depends in large degree the feasibility of film cameras for aerial work4 ,it presents many difficulties3 a long film is unwieldy1 is inherently subject to curling1 and takes up much space if it is handled entire4 ,for small scale operations roll film can be cut into short strips and developed either by drawing through a tray or1 if cost of developer is no object1 in a deep tank4 ,in order to make the cutting apart of exposures easy in the dark1 film cameras should make some form of punch mark in the film between the exposed parts1 or the space between exposures should be uniform1 so that a print trimmer set to a definite mark may be used4 ,racks for holding two or three feet of film1 folded back on itself and clasped by spring clothes-pins1 are fairly practical4 ,one object of the use of film1 however1 is to greatly increase the number of possible exposures2 and where hundreds instead of dozens of exposures are to be developed1 this method takes up entirely too much time4 ,following the practice in moving picture development1 film developing machines of various designs have been devised4 ,among these may be described the ,g4 ,e4 ,m4 machine2 the ,ansco machine2 the ,eastman apron machine2 the ,brock frame and tank apparatus2 the ,eastman reel machine2 and a modification of the latter by the ,united ,states ,air ,service4 ,the ,g4 ,e4 ,m4 film developing apparatus1 similar in idea to the ,eastman 8apron0 method of film developing1 as exemplified in the familiar amateur film developing machines1 has the film wound in a spiral on a long linked metal frame or chain4 ,after being wound it is placed in a tub of developer1 from that to a tub of water1 thence to a tub of hypo1 and finally to a tub of water1 where it is washed in several changes4 ,the objections to the method are that it takes up much floor space for the various tubs1 and that it requires such large quantities of solution4 ,to develop a thirty-five foot length of #ah #bd centimeter exposures requires approximately #bh gallons of developer2 for the rinsing1 #bh gallons of water1 and the same for hypo1 and at least three times that for washing4 ,in all #afh gallons of water must be brought to the developing hut or lorry4 ,the ,ansco machine makes use of an idea frequently applied in the moving picture industry4 ,the film is carried spirally1 upon two cross-arms which bisect each other at right angles1 and which contain vertical pins around which the film is looped1 beginning at the center and working out4 ,after it is wound it is placed in a tub of developer1 as in the ,g4 ,e4 ,m4 machine4 ,it has an advantage over this apparatus in that the shape of the tubs or tanks is square instead of round4 ,but it is equally extravagant of space and water4 ,this same criticism may be made of the ,eastman apron apparatus for film developing4 ,this is similar to the ,g4 ,e4 ,m4 machine1 but differs from it in using a perforated celluloid apron to support the film during the various operations1 instead of a metal chain4 ,the ,brock developing outfit consists of a rectangular wooden frame and a three-compartment tank4 ,the frame1 which is approximately #c by #d feet in size1 is used as a support for the #d inch wide film1 which is wound spirally around it1 between guiding pins4 ,a special support is provided1 on which the frame may be rotated as the film is fed off the camera spool4 ,the frame1 with the film on it1 is lowered successively into the three narrow but deep compartments of the developing tank4 ,the first compartment holds developer1 the next water1 the next hypo4 ,the amount of developing solution required is rather large 7#if gallons of water in all for a strip of #ajj #d #e inch exposures71 but because of the small surface exposed to the air1 it keeps for a considerable period4 ,the chief demand for floor space with this apparatus is for feeding the film on to the frame4 ,in the ,eastman twin reel machine the film is wound on a wooden drum or reel of large diameter1 to form a helix4 ,the drum is suspended so that the bottom edge touches the developing solution1 and1 upon revolving the drum1 every portion of the helix of film is brought into contact with the developer4 ,by shaping the developing tank so that it closely conforms to the shape of the reels1 a high economy in quantity of developing agent can be achieved4 ,when developing action is finished1 the developer is emptied out1 rinse water put in2 hypo follows1 and then comes the final washing with water4 ,with this apparatus the whole cycle is completed1 for the #ce feet length of film above considered1 with seven gallons of water4 ,the ,air ,service apparatus differs from the above only in the drying method1 which will be described below4 ,heavy cut film1 such as is marketed under the name of ,portrait ,film1 has not thus far been used in aerial work1 except for printing transparencies4 ,it is conceivable1 however1 that film in the cut form may be used in some future design of camera4 ,this may be developed expeditiously in a tray1 six or eight films being handled at a time1 in a pile1 pulling out the lower one frequently and placing it on top4 ,the core rack system is also available for film in this form1 special racks with clips to hold the film being necessary4 ,plate ,drying4--,the drying of negatives on glass is a comparatively simple matter1 owing to the rigid nature of the emulsion support4 ,a large number of plates may be placed in a compact mass in the ordinary plate racks of commerce with the wet sides accessible to a draft of air4 ,two dozen plates separated from each other by a quarter of an inch and left to dry spontaneously in a room of ordinary humidity and living temperature will dry in two hours and a half4 ,if the surface be wiped with soft cheese-cloth or chamois1 so as to absorb all the surface moisture before the plates are placed on the rack1 this time may be appreciably reduced4 ,by placing the plates in a forced draft of air1 from an electric fan1 this time may be reduced to an hour4 ,extra rapid drying of plates may be accomplished by placing them in a bath of alcohol before putting them in the racks4 ,the alcohol displaces all the water in the film1 and is itself very quickly dissipated into the atmosphere when the plate is taken from the tray4 ,the plate must be left in the alcohol tray long enough for the substitution of the alcohol for the water in the film to take place4 ,five minutes is long enough4 ,the alcohol before use must be as nearly free from water as possible4 ,the best way to make sure of this is to place in the bottle of alcohol some lumps of calcium oxide1 which will take up the water and form calcium hydroxide1 which settles at the bottom of the bottle4 ,another method of quick plate drying takes advantage of the extraordinary greediness of potassium carbonate for water4 ,the wet plates are placed in a saturated solution of potassium carbonate and left for a minute4 ,if a plate be now taken from the solution and its surface wiped with a soft cloth1 it will be found that the film has a greasy1 slippery feeling1 but that it contains no water and can be printed from at once4 ,plates so treated should be washed1 however1 at some time in the succeeding four months1 or the traces of potassium carbonate left in the film cause deterioration4 ,film ,drying4--,unlike the drying of plates1 drying of film negatives is a very puzzling problem1 and may be considered as the crux of the successful use of film in aerial cameras4 ,apron and similar machines have very poor drying efficiency if the film is left in place1 for not only the film but the apron or chain must be freed of water4 ,this may be hastened1 as in the ,g4 ,e4 ,m4 machine1 by blowing air through with fans1 but even with their help drying a #ce foot film is a matter of two hours or more4 ,passing the film through wringers or a squeegee to remove excess water is a considerable aid2 the film may either be re-wound on a dry reel1 to be put in a forced draft of air1 or may be hung up in short lengths or festooned1 either method taking up a great deal of space4 ,the use of alcohol is not advisable as it may abstract camphor from the celluloid and cause the film to become distorted4 ,the ,eastman twin reel machine had an upper reel joined to the lower or developing reel1 with a chain and sprockets1 so that the upper reel revolved at the same time and rate of revolution as the lower1 when the lower was being revolved at the gentle speed appropriate to the developing process4 ,fans blew a draft of air over the upper reel4 ,this method necessitated over an hour for drying4 ,illustration3 ,,fig4 #aag4--,u4 ,s4 ,air ,service film developing machine for film #bd centimeters wide4 ,the ,air ,service model of film developing and drying machine 7,fig4 #aag7 introduces an essential modification in the drying scheme of the ,eastman apparatus4 ,the upper reel is quite independent of the lower reel and is revolved at a high rate of speed1 so that a whirling action is introduced into the drying4 ,large rotating fans at the same time drive a considerable volume of air across the film surface1 and the combination of the two agencies makes it possible to dry #ce feet of #ah #bd centimeter film in #bj to #cj minutes4 ,this for large numbers of pictures makes the use of film even quicker than that of plates4 ,the only practical drawback to the apparatus is its bulk1 which calls for a separate room or trailer4 ,this1 however1 seems to be inevitable in the use of large roll film4 ,cut film can be dried with speed only if placed in a draft of warm air4 ,drying boxes1 with a chute or chimney and with fans to drive the air through from an alcohol stove1 will dry several dozen films in an hour4 ,the films must not be closer together than about one inch1 which makes the drying boxes rather cumbersome4 ,marking ,negatives4--,after development and drying1 and before filing or printing1 each plate should be marked with data for purposes of future identification4 ,this is most easily done with pen and ink on the film side 7in reversed lettering7 either along an edge in the unexposed portion covered by the sheath or in a corner1 so as to lose as little of the photograph as possible4 ,just what data shall be inscribed is dictated by the purpose for which the negative was made4 ,the date1 altitude1 time of day1 true north 7from known permanent features or from shadow direction and time of day71 number of the camera used1 the focal length of the lens4 ,other records1 such as the plane and squadron numbers1 or even the pilot's and observer's initials1 may be called for 7,fig4 #ge74 ,for mapping work the scale of each of a set of negatives1 once found1 may be marked1 either in figures or by means of a line of length corresponding to a fixed distance on the ground4 ,rectifying data can similarly be inscribed1 so that the negative can be printed in the enlarging and rectifying camera with the minimum of delay4 ,,chapter ,,xxiv ,,printing ,,and ,,enlarging ,contact ,printing4--,single prints are made most simply in a printing frame held at a short distance from a light source4 ,when any quantity must be made1 as in turning out prints at high speed for distribution to an army before an attack1 printing machines are employed4 ,these consist essentially of a light box1 a printing frame of plate glass1 and a pressure pad4 ,in the commercial models1 such as the ,crown and the ,ansco1 which are equipped with electric light1 merely bringing the pressure pad down and clamping it automatically turns on the light1 while release of pressure terminates the exposure4 ,the question of regulating the distribution of light is of considerable importance with negatives taken by focal-plane shutters of non-uniform rate of travel4 ,in the ,mc,intire printer 7,fig4 #aai71 the separate electric bulbs are on long necks in ball and socket joints1 so that they can be brought individually closer to the printing surface or farther away from it1 thus permitting a wide range of 8dodging40 ,this printer also has an automatic time control for the light1 a valuable device where many prints from the same negative are desired4 ,these machines are well suited for printing aerial negatives1 either plate or cut film1 if used where a source of electric current is available4 ,the chief defect1 which may be caused by faulty construction1 is imperfect contact between paper and negative1 a cause of serious unsharpness on prints destined for minute study in interpretation4 ,illustration3 ,,fig4 #aah4--,printing machine4 ,the printing of aerial negatives may be done either on roll or cut paper1 and if films are used1 a further alternative is offered of handling it either in the roll or in cut form4 ,where many prints are to be made from one negative roll paper has some advantages1 particularly if a developing and drying machine is available4 ,but for moderate numbers the advantage is small1 since cut prints can be developed quite conveniently in goodly numbers in the ordinary trays4 ,but the advantages of keeping film in the roll form are very great1 both in respect to storage and in respect to handling during printing1 as the rollers provide the necessary tension and prevent the film 8getting away40 ,illustration3 ,,fig4 #aai4--;,interior of ,mc,intire printer1 showing lamps adjustable in position for 8dodging40 ,illustration3 ,,fig4 #abj4--;,film printing machine4 ,for the ,american ,air ,service1 cut paper has been used exclusively4 ,for film printing1 the ,ansco machine has been equipped with roll pivots to take film #bd centimeters wide which may be advanced in either direction by turning large milled heads 7,fig4 #abj74 ,if we put rollers on the two remaining sides of the box to handle paper we transform the printer into the same form as a ,french machine1 in which paper and film are moved at right angles to each other4 ,a disadvantage of this modification1 however1 is the difficulty of examining the negative to be printed4 ,stereo ,printing4--,to make separate prints from the two elements of a stereoscopic pair and mount them side by side after proper orientation is too slow a process if quantities of prints are needed4 ,one method of multiple production is to make a master stereogram1 and then produce photographic copies of it1 but there is inevitable loss of quality in this copying process4 ,an intermediate method is to print from both negatives on the same sheet of paper4 ,in order to do this the negatives must be placed in rather large frames1 with mats properly located to guide the placing of the paper4 ,the ,richard double printing frame is a practical device which simplifies the necessary manipulations4 ,it consists essentially of a platform pierced with three illuminated openings4 ,the two negatives are compared1 superposed1 and orientated over the central opening and then shifted laterally1 one to each of the two side openings1 which serve both as printing frames and masks4 ,the printing back slides on a rod1 permitting the paper to be lifted up and moved between exposures4 ,once the negatives are properly placed1 stereo prints can be turned out quickly and easily4 ,enlarging4--,in the ,french service contact printing was the rule during the war4 ,the ,english practice1 on the other hand1 was to take small negatives--#d #e inches1 with #h to #ab inch lenses--and enlarge them1 usually to #f #h inches4 ,for this purpose a regular part of the ,english photo section equipment was the enlarging camera 7,fig4 #aae74 ,this may be briefly described as a short focus camera in which the subject to be photographed is a negative1 illuminated by transmitted light1 whose image is thrown by the camera lens on the paper or other sensitive surface4 ,by making the distance between negative and lens less than that between lens and paper1 the resulting print is an enlargement1 and vice versa4 ,the scale of enlargement or of reduction is varied over limits set only by the length of the camera and the amount of light available4 ,the lens employed must of course possess sufficiently high quality to preserve all the sharpness of the negative1 and focussing must be done with accuracy4 ,next to the lens the most important element is the light source4 ,this may be of the point form1 such as a concentrated filament electric lamp1 an oxy-acetylene lime light1 or an acetylene flame4 ,the latter was extensively used in the ,english service1 while acetylene generators for emergency purposes formed part of each ,american photo truck equipment4 ,with point light sources we must use condensers to focus the light into the projecting lens4 ,much less efficient1 but the only recourse where large condensers are not available1 is a diffusing glass behind the negative1 illuminated either by a bank of electric lamps with mirrors or by a ,u tube mercury vapor lamp1 where proper current can be got4 ,the device for holding the printing paper must permit quick changing1 but insure good contact4 ,we may use either a spring plate to hold the paper against plate glass from behind1 or else a weight acting on a lever arm of sufficient length4 ,the need for some automatic means of focussing an enlarging camera has been very generally felt4 ,an illustration of such an enlarging camera is that put out by ,williams1 ,brown ,earle1 of ,philadelphia1 known as the 8,semperfocal0 7,fig4 #aba74 ,in this camera the movements of the lens1 paper easel and negative are so inter-related and actuated with respect to each other that the correct focus of the instrument is maintained for any degree of enlargement or reduction4 ,this feature is a great help in making up mosaic maps1 where prints of continuously varying scale ordinarily occasion serious delay for individual focussing4 ,illustration3 ,,fig4 #aba4--8,semperfocal0 enlarging camera1 with mechanism for holding image in focus at any enlargement4 ,determining the correct enlargement for each negative of a mosaic is perhaps the most important problem in the use of the enlarging camera for aerial work4 ,the correct setting of the camera may be found by either of two methods3 the negative may be previously scaled and marked with a line on its edge1 which must be projected to a definite size2 or the true location of several points in the picture as obtained from an accurate map may be marked on the enlarging camera easel according to the desired scale1 and the negative image projected to coincide with these4 ,in either case1 if an exact scale is desired1 allowance must be made for paper shrinkage1 a matter which must be determined by previous experiment4 ,rectifying4--,negatives taken when the plane is not flying level will be distorted 7,figs4 #acd and #ace74 ,contact prints from these will not fit into a mosaic1 and no mere enlargement or reduction will make them available4 ,it is necessary with these negatives to resort to a rectifying camera4 ,this is an enlarging camera built so that the negative and print easel may be inclined about vertical and horizontal axes1 thereby purposely introducing a distortion sufficient to offset the distortion of the negative4 ,thus1 if the bottom of the printing surface is moved away from the lens1 that part of the picture will be enlarged2 if moved toward the lens1 reduced4 ,for small rectifications the common practice is to tilt the printing surface alone1 a method that is practical as long as this tilting does not affect the focus so much as to require prohibitive stopping down of the lens4 ,for great distortions1 such as that inherent in the principle of the ,bagley camera1 it is necessary to tilt both negative and print in order to preserve an approximate focus1 a given portion of the negative moving toward the lens as the corresponding portion of the print is moved away4 ,both schemes for rectification are shown diagrammatically in ,fig4 #abb4 ,developing and ,drying ,prints4--,the developing of prints follows closely that of cut or roll film1 and so need not be treated separately4 ,the drying of emulsions on paper is more easily accomplished than the drying of emulsions on glass1 for two reasons3 the emulsions on paper are much more thinly coated1 and there is diffusion of moisture into the atmosphere from front and back of the printing medium4 ,in the field a common method has been to soak the prints in water-free alcohol and then burn off the alcohol1 thus securing a dry print within two or three minutes after the conclusion of washing4 ,a later method very generally employed is to cover wooden frames three or four feet above the ground with chicken wire or muslin1 and on these lay the prints after soaking them in alcohol4 ,below the frames currents of warm air rise from pans of burning alcohol1 previously used to soak the prints and now useless as alcohol because of their high water content4 ,illustration3 ,,fig4 #abb4--;,diagram showing enlarging with and without distortion3 ,a1 enlarging without distortion2 ,b1 distortion for rectification of print1 by inclining printing surface2 ,c1 distortion1 for rectification of print1 by inclining both negative and printing surface4 ,before putting them in alcohol it is advisable to squeegee all the surface water from the prints4 ,this may be expeditiously done by removing them in mass from the final wash water upon a large ferrotype plate1 and either running the plate and prints together through a wash wringer with light pressure1 or covering the whole with a sheet of blotting paper and pressing out the water underneath by means of a rubber squeegee vigorously applied4 ,for base work one of the modern automatic print-drying machines used in commercial photography would be desirable4 ,glossy surfaces are given prints by the usual ferrotype plate method4 ,but this is too time-consuming for war practice1 and besides has but doubtful advantage where papers of the glossy type are chosen4 ,,vi ,,practical ,,problems ,,and ,,data ,,chapter ,,xxv ,,spotting 8,spotting10 as distinct from mapping or from the photography of continuous strips1 is the photography of a definite individual objective4 ,in military work spotting or 8pin pointing0 includes the photography of particular trenches or pivotal points in a trench system before an attack 7,fig4 #abc71 of roads or bridges along which an advance must pass 7,fig4 #abd71 of batteries or big guns which are the subject of artillery fire 7,fig4 #abe71 both before and after their bombardment 7,fig4 #abf71 of gun puffs or exploding bombs 7,fig4 #aca74 ,the technique of spotting consists largely in getting properly over the target and then securing the exposure at just the right moment4 ,this is chiefly a question of proper piloting2 but the aid which can be offered to the pilot by camera auxiliaries designed particularly for spotting needs is very large4 ,discussion of the task of the pilot who must steer a photographic plane accurately over a previously selected point of interest cannot be undertaken without raising the question of who should take the picture1 pilot or observer8 ,in the ,english service the most general practice was for the pilot to be charged with the responsibility both of covering the objective and of exposing4 ,if a propeller drive was used on the camera1 this left to the observer only the task of changing magazines4 ,if the camera was hand operated the plates were changed either by the observer1 or else1 as was frequently the case1 distance operating devices were attached1 so that the pilot even then did everything except change the magazines1 and the observer was kept free to watch the sky for enemy aircraft4 ,a very desirable adjunct to the camera when plates are shifted automatically or by the observer is a distance indicator1 to show the pilot when the shutter is set4 ,electrical indicators for this purpose have been devised4 ,illustration3 ,,fig4 #abc4--,low view of trenches on the ,yser1 showing concrete structures undamaged by bombardment4 ,illustration3 ,,fig4 #abd4--,the ,piave ,river4 ,to left1 destroyed bridge1 to right1 pontoon bridge under construction4 ,italian aerial photographic service4 ,illustration3 ,,fig4 #abe4--,showing big gun hidden in forest4 7,upper left-hand corner74 ,illustration3 ,,fig4 #abf4--;,example of spotting4 ,battery before and after bombardment4 ,if the camera is completely hand operated1 as were most of those in the ,french and ,german services1 there is little choice but for the observer to perform the entire operation4 ,the exposing operation could have been delegated to the pilot1 but such was not the custom with the ,french or with the ,american squadrons using ,french apparatus4 ,in this method of operation the observer depends on the pilot to get the plane over the target1 while the pilot depends on the observer to get the picture when the target is covered4 ,ample opportunity is thus offered for misunderstanding and disagreement4 ,this can be avoided only by excellent sights properly aligned1 for both pilot and observer1 and by some means of communication between the two men concerned4 ,illustration3 ,,fig4 #abg4--,photograph1 made with long focus lens to determine the results of aerial bombing4 ,the 8,tirpitz0 battery of long range naval guns directed on ,dunkirk4 ,the simplest means of communication is of course direct conversation4 ,but this is only possible in those planes1 such as the ,,dh-#i1 in which pilot's and observer's cockpits are immediately together1 so that1 by shouting1 any desired information can be conveyed with fair ease4 ,when the distance is increased to four or five feet1 as in the ,,dh #d1 the loudest shouts are totally lost in the roar of the engine and the blast of the wind4 ,speaking tubes and telephones are now fairly good1 but are none too comfortable or convenient to have strapped on one's head and face4 ,a primitive device used to some extent in the war was merely a pair of reins attached to the pilot's arms1 by which he could be directed which way to steer4 ,there is much to be said for a simple semaphore system1 where an indicator in the observer's cockpit actuates a similar dial in front of the pilot1 indicating 8right0 or 8left10 8picture obtained10 8try again10 etc4 ,if the observer has a sight by which he can see far enough ahead to correct the pilot's error of pointing1 the need for an accurate sight for the pilot is diminished4 ,illustration3 ,,fig4 #abh4--;,diagram showing relationship between focal length and area covered by plate4 ,illustration3 ,,fig4 #abi4--;,diagram giving data on area covered at various altitudes by representative lens4 ,illustration3 ,,fig4 #acj4--;,burchall ,slide ,rule1 for calculating intervals between exposures1 and for other aerial photographic data4 ,in considering the question of sights1 attention may again be called to the poor 8visibility0 from the pilot's seat in the present prevailing type of two-seater tractor plane4 ,blind directly in front1 beneath1 and to either side 7,figs4 #g1 #h and #i71 it is no unusual thing for a pilot to entirely miss an objective1 such as a railway line1 which he can only estimate to be beneath him by judging its distance from those objects to either side which he can actually see4 ,the ,english practice of leaving a clear space of six inches to a foot between the fuselage and the beginning of the wing fabric1 allows the pilot to look down over the side1 a decided advantage4 ,but for photographic purposes nothing can compare with a good negative lens carrying fore and aft lines or wires1 so that the pilot can see his objective in ample time to head directly for it4 ,the lens should either be large enough so that its rear edge gives the view directly downward1 or supplemented by an additional lens pointing directly down1 so that the covering of the target is assured4 ,to locate such a lens in the front cockpit1 free of all controls1 is a very hard task2 even so its view is likely to be badly interrupted by the landing gear4 ,nevertheless1 so important is it1 both in photography and in bombing1 to have a sight by which the plane can be accurately directed that designers of planes should recognize this need and make every effort to provide a suitable location4 ,illustration3 ,,fig4 #aca4--;,aerial bombardment of ,trieste4 ,note falling bombs in center of picture2 and exploding anti-aircraft shells over the water4 ,italian official photograph4 ,illustration3 ,,fig4 #acb4--;,example of spotting requiring exposure at exact instant4 ,explosion following burst of bomb in ammunition dump4 ,british official photograph4 ,sights for the observer have been discussed already4 ,here again the negative lens is to be preferred1 but while the pilot's lens needs only directing lines in the axis of the plane 7unless he takes the picture71 the observer's lens needs both an accurate center mark and an additional upper or lower sighting point4 ,accurate alignment of these marks with the camera axis must be arranged for in precise spotting4 ,accurate spotting work requiring the delineation of fine detail calls for cameras of considerable focal length4 ,the camera of longest focal length used in the war was the ,french #abj centimeter 7,fig4 #da74 ,this was employed with great success in such work as regulating the fire of heavy railway guns brought into range only at night1 to fire a few shots at chosen angles4 ,photographs taken the next day would then show the exact spot where each shell fell1 and the damage it did1 to serve as a guide for the next night's operations 7,fig4 #abg74 ,the field of these cameras is quite small--#h to #ab degrees--and so not only must sighting be exact but the area covered on the ground must be accurately known4 ,this is to be calculated from the altitude1 focal length1 and plate size1 by the relation-- distance on ground altitude ------------------------------------ ------------------------ plate length focal length ,data derived from such calculations may be incorporated in tables1 or graphically in diagrams such as ,figs4 #abh and #abi4 ,illustration3 ,,fig4 #acc4--,the same subject a few minutes later4 ,height of smoke shown by shadow4 ,british official photograph4 ,these calculations and others required in mapping and stereo-work are simply and quickly made by slide-rule devices4 ,one of these1 the ,burchell ,photographic ,slide ,rule1 developed in the ,english service1 is shown in ,fig4 #acj4 ,this consists of two dials1 the center one of which is mounted--usually by a pin pushed into a cork behind--so as to turn freely1 to permit its being set for altitude1 focal length1 ground speed1 plate size1 etc41 whereupon the area covered1 or the appropriate interval between exposures may be read off4 ,cameras for spotting work should be capable of exposure at the exact moment desired4 ,for if the camera is ever to catch the gun as it discharges1 the bomb as it falls 7,fig4 #aca71 or the shell as it explodes 7,fig4 #acb71 the photograph must be taken within the instant4 ,automatic cameras1 exposing at regular intervals1 while adequate for mapping1 are not fitted for many kinds of spotting4 ,,chapter ,,xxvi ,,map ,,making ,technique of ,negative ,making4--,stated in its simplest terms1 the whole problem of making a photographic map from the air consists in taking a large number of slightly overlapping negatives1 all from the same altitude1 with the plane flying uniformly level4 ,when trimmed and mounted in juxtaposition1 or pasted together so as to overlap in their common portions1 the prints from these negatives constitute a complete pictorial map4 ,there is thus furnished by a few hours' labor topographic information which would be the work of months to obtain by other means4 ,the making of map photographs involves all the special technique of spotting1 with much in addition4 ,the pilot's task is not merely to go over one object2 he must navigate a narrow path1 at a constant altitude1 on an even keel4 ,if he is to make not merely a ribbon1 but a map of considerable width1 he must take successive trips parallel to the first1 each displaced just far enough from the previous course to insure that no portion is missed--a difficult task indeed4 ,it is the observer's duty to so time the intervals between exposures that they overlap enough1 but not so much as to be wasteful of plates or film4 ,he must also change magazines or films so quickly as to miss no territory1 or if some be missed1 his is the task of directing the pilot back to the point of the last exposure1 where they begin a new series4 ,level flying is entirely a pilot's problem4 ,its importance will be realized when we consider the accompanying diagrams 7,figs4 #acd and #ace71 where the effect on the resultant picture is shown of climbing1 gliding1 or banking to either side4 ,prints from negatives distorted in this way neither will be true representations of the territory photographed1 nor will they match when juxtaposed4 ,in fact1 they can be utilized only if special rectifying apparatus is available for printing4 ,flying at a constant altitude is similarly necessary if the prints are to be utilized without enlargement or reduction in order to make them fit4 ,illustration3 ,,fig4 #acd4--;,diagram showing effect of banking on aerial photograph4 ,illustration3 ,,fig4 #ace4--;,diagram showing effect of climbing and diving on aerial photograph4 ,assuming a skilled pilot who will do his part1 the next step is to calculate the exposure intervals in order to insure an adequate overlap4 ,if a negative lens is installed which has been marked with a rectangle the size of the camera field1 the simplest method is to estimate the proper instant for exposure by watching the progress of objects across the lens face4 ,this of course requires constant attention1 and it is easier to do this only occasionally1 in order to determine the ground speed in terms of camera fields traversed per minute4 ,thereafter exposures are to be made by time1 as determined by a watch or clock4 ,any desired degree of overlap can be chosen1 and either estimated1 or more or less accurately fixed by lines marked on the negative lens at a shorter distance apart than the edges of the field4 ,the most usual overlap is #bj per cent41 except for stereos1 which call for #ej to #ge per cent4 ,in the absence of a negative lens or some other sight to show the whole camera field1 it is necessary to resort to calculation from the speed and altitude of the plane1 the focus of the lens and the dimensions of the plate4 ,if ,a is the altitude1 a the focal length of the lens1 d the diameter of the plate in the direction of travel 7usually the short length is chosen for economy of flights to cover a given width71 f the fractional part by which one negative is desired to overlap the next1 and ,v the ground speed of the plane1 then we have1 by simple proportion1 that the interval between exposures1 t1 must be-- ,ad7#a - f7 t -------------------------- a,v ,if ,a #bjjj meters1 d #ah centimeters1 f 1 a #ej centimeters1 and ,v #bjj kilometers per hour1 this relation gives-- #bjjj 4#ah 4#h #cfjj t ---------------------------------------- ---- #aj4c seconds 4#e #bjj1jjj ,the principle of overlapping map exposures is shown in the accompanying diagram 7,fig4 #abi71 together with data calculated as above for a #d #e inch plate4 ,it is particularly to be noted that it is the ground speed of the plane that is used4 ,this may be calculated by knowing the air speed and the wind velocity and direction4 ,fig4 #acf shows the method of doing this graphically4 ,first an arrow is drawn representing the direction it is desired to fly4 ,next a second arrow is drawn of length to represent the wind velocity4 ,this must be inclined toward the first arrow in the direction of the wind1 and its head is to touch the head of the first arrow4 ,then with the farther end of this second arrow as a center1 describe a circle of such a length as to represent the air speed of the plane1 in the same units as the wind velocity4 ,connect the point where this circle cuts the arrow of flight direction to the center of the circle by a straight line4 ,this line constitutes the air speed arrow1 giving the direction it is necessary to fly1 at the given air speed1 to make the course desired4 ,the length of the flight direction arrow between its head and its point of intersection with the air speed arrow gives the ground speed4 ,illustration3 ,,fig4 #acf4--;,diagram showing method of calculating ground speed from air speed and wind velocity4 ,when the wind is ahead or astern this calculation reduces to the simple subtraction or addition of the wind velocity to the air speed of the plane4 ,whenever possible1 mapping should be done up and down the wind 7,fig4 #acg74 ,if the plane is 8crabbing10 the above calculations for overlap are only valid if the camera can be turned normal to the direction of travel over the ground4 ,if the camera cannot be so turned the corners of the successive pictures overlap instead of their sides1 with quite unsatisfactory results 7,fig4 #ach74 ,calculation of the distance apart of the parallel flights necessary to make a map of any width is done by the use of a formula similar to the longitudinal overlap formula above1 distance figuring instead of time4 ,using the same symbols1 and denoting the distance by ,d1 we have-- ,ad7#a - f7 ,d -------------------------- a ,with the same figures as before1 but substituting #bd centimeters for the plate dimension1 this relation gives-- #bjjj 4#bd 4#h ,d ------------------------------ #gfh meters 4#e ,illustration3 ,,fig4 #acg4--,overlaps made when flying with or against the wind4 ,illustration3 ,,fig4 #ach4--,unsatisfactory overlaps made when plane is 8crabbing40 ,it is of course largely a pilot's problem to steer the plane over parallel courses at a given distance apart1 although the observer1 noting conspicuous objects through a properly marked negative lens1 may direct the pilot by any of the means of communication already mentioned4 ,an alternative method of securing parallel strips1 which is to be highly recommended where enough photographically equipped airplanes are available1 is for several planes to fly side by side1 maintaining their proper separation 7,fig4 #aci74 ,cameras and ,auxiliaries for ,map ,making4--,mapping can be done quite satisfactorily by hand operated or semi-automatic cameras1 provided the observer has not too many other duties4 ,on the other hand1 the operation of exposing at more or less definite intervals of time1 irrespective of the object immediately presented to the camera1 is a largely mechanical one4 ,it naturally suggests the employment of an automatic mechanism1 whose speed of operation only is it necessary to watch4 ,if a non-automatic camera is used the timing of exposures may be done by watching a negative lens1 as described above1 or by reference to a clock1 assuming that the ground speed is known through calculation4 ,a very practical advance over the ordinary use of a clock is to attach a stop-watch to the shutter release1 so that it is turned back to zero and re-started at each exposure 7,fig4 #gj74 ,in passing1 it may be noted that if the stop-watch hand makes an electric contact which throws the shutter release1 then the device constitutes an attachment for turning any semi-automatic camera into an automatic4 ,the most suitable cameras for mapping are unquestionably those of the entirely automatic type4 ,the use of such cameras always demands a knowledge of the ground speed4 ,this demand has led to many suggestions for ground speed indicators4 ,the common idea of these is to provide a moving part on the plane--either a disc of large diameter1 or a chain1 or a revolving screw--whose speed may be varied until any point upon it appears to keep in coincidence with a point on the moving landscape below4 ,the ground speed is then to be read off a properly calibrated dial4 ,or1 as a further step1 the frequency of the exposures may be directly controlled by the ground speed indicator mechanism4 ,the entire control of the camera would then consist merely in occasional adjustment of the ground speed indicator4 ,illustration3 ,,fig4 #aci4--,planes starting out to make a map by flying in parallel4 ,while entirely possible in theory1 these devices are not easy to work with in practice1 because the plane is always subject to some pitching and rolling1 which make it difficult to hold any object constantly on the moving point4 ,this is especially true at high altitudes1 where the apparent motion of the earth is quite slow compared to the swervings of the plane4 ,this objection is in part removed if the ground speed indicator is carried by a gyro stabilizer4 ,ordinary mapping does not demand such exquisite rendering of detail as does trench mapping4 ,nor is it necessary to fly in peace-time at such high altitudes as in war4 ,in consequence1 mapping cameras are preferably of the short focus1 wide angle type1 say1 #be centimeter focus for an #ah #bd centimeter plate4 ,film is to be preferred over plates because of the greater number of exposures it is possible to make on a flight4 ,the shutter of the mapping camera must be extremely uniform in its rate of travel so that the elements of the map may match in tone 7,fig4 #adj74 ,a mount which permits the camera to be turned normal to the direction of flight1 such as the ,british turret mount 7,fig4 #hg71 is particularly desirable if flying across the wind is necessary1 as will often be the case in mapping strips between towns or between flying fields4 ,devices to indicate compass direction and altitude are called for in new and poorly mapped territory1 and may be expected to receive intensive study in the future4 ,the question of their utility is1 however1 bound up with the whole question of the sphere of aerial photographic mapping4 ,up to the present this has been almost entirely a matter of filling in details on maps obtained by the regular surveying methods1 or of making pictorial maps for aviators4 ,to what extent primary mapping can be done by the airplane is yet to be determined4 ,illustration3 ,,fig4 #adj4--;,a strip map1 showing effect of uneven focal plane shutter action4 ,at this point mention must be made of special cameras for securing extremely wide angle views1 thereby minimizing the number of flights4 ,the ,bagley camera1 devised by ,major ,bagley of the ,u4 ,s4 ,engineers1 is an example4 ,it has three lenses1 a middle one pointing directly downward1 and one to either side at an angle of #ce degrees4 ,the pictures obtained with the side cameras are of course greatly distorted1 and must be rectified in a special rectifying camera4 ,the resultant definition is not good1 but as the maps are made on a much smaller scale than the original pictures1 this is not a serious objection4 ,it is a matter for the future to decide whether the additional labor on the ground necessary for the rectifying process is to be more expensive than the extra flights which must be made with the ordinary types of cameras covering a smaller angle4 ,printing and ,mounting ,mosaics4--,with an ordinary set of overlapping negatives the first step toward producing a map is to scale the negatives4 ,for this purpose one should be selected which by comparison with a map shows no distortion1 and which is on the desired scale1 or is known to have been made at the average altitude of flight4 ,a sketch map of the territory should then be drawn1 on this scale1 based on available maps4 ,this sketch is preferably made on a large ground glass illuminated from behind 7,fig4 #ada74 ,on this all the negatives should be laid1 and their proper relative positions sought4 ,when this is done it is evident at once whether all the territory has been covered1 and whether there are any superfluous negatives4 ,each negative should then be examined as to its scale and distortion4 ,if it can be made to fit the scale by simple enlargement or reduction1 a line can be drawn on one edge of a length indicating its scale4 ,this line will later be used as a guide in the enlarging camera4 ,if the picture is badly distorted it must either be replaced by another negative1 or if rectifying apparatus is available1 it must be set aside for the making of a rectified print4 ,illustration3 ,,fig4 #ada4--,scaling negatives for mosaic map-making4 ,the next step is to make prints from the negatives1 which may be done either by contact1 or1 necessarily if differences of scale must be compensated1 in the enlarging camera4 ,if prints to an exact scale are required the shrinkage of the paper must be determined and allowed for4 ,the prints must all show the same tone1 and must be uniform from edge to edge4 ,if the focal-plane shutter is not uniform in its travel1 as is frequently the case1 this means that the print must be 8dodged10 or exposed more at one edge than the other1 by locally shielding the plate and paper during exposure4 ,a case of the step-like effect caused by uneven shutter action is shown in ,fig4 #adj4 ,the effect due to uneven shutter action is of course absent with a between-the-lens shutter1 which constitutes a strong argument in favor of that type for use in mapping cameras4 ,illustration3 ,,fig4 #adb4--;,arranging prints for a mosaic map4 ,when the prints are made they must be mounted together on a large card or cloth background4 ,for a very small mosaic they may be juxtaposed by simple examination1 matching corresponding details in successive prints4 ,for a mosaic of any size an accurate outline map must be drawn on the surface to which the prints are to be attached4 ,the prints are then laid out on this outline1 moved to their correct positions1 and held down by pins 7,fig4 #adb74 ,when they are all arranged the final mounting may be begun4 ,the excess paper1 beyond what is necessary for safe overlaps1 may be trimmed off1 exercising judgment as to which print of each adjacent pair is of the better quality1 and utilizing it for the top one at the overlapping junction4 ,if one print shows serious distortion it may be placed under its fellows on all four edges1 thus minimizing its weight4 ,the edges are best made irregular by tearing4 ,straight edges are apt to force themselves on one's attention in the final mosaic and give an erroneous impression of the existence of straight roads or other features4 ,both forms of edging are shown in ,figs4 #abd and #adc4 ,an alternative method of securing the final print mosaic1 where film negatives are used1 is to trim successive film negatives so that the trimmed sections will exactly juxtapose1 instead of overlap4 ,the sections are then mounted1 by stickers at their edges1 on a large sheet of glass1 and printed together4 ,captured ,german prints show that this was the method commonly used with the ,german film camera 7,fig4 #fb74 ,it will be noted that the procedure which has been described and illustrated by ,figs4 #adb and #adc assumes the previous existence of a map accurately placing at least the chief features of the country covered4 ,this draws attention at once to the limitations and true sphere of aerial photographic mapping at the present time4 ,with the cameras thus far it is not possible1 nor is it attempted1 to do primary mapping of unknown regions4 ,distortions due to lens1 shutter1 film warping and paper shrinkage considerably exceed the figures permitted in precision mapping4 ,from the standpoint of geodetic accuracy the cumulative errors of deviations in direction1 altitude and level1 peculiar to flying1 would soon become prohibitive4 ,illustration3 ,,fig4 #adc4--;,a partly completed map4 ,prints mounted over an outline sketch map to proper scale4 ,the great field for aerial photographic mapping in the near future lies in filling in detail on maps heretofore completed as to general outlines1 or1 as in the war1 on maps far out of date4 ,the war-time procedure in country largely unknown1 such as ,mesopotamia1 was probably closely that which will be necessary in peace4 ,conspicuous points in the landscape were first triangulated from friendly territory1 and from these the outline map was drawn1 whose details were to be supplied by aerial photographs4 ,much of the 8mapping0 of cross country aerial routes so far done is frankly of a pictorial nature1 showing conspicuous landmarks and good landing fields--extremely valuable and useful1 but not to be confused with precision mapping4 ,in assembling mosaics of this kind the elaborate procedure described above is not followed4 ,the process is the simple one of juxtaposing adjacent prints as accurately as possible by visual examination4 ,errors are of course cumulative1 but as long as exact distances are not in question this is no matter4 ,,chapter ,,xxvii ,,oblique ,,aerial ,,photography ,oblique views from the airplane are of very great value4 ,while vertical views are more searching in many respects1 they do nevertheless present an aspect of the earth with which ordinary human experience is unfamiliar4 ,consequently they are difficult to interpret without special training4 ,they suffer1 too1 from the military standpoint1 from the limitation that it is with vertical extension just as much as with horizontal that an army has to contend in its progress4 ,elevations and depressions of land show on an oblique view where they would be entirely missed in a vertical one4 ,for illustration1 study the picture of part of the outskirts of ,arras 7,fig4 #add71 presenting moat1 walls and embankments1 all of which would be serious obstacles1 but would hardly be noticed on a vertical view4 ,pictures taken from directly overhead are eminently suited to artillery use1 but oblique views of the territory to be attacked1 taken from low altitudes1 formed an essential part of the equipment of the infantry in the later stages of the war4 ,pictorially1 oblique views are undoubtedly the most satisfactory4 ,the most revealing aspect of any object is not one side or face alone1 but the view taken at an angle1 showing portions of two or three sides4 ,best of all is that taken to show portions of front1 side and top--the well-known but heretofore fictitious 8bird's-eye view0 7,fig4 #ade74 ,this possibility is ordinarily denied the surface-of-the-earth photographer1 but the proper vantage point is attained in the airplane4 ,aerial obliques may be taken at any angle1 although a distinction is sometimes made between obliques of high angle and panoramic or low angle views 7,fig4 #adf74 ,in addition to ordinary obliques1 a very beautiful development is the stereo oblique4 ,both kinds of oblique photography call for special instrumental equipment and technique4 ,illustration3 ,,fig4 #add4--,the outskirts of ,arras4 ,low oblique showing contours4 ,methods and ,apparatus for ,oblique ,photography4--,the simplest method of taking oblique pictures from a plane is to use a hand camera pointed at the desired angle4 ,its limitations are in the size and scale of the picture obtainable1 and in the inherent limitations to the method of camera support4 ,a step in advance of this is to mount the camera above the fuselage1 on the machine gun ring or turret1 in place of the gun4 ,considerably greater rigidity is thus obtained1 and heavier cameras can be utilized1 although the wind resistance is a serious factor4 ,excellent obliques have been made in this way1 even with #ej-;centimeter cameras1 but the scheme is impractical in military planes1 because of the removal of machine gun protection4 ,illustration3 ,,fig4 #ade4--,oblique view of ,capitol and ,congressional ,library1 ,washington4 ,illustration3 ,,fig4 #adf4--;,fort ,alvenslegen1 near ,metz4 ,photo by ,photographic ,section ,a4 ,e4 ,f4 ,if the camera is fixed in the fuselage in its normal vertical position1 obliques may be and have been taken by the simple expedient of banking the plane steeply4 ,this is not to be recommended as a standard procedure1 especially for taking a consecutive series of exposures4 ,the most satisfactory arrangements for taking obliques are two2 first1 to mount the camera obliquely in the plane1 and second1 to use a mirror or prism1 in front or behind the lens of the vertically mounted camera4 ,the first method has been employed chiefly by the ,french1 the latter by the ,english1 whose gravity fed cameras could not be mounted obliquely4 ,taking up first the oblique mounting of cameras1 we find two ways of doing this3 longitudinal mounting and lateral mounting4 ,in longitudinal mounting the camera projects forward and downward1 usually from the nose of a pusher or bi-motored plane4 ,with this form of mounting 7,fig4 #adg7 it is necessary of course to fly directly toward the objective4 ,if this is a portion of enemy trench1 which must be photographed from a height of #djj or #ejj meters1 the plane will be directly on top of its objective a few seconds after the exposure is made1 and be a conspicuous target1 in imminent danger of destruction4 ,moreover1 only a single short section of the trench would be obtained for each crossing of the line4 ,the one case where resort to this method is practically forced is with the #abj-;centimeter cameras which simply cannot be slung athwart the plane4 ,there is a slight advantage in this method of carrying in that the motion of the image is less if the objective is approached1 instead of being passed at the side1 and so longer exposures can be made4 ,the longitudinal mounting has1 however1 been very generally superseded by the lateral4 ,illustration3 ,,fig4 #adg4--abj-;centimeter camera mounted obliquely in the fore-and-aft position4 ,methods for mounting cameras obliquely for taking pictures through the side of the plane have been discussed in detail in connection with camera mountings and installations 7,fig4 #ic74 ,the chief difficulties are want of space1 obstacles at the side such as control wires and longerons1 and failure of the camera to function properly at an angle4 ,even in the broad circular sectioned fuselage of the ,salmson plane1 quarters are so cramped that the ,french #ej-;centimeter camera when obliquely mounted cannot be used with the #ab-plate magazine1 and recourse is made to thin flat double plate-holders4 ,holes in the side of the fuselage should clear all wires and should command a view unobstructed by the wings--which often means that the camera must be carried behind the observer's cockpit1 irrespective of the suitability of that space from other standpoints4 ,cameras dependent for their action on gravity1 such as the de,ram and ,english ,l type1 are unsuited for oblique suspension4 ,illustration3 ,,fig4 #adh4--,mirror on camera cone for taking oblique views4 ,for cameras which1 because of their method of operation or shape cannot be slung obliquely1 the only way to take obliques is to employ mirrors 7,fig4 #adh7 or prisms4 ,these must be of the same optical quality as the photographic lens4 ,they are both necessarily of considerable weight because they must be of large area of face to fill the entire aperture of an aerial lens4 ,mirrors are lighter than prisms1 but must be quite thick to prevent distortion of the surface due to any possible strains to their mount4 ,right angle glass prisms have been used by the ,english with the #h and #aj inch ,l cameras4 ,the prisms were uniformly tilted to an angle of #ab degrees from the horizontal4 ,glass mirrors can be silvered either on the rear or front surface4 ,if on the rear1 both surfaces must be accurately parallel1 which means much greater labor and expense than if the front surface can be utilized4 ,the difficulty with front surface mirrors is that the metallic coating is easily tarnished or scratched1 especially if silver is used1 which is almost imperative1 since all the other metals have considerably lower reflecting powers4 7,gold might serve both as mirror and color filter1 because of its yellow color47 ,placing the mirror inside the camera body in part obviates this trouble1 but means the use of a special elbow lens cone4 ,in any case the mirror or prism occasions at least a #aj per cent4 loss of light4 ,pictures taken by reflectors of any kind are reversed1 and must either be printed in a camera1 or on transparent film which may be viewed from the back4 ,the most usual condition for making obliques is to fly very low 7#cjj to #fjj meters71 with the line of sight of the camera from #ab to #de degrees from the horizontal4 ,this low altitude necessitates very short exposures1 to avoid movement of the image4 ,the picture may be taken either the long or the short way of the plate1 depending on the character of the object and the information desired4 ,it is to be noted that successive oblique pictures cannot be mounted to form a continuous panorama--this being possible with obliques only if they are taken from one point1 as from a captive balloon4 ,if successive views are made on a straight flight at intervals so as to exactly juxtapose in the foreground1 they overlap by a large margin the middle1 and a point on the horizon1 if that shows1 will be in the same position in every picture4 ,mosaics of obliques could be made only by some system of conical mounting4 ,sights for ,oblique ,photography4--,any of the sights previously discussed for vertical work1 such as the tube sights1 are applicable to obliques4 ,they must1 however1 be suited for mounting at an angle1 in a position convenient for the observer4 ,in addition1 provision must be made for adjusting the angle so that the lines of sight of camera and finder are parallel4 ,mounting outside the fuselage is practically the only feasible way1 and is less objectionable with oblique than with vertical sights1 as oblique sighting does not require the observer to stand up and lean over the edge of the cockpit4 ,windows in the side of the fuselage1 either of celluloid or non-breakable glass1 are a great aid to oblique observation4 ,marks upon the transparent surface can be utilized for the rear points of a sight of which the front point is a single fixed bead or rectangle4 ,,chapter ,,xxviii ,,stereoscopic ,,aerial ,,photography ,one of the most striking and valuable developments in aerial photography has been the use of stereoscopic views4 ,pairs of pictures1 taken with a considerable separation in their points of view and studied later by the aid of the stereoscope1 show an elevation and a solidity which are entirely wanting in the ordinary flat aerial vista4 ,often1 indeed1 these attributes are essential for detecting and recognizing the nature of objects seen from above4 ,stereoscopic aerial photography has been justly termed 8the worst foe of camouflage40 ,principles of ,stereoscopic ,vision4--,the ability to see objects in relief is confined solely to man and to a few of the higher animals in whom the eyes are placed side by side4 ,when the eyes are so placed they both see1 to a large extent1 the same objects in their fields of view4 ,owing to the separation of the eyes the actual appearance of all objects not too far away is different1 and it is by the interpretation of these differences that the brain gets the sensation of relief4 ,thus in ,fig4 #adi the two eyes are shown diagrammatically as looking at a cube4 ,the right eye sees around on the right-hand face of the cube1 the left eye on the left-hand face of the cube4 ,the two aspects which are fused and interpreted by the brain are shown in the lower diagram4 ,stereoscopic views or stereograms1 made either by photography or1 in the early days1 by careful drawing1 consist of pairs of pictures made of the same object from two different points4 ,for ordinary stereoscopic work these points are separated by the distance between the eyes1 approximately #fe millimeters or #b inches4 ,these two pictures are then so viewed that each eye receives its appropriate image from the proper direction1 whereupon the object delineated stands out in relief4 ,illustration3 ,,fig4 #adi4--,the principle of stereoscopic vision4 ,fusion of the two elements of the stereoscopic picture can take place without the assistance of any instrument1 if the eyes are properly directed and focussed1 but this comes only with practice4 ,holding the stereogram well away from the face the eyes are directed to a distant object above and beyond1 in order to diverge the axes4 ,then without converging1 the eyes are dropped to the picture1 which should spring into relief4 ,it is necessary in moving the eyes from the distant object to the near stereogram to alter their focus somewhat1 depending on how near the stereogram is held2 and the success of the attempt to fuse the images depends on the observer's ability to maintain the eyes diverged for a distant object while focussing for a near one4 ,near-sighted people 7on taking off their glasses7 fuse the stereoscopic images quite easily1 since their eyes do not focus on distant objects even when diverged for them4 ,transparencies are easier to fuse than paper prints1 but in any case where a stereoscope is not used the separation of image centers should not be more than that of the eyes4 ,illustration3 ,,fig4 #aej4--;,common form of prism stereoscope4 ,stereoscopes4--,the easier and more usual method of fusing the stereoscopic images is by a stereoscope4 ,the simplest form consists merely of two convex lenses1 one for each eye1 their centers separated by a distance somewhat greater than that between the eyes4 ,their function is to bring the stereogram to focus1 and1 by the prismatic action of the edges of the lenses1 to converge the lines of sight which pass through the centers of the two pictures to a point in space in front of the observer4 ,the two lenses should be mounted so as to provide for the adjustment of their separation to fit different eyes and print spacings4 ,the most common form of stereoscope is that designed by ,holmes1 for viewing paper print stereograms 7,fig4 #aej74 ,it has prismatic lenses of an appropriate angle to converge pictures whose centers are three inches apart1 instead of the lesser distance appropriate to stereograms intended for fusing without an instrument4 ,no adjustment is provided for varying the lens separation1 but the print can be moved to and fro for focussing4 ,illustration3 ,,fig4 #aea4--;,box stereoscope4 ,illustration3 ,,fig4 #aeb4--;,diagram of mirror stereoscope4 ,another form of stereoscope1 one of the first produced1 is the mirror stereoscope 7,fig4 #aeb71 now used extensively for viewing stereo ,x-ray pictures4 ,it consists of two vertical mirrors at right angles to each other1 with their edge of contact between the eyes4 ,the two prints to be studied are placed to right and left1 an arrangement that permits the use of prints of any size4 ,the convergence point is controlled by the angle between the mirrors4 ,the ,pellin stereoscope 7,fig4 #aec7 utilizes two pairs of mirrors in a way to permit the use of large prints4 ,the prints are1 however1 placed side by side on a horizontal viewing table1 which avoids certain difficulties of illumination met with in the simpler mirror form4 ,the box form of stereoscope 7,fig4 #aea7 using either prisms or simple convex lenses1 is particularly adapted for viewing transparencies1 although the insertion of a door at the top provides illumination for paper prints4 ,the ,schweissguth design 7,fig4 #aed7 is intended primarily as an aid to selecting the portions of the prints to be cut out for mounting4 ,the platform on which the pictures rest is composed of two long rectangular blocks1 on which are plates of glass raised sufficiently to permit the prints to be slid underneath4 ,the space between the blocks allows the unused portion of the photograph to be turned down out of the way4 ,prints of any size can thus be moved about until the proper portions for stereo mounting are found4 ,either block can be moved in its own plane and also to and from the eye1 whereby two prints of somewhat different scales can be fused4 ,illustration3 ,,fig4 #aec4--,pellin double mirror stereoscope4 ,the ,taking of ,aerial ,stereograms4--,the normal separation of the eyes is altogether too small to give an appearance of relief to objects as far away as is the ground from a plane at ordinary flying heights4 ,in order to secure stereoscopic pairs it is therefore necessary to resort to a method originally employed for photographing distant mountains and clouds4 ,this is to take the two pictures from points separated by distances much greater than the interocular separation--by meters instead of millimeters--corresponding to the positions of the eyes on a veritable giant4 ,in the airplane this is accomplished by making successive exposures as the plane flies over the objective1 at intervals to be determined by the speed1 the altitude and the amount of relief desired 7,fig4 #aee74 ,an all important question which arises immediately is3 ,what separation of points of view shall we select8 ,if the exposures are too close together there will be little relief2 if too distant the relief will be so great as to be unnatural1 even offensive4 ,obviously we cannot here establish a criterion of natural appearance1 since the natural appearance to ordinary human eyes is devoid of relief4 ,we may1 however1 define correct relief as that obtained when the apparent height of elevated objects is right as compared with their extension or plan4 ,illustration3 ,,fig4 #aed4--,schweissguth stereoscope1 used for selecting portions of prints to be mounted4 ,illustration3 ,,fig4 #aee4--,method of taking stereoscopic pictures4 ,in order to secure this condition it is necessary1 first1 that each element of the stereoscopic pair be correct in its perspective4 ,this is fortunately an old photographic problem1 already well understood4 ,its solution is to view the photograph from a distance exactly equal to the focal length of the camera lens4 ,since the normal viewing distance is not less than #be centimeters1 lenses of this focal length at least are requisite for correct perspective4 ,secondly1 it is necessary for correct relief that the two views be taken with a separation equal1 on the plane of the plate1 to the separation of the eyes1 or #fe millimeters4 ,if d is the interocular distance1 a the viewing distance1 identical with the focal length of the lens used1 and ,a the altitude1 then ,d1 the distance between exposures1 is given by the relation-- d ,d ------ ------ a ,a ,for a #be centimeters1 ,d_/,a #f4e_/be1 approximately 1 or the interval between exposures must be a quarter the altitude4 ,with a #ej centimeter lens this becomes 1 and so on4 ,these figures show the fallacy of the suggestion sometimes made that we take stereoscopic pictures by two cameras placed one at the extremity of each wing4 ,when lenses of more than #be centimeters focal length are employed1 the stereoscope should be one capable of throwing the convergence point farther away than the customary #be centimeters4 ,in the simple lens type of instrument we can do this by bringing the centers of the lenses closer together1 and by making the focus agree with the convergence point by adjustment of the distance between lenses and stereogram4 ,if enlargements are used they should be treated in all respects as originals made by lenses of the greater foci corresponding to the scale of the enlargement4 ,when all the conditions are covered1 the appearance presented in the stereoscope is that of a model of the original object at a distance a1 and a_/,a times natural size4 ,if pictures are made at exposure intervals less than those indicated for correct relief1 they show insufficient relief4 ,this does not1 however1 give an unnatural effect1 because anything between no relief and 8correct0 relief appears natural with large objects which are not ordinarily seen in relief by eyes not ,brobdignagian4 ,conversely1 stereograms made with too large exposure intervals show exaggerated relief4 ,yet this is often no objection4 ,it is indeed rather an advantage if we wish to bring objects of interest to notice4 ,consequently1 so long as the exaggeration of relief is not offensive1 the permissible limits of exposure interval are pretty large4 ,actually1 the eye tolerates such great deviations from strictly normal conditions that satisfactory stereoscopic effects are obtained for pictures viewed at very different distances from the focal length of the taking lens1 and with the axes of the eyes parallel or even diverging1 although there is some strain whenever focus and convergence points differ4 ,on the whole1 therefore1 it may be said that the conditions above laid down for correct relief are only a normal1 to be approximated as nearly as is practicable4 ,having established the correct relation of taking points for stereos the next problem is how to determine these when in the plane4 ,the simplest way is by means of a stereoscopic sight4 ,this consists essentially of two lines of sight 7fixed by beads1 crosses1 or other objects71 inclined toward each other at the angle determined by the ratio of the ocular separation to the focal length of the lens4 ,if the back sight is made a single bead or cross1 the rest of the stereo sight will consist of two beads or crosses1 separated from each other by the ocular distance of #fe millimeters1 and distant from the back sight by the focal length of the lens 7,fig4 #aeg74 ,the first picture is taken when the object is in line with the forward pointing line of sight1 the second when it lies along the backward pointing one4 ,like other sights1 the stereoscopic sight may be attached either to the camera1 or if this is fixed in position1 to any convenient part of the plane4 ,a very simple sight for vertical stereoscopic photography consists of an inverted ,v painted on the side of the fuselage1 so that the eye can be placed at the vertex and sighted along either leg4 ,the common method of determining the space between exposures is by the time interval4 ,if ,v is the speed of the plane1 and t the desired time interval1 we have1 from the last equation-- ,d d,a t ------ -------- ,v a,v ,if ,a #bjjj meters1 d #fe millimeters1 and a #be centimeters1 and if the plane is traveling #bjj kilometers per hour1 the time interval must be-- 4#jfe #bjjj #cfjj ------------------------------------ #i4d seconds 4#be #bjj1jjj ,at #ajjj meters altitude the interval will be half this1 and so on in proportion4 ,if the pictures are taken with a #ej centimeter focus camera1 and are hence to be viewed at #ej centimeters convergence distance instead of at #be1 the time will again be halved4 ,these relations are clearly shown in the diagram 7,fig4 #aef74 ,here the left-hand portion shows how to find the stereoscopic base line at each altitude for each focal length2 while the right-hand portion shows how to translate this into time interval for any plane velocity4 ,the ,burchall slide rule 7,fig4 #acj7 shows another way to arrange these data in form for rapid calculation4 ,illustration3 ,,fig4 #aef4--;,chart for calculating intervals between exposures for stereoscopic pictures4 ,plates used for stereoscopic negatives should be at least twice as long as the ocular separation1 if correct relief is desired1 and the full size of the stereoscope field is to be utilized4 ,this relation follows at once if we consider that we wish to cut from each negative a rectangle #fe millimeters wide1 and that the image of the target has shifted #fe millimeters between exposures4 ,if the plate is larger than this there is opportunity to select the view1 or to pick several4 ,if the plate is smaller the elements of the stereogram must be narrow strips4 ,this1 however1 holds only for contact prints4 ,the ordinary ,english practice in making stereo negatives is to take successive pictures with an overlap of #fj to #ge per cent4 ,this practice is probably dictated by the #d #e inch plate1 since #fj per cent4 overlap on #d inches means a separation of just over an inch and a half instead of #b1 but it leaves #b inches of picture common to the two negatives4 ,with overlap the common portion is #c inches1 which permits of cutting #b inch prints1 and allows some latitude for irregular motion of the plane or for chance error in calculation of intervals4 ,data on the basis of overlaps for a #d-;inch plate are shown in connection with ,fig4 #aee which shows in diagrammatic form the variation of exposure interval with height1 together with other points of interest4 ,elevation ,possible to ,detect in ,stereoscopic ,views4--,can the actual difference in elevation be discovered by the use of stereoscopic views8 ,an approximate idea may be obtained from the following considerations3 ,suppose we have two small point-like objects1 one above the other1 such as a street lamp globe and the base of the lamp pillar4 ,in a view taken from directly overhead these will be superposed1 and so will not be capable of separation4 ,but1 as the point of view is shifted sideways1 the two objects separate1 until a point is reached where they can just be distinguished as double4 ,when this condition holds for either picture of the stereoscopic pair it will be possible to obtain stereoscopic relief4 ,now the separation which can just be distinguished is commonly assumed to be one minute of arc4 ,this angle corresponds to about #a_/cdjj the distance from the eye to the object4 ,if the object is assumed at a distance a from the face1 and on a line with one of the eyes1 which are separated by the distance d1 then 7all angles being small7 the object must be of height a_/d times the horizontal distance which corresponds to one minute4 ,for #be centimeters' viewing-distance this quantity is about #d1 so that the least perceptible elevation is #d_/cdjj or about #a_/ijj4 ,the stereogram having been made under conditions giving correct relief1 this fraction is also the fraction of the altitude of the plane when the photograph was taken which may be detected4 ,an object as high as a man 7#f feet7 should be visible as a projection in a stereoscopic view taken at #f #ijj #edjj feet4 ,this relation--#a_/ijj--;holds 7irrespective of the focal length of the lens71 as long as the conditions for correct relief are maintained4 ,stereoscopic ,aerial ,cameras4--,cameras for aerial stereoscopic photography need in no way differ in construction from those made for mapping or spotting1 provided only they permit exposures to be made at short enough intervals4 ,the addition of special sights1 as already discussed1 constitutes the only real difference between single view and stereoscopic aerial cameras4 ,but even without such sights ordinary aerial cameras are applicable to stereo work by the usual procedure of determining the exposure spacing by time4 ,one scheme employed for taking low stereos1 where the interval is only two or three seconds1 is to mount two cameras in the plane1 exposing them one after the other at the correct interval4 ,another method which has been tried with success is the use of a double focal-plane shutter in a single lens camera 7,fig4 #aeg74 ,the two shutters are side by side1 with their slots parallel to the line of flight4 ,to take a stereo negative we expose first the shutter nearer the tail of the plane1 and then the other1 after an interval which can be calculated from the speed and altitude1 or1 better1 determined by a stereoscopic sight4 ,the two views are thus obtained on a single plate4 ,prints from these negatives are transposed right and left1 and1 if the prints are viewed in an ordinary stereoscope1 have to be cut apart and transposed for mounting1 or else this may be done to the negatives4 ,illustration3 ,,fig4 #aeg4--;,aerial hand camera fitted with two complimentary shutter slits and double sight1 for stereoscopic photography4 ,in this connection attention may be drawn to an alternative method of viewing stereograms1 which may be used on transposed prints--a method which needs no instrument1 and so has sufficient advantage to even warrant mounting ordinary stereoscopic pairs in the transposed position for observation4 ,this method consists in crossing the optic axes1 in the fashion illustrated in ,fig4 #aeh4 ,a finger is held in front of the face in such a position that the left stereogram element and the finger are seen in line by the right eye2 the right element and the finger by the left eye4 ,the proper position is found by alternately closing each eye1 and advancing or retracting the finger4 ,then both eyes are opened and converged on the finger tip1 which is thereupon dropped1 leaving the picture standing out in relief4 ,an opportunity to try this method is afforded by ,fig4 #aei4 ,illustration3 ,,fig4 #aeh4--,method of fusing transposed stereoscopic images by crossing the optic axes4 ,stereo ,obliques4--,the theory of making oblique stereo pictures is identical with that of other stereos4 ,the only problem peculiar to obliques is that of making the exposures at short enough intervals apart4 ,this problem is due largely to the fact that oblique views are ordinarily taken from low altitudes1 for the purpose of 8spotting0 particular objects1 rather than for mapping the gross features of an extended area4 ,the same problem of how to secure a short exposure interval is met with when we attempt to take vertical stereos from a low altitude1 but as already discussed1 it is much preferable from the pictorial standpoint that pictures of definite small objectives be made obliquely4 ,another reason for taking stereo obliques from points but little separated is of some interest in connection with the discussion above given of 8correct0 and 8natural0 relief4 ,when the relief is 8correct0 the object appears1 as already stated1 to be a small model in its true proportions1 standing at the convergence distance4 ,when the eyes are converged to a small object #be to #ej centimeters away all objects beyond are hopelessly transposed and confused4 ,this does not happen when we look at large distant objects1 since their background is at a distance effectively but little beyond them4 ,as a result1 when a stereo oblique is made in 8correct0 relief of such an object as the ,washington monument with buildings beyond1 the confusion of the background presents an appearance entirely contrary to our visual experience with objects as large as the neighboring buildings are known to be4 ,this effect may be avoided by choosing a uniform background such as grass1 or by taking the pictures very much closer together1 at the expense of 8correct0 but at a gain in 8natural0 relief4 ,stereo obliques can of course only be made with any facility by laterally pointing cameras4 ,from the calculations already given it appears that a 8correct0 stereo oblique of an object #ejj meters away will mean exposures only two or three seconds apart1 too short an interval for any of the ordinary plate-changing and shutter-setting mechanisms2 and the case is even worse should less relief be desired4 ,one solution of this problem has been the use1 already mentioned1 of two cameras mounted together1 either side by side or one over the other1 with separate shutter releases4 ,both releases may be controlled by the observer1 using a sight1 or else pilot and observer may work in harmony as has been recommended in the ,english service1 where the pilot releases one shutter and the observer counts time from the instant he sees the first shutter unwind and releases the second4 ,illustration3 ,,fig4 #aei4--,oblique stereogram made with stereoscopic aerial camera 7,fig4 #aeg74 ,to be viewed by crossing the optic axes 7,fig4 #aeh74 ,a very satisfactory apparatus for the taking of stereo obliques consists of a #aj-;inch focus hand-held camera 7,fig4 #aeg71 provided with a two-aperture focal-plane shutter4 ,the right-hand half of one curtain aperture is blocked out1 the left-hand half of the other4 ,the first pressure on the exposing lever exposes one-half of the plate1 the second the other4 ,a stereoscopic sight of the type already described is placed on the bottom4 ,to make an oblique stereo negative the camera is held rigidly by resting the elbows on the top of the fuselage and the first exposure is made when the object comes in line with the rear sight and the leading front sight4 ,the eye is then moved so as to look along the line of the rear sight and the following front sight1 and when the object is again in alinement the second pressure is given the exposing lever4 ,fig4 #aei shows a stereo oblique made by this camera4 ,the elements are transposed right and left1 and the stereogram may be viewed by crossing the optic axes as shown in ,fig4 #aeh1 or the two pictures may be cut apart and remounted4 ,the ,mounting of ,aerial ,stereograms4--,the first step in making the printed stereogram is to select two pictures taken on the same scale1 but from slightly different positions4 ,these may be two chosen from a collection made for other purposes1 or else a pair taken at distances calculated to fit them for stereoscopic use4 ,the next step is to mark the center of each picture1 either with easily removed chalk or with a pin point4 ,they are then superposed1 and afterward carefully moved apart by a motion parallel to the line joining their centers when superposed4 ,the final step before mounting is to mark out and cut the two elements1 their bases being parallel to the line of centers1 their horizontal length the distance between the optic axes of the stereoscope 7or as near this as the size of the prints will permit74 ,they are then mounted on a card1 with their centers separated by approximately #fe millimeters4 ,the right-hand view is the one showing more of the right-hand side of objects1 and vice versa4 ,this process of arranging1 cutting1 and mounting is shown clearly in ,fig4 #afj4 ,in this case the stereoscopic elements lie symmetrically about the line joining the centers of the original prints4 ,this is not necessary1 as they may be selected from above or below this line so long as their bases are parallel to it4 ,a simplification of this method consists in superposing the two prints1 laying over them a square of glass of the size to which they are to be cut1 then turning it so that a side is parallel to the line of centers1 and cutting around it through both prints with a sharp knife4 ,the principle and results are of course the same with both methods4 ,illustration3 ,,fig4 #afj4--,method of arranging stereoscopic prints for cutting4 ,if large numbers of stereoscopic prints are required it is necessary1 for economy of time1 either to photograph a finished stereogram and make prints from this copy negative1 or to set up special printing machines4 ,under the general discussion of printing devices a stereoscopic printer is described 7the ,richard7 in which the two negatives are placed so that stereo prints can be got by two successive printings on one sheet of paper4 ,uses of ,stereoscopic ,aerial ,views4--,attention has already been called to the characteristic flatness of the aerial view4 ,neither the picture on the retina nor that on the photographic plate affords any adequate idea of hills and hollows4 ,unless shadows are well defined1 small local elevations and depressions cannot be distinguished from mere difference in color or marking4 ,even in the presence of shadows it is often only by close study that differences of contour are noticeable4 ,but with stereoscopic views these features stand out in a striking manner4 ,taking our illustrations from military sources1 we may note the use of stereoscopic pictures to detect undulations of ground in front of trenches 7,fig4 #afa74 ,they reveal the hillocks1 pits1 small quarries1 streams flowing behind high banks1 and other features which make the attack hard or easy4 ,commanding positions are shown1 the boundaries of areas exposed to machine-gun fire1 and the defilades where the attackers may pause to reform4 ,concrete 8pill boxes0 are located in the midst of shell holes of the same size and outline1 and can be differentiated from them4 ,railway or road embankments and cuts can be detected and studied to extraordinary advantage in stereoscopic pictures4 ,thus what appears to be a mine crater on a level road1 easily driven around1 may be a gap blown in an embankment1 a serious obstacle indeed4 ,bridges1 observation towers and other elevated structures jump into view in the stereoscope when often they have entirely eluded notice in the ordinary flat picture4 ,once presented in relief1 camouflaged buildings or gun emplacements1 however carefully painted1 are ridiculously easy to pick out4 ,illustration3 ,,fig4 #afa4--,typical stereogram of military detail4 ,fuse by looking at a distant object over the top of the page1 and quickly dropping the eyes to the print4 ,practical peace-time applications of stereoscopic views can easily be foreseen following the lines of war experience4 ,such1 for instance1 would be the study of proposed railway or canal routes4 ,a series of stereograms would obviate the necessity of contour surveys1 at least until the exact route was picked and construction work ready to start4 ,apart from their utilitarian side1 however1 stereoscopic views have very great pictorial merit4 ,stereoscopic pictures of cathedrals1 public and other large buildings1 have often great beauty1 and afford opportunities for the study of form given by no other kind of representation1 short of expensive scale models4 ,they may very well lead in the near future to a revival of the popularity of the stereoscope4 ,impression of ,relief ,produced by ,motion4--,an appearance of solidity can be obtained in moving pictures by the simple expedient of slowly moving the camera laterally as the pictures are taken4 ,as an illustration1 if the moving picture camera is carried on a boat while structures on the shore are photographed1 when these are projected on the screen they appear in relief1 due to the relative motion of foreground and background4 ,as relief of this sort is not dependent on the use of the two eyes1 it demands no special viewing apparatus4 ,this idea has been utilized to a limited extent in ordinary moving picture photography by introducing a slow to-and-fro motion of the camera1 but this can hardly be considered satisfactory1 since this motion is so obviously unnatural4 ,in moving pictures made from the airplane the normal rapid motion of the point of view is ideal for the production of the impression of relief in the manner just described4 ,for instance1 in moving pictures of a city made from a low flying plane1 the skyscrapers and spires as they sweep past stand forth from their more slowly moving background in bold and satisfying solidity4 ,in fact1 such pictures probably constitute the most satisfactory solution yet found of the vexing problem of 8stereoscopic0 projection4 ,no better medium can be imagined for the travel lecturer to introduce his audience to a foreign city than to throw upon his screen a film made in a plane approaching from afar and then circling the architectural landmarks at low altitudes4 ,,chapter ,,xxix ,,the ,,interpretation ,,of ,,aerial ,,photographs ,oblique aerial photographs if on a large enough scale are even easier to interpret than are ordinary photographs taken from the ground1 since they practically preserve the usual view1 and add to it the essentials of a plan4 ,with verticals1 however1 this is far from the case4 ,in them all natural objects present an appearance quite foreign to the ordinary mortal's previous experience of them4 ,this may be easily demonstrated by taking any aerial view containing a fair amount of detail and trying systematically to identify each object4 ,a necessary preliminary to doing this accurately is acquaintance with and study of the ground photographed1 or of similar regions1 and of objects of the same character as those likely to be included4 ,the interpretation of military aerial photographs is of such importance1 and has become such an art1 that it is the function of special departments of the intelligence service4 ,extended courses in the subject are now given in military schools4 ,this instruction must cover more than the interpretation of aerial photographs as such4 ,general military knowledge is essential1 so that not only may photographed objects be recognized1 but the significance of their appearance be realized4 ,whether attack or retreat is indicated2 whether a long range bombardment is in preparation1 or a mere strengthening of local defences4 ,the natural difficulties of interpreting aerial views are enormously increased by the unfamiliar nature and frequently changed character of the military structures1 and particularly by the attempts made to conceal these from aerial observation by selection of surroundings and by camouflage4 ,the small scale of the photographs1 in which a machine gun shows as a mere pin point1 adds to the uncertainty1 with the net result of making interpretation a task of minute study and deduction worthy of a ,sherlock ,holmes4 ,little detailed information on interpretation can be profitably written in a general treatise1 partly because the illustrations available are of a highly technical military character1 partly because original photographs instead of halftone reproductions are practically imperative for purposes of study4 ,nevertheless some general instructions1 applicable to any problem of interpretation1 may be given1 as well as a few illustrations1 drawn from military sources1 which will serve to show the detective skill necessary4 ,first of all it is important that the print or transparency be held in the right position4 ,the shadows must always fall toward the observer2 otherwise1 reliefs will appear as hollows and hollows will show as hills4 ,the reason for this is that the body ordinarily acts as a shield1 preventing the formation of shadows except by light falling toward the beholder4 ,thus in ,fig4 #afb the slag heap looks like a quarry when the shadows fall away from one4 ,the necessity for proper direction of shadows is1 it may be noted1 in conflict with the ordinary convention for the orientation of maps--at least in the northern hemisphere4 ,a city map1 made by sunlight falling from the south1 presents its shadows as falling away from the observer1 when it is mounted with its north point at the top1 as is customary4 ,as a consequence buildings in aerial photographic mosaics of cities occasionally look sunken instead of standing out4 ,illustration ,wrong way4 ,shadows falling away ,right way4 ,shadows falling toward from observer4 observer4 ,,fig4 #afb4--,the wrong way and the right way to hold a photograph for interpretation4 ,illustration3 ,,fig4 #afc4--;,guide to interpretation of trench details4 ,the relation between the shape of the shadow and the object casting it must be well learned4 ,this is a part of the training of every architectural draftsman1 but the appearance of shadows from above has not heretofore been a matter of importance4 ,the difference between high and low trenches1 between cuttings and embankments1 between shell holes1 occupied or unoccupied1 and 8pill boxes10 must be detected largely from the character of the shadows4 ,which elevations and depressions are of military and which of merely accessory nature1 whether this black dot is a machine gun or a signaling device1 whether that dark spot is an active gun port or an abandoned one--these are all matters of shadow and of light and shade study4 ,several illustrations of these points appear in ,figs4 #afc1 #afd and #afe4 ,illustration3 ,,fig4 #afd4--;,guide to interpretation of shell holes and other pits4 ,illustration3 ,,fig4 #afe4--;,illustrating the importance of distinguishing between objects of similar appearance but different military importance4 ,shadows may be used to get exact information as to directions and magnitudes4 ,if we know the time of day at which a picture is taken1 the direction of the shadows will give the points of the compass4 ,a chart for doing this is shown in ,fig4 #aff4 ,the length of a shadow is a measure of the height of the object casting it1 and the exact relation between the two dimensions is determined by the day and hour4 ,fig4 #afg embodies in chart form the values of this relationship for all times of the year and day1 while ,fig4 #afh shows the kind of picture in which shadow data could be utilized to great profit4 ,illustration3 ,,fig4 #aff4--,location of true north from direction of shadows4 ,place the dial on the photograph1 the hour line corresponding to the time it was taken being pointed in the direction of the shadows4 ,north lies between the two arrows1 the exact direction being obtained by joining the center of the dial to the point on the figure of eight corresponding to the date on which the picture was taken4 7,number on figure of eight represent the #ast of the month47 ,minute changes1 both in light and shade and in position1 must be watched for with great care4 ,naturally growing foliage and the cut branches used for camouflage differ in color progressively with the drying up of the leaves4 ,hence a mere spot of lighter tone in a picture of a forest1 especially if the picture is taken through a deep filter1 becomes instant object for suspicion4 ,the complete study of any position calls for photographs of all kinds--verticals1 obliques1 and stereos4 ,stereoscopic views are the worst foe to camouflage4 ,a bridge painted to look like the river beneath is labor thrown away if the stereo shows it to be a good ten feet above the real river6 ,illustration3 ,,fig4 #afg4--,length of shadow of object one meter high1 at different times of the day and year1 for latitude of ,paris4 ,illustration3 ,,fig4 #afh4--;,bethune1 ,august1 #aiah1 illustrative of interpretation by shadows4 ,illustration3 ,,fig4 #afi4--,typical trench photograph showing first and second lines1 communicating trenches1 listening posts1 machine gun emplacements1 and barbed wire4 ,a few illustrations of the more ordinary and obvious objects whose detection is the subject of aerial photography are shown in accompanying figures4 ,fig4 #afi pictures a typical trench system1 with barbed wire4 ,the trenches show as narrow castellated lines1 from which run the zigzag lines of communicating trenches1 saps1 and listening posts4 ,the minute pockmarks behind the main trench lines are shell holes and machine gun pits4 ,the barbed wire shows as double and triple gray bands1 intricately criss-crossed at strategic points4 ,another form of defence1 intended for the same purpose as the barbed wire of the western front1 is that furnished by overthrown trees in forest regions4 ,fig4 #agj reveals a mountain fortress surrounded by a zone of felled trees1 and indicates in striking manner the value of the information a single aerial photograph may furnish to an attacking force4 ,fig4 #abc shows on a comparatively large scale opposing trench systems in which a natural obstacle--a river--separates the adversaries4 ,nicks and dots indicate machine guns to the skilled eye1 and several rectangular structures are revealed as concrete buildings which have survived unscathed the shell fire which has obliterated1 and caused to be rebuilt1 nearly every other element of the trench system4 ,illustration3 ,,fig4 #agj4--;,a mountain fort surrounded by felled timber4 ,isolated battery emplacements 7,fig4 #aga7 must be carefully studied to learn if they are in use4 ,the chief indication is given by the paths the men make in going and coming2 these show as fine light lines1 obliterated by growing vegetation if long disused4 ,another indication is the blast marks in front of the gun muzzles2 occasionally the sensitive plate will catch the actual puff of smoke as the gun is discharged4 ,illustration3 ,,fig4 #aga4--,three stages in the life of a battery4 ,railways of various gauges show as thin lines1 crossed by ties1 and exhibiting the characteristic curves and switches4 ,they are particularly important to detect because they naturally lead to guns or supplies of importance4 ,abandoned railways from which the rails and ties have been removed leave their marks on the ground and must be carefully distinguished from lines in actual operation4 ,aviation fields are easily recognized by the hangars1 often with 8funk hole0 trenches alongside for the men to take shelter in during air raids 7,fig4 #agb74 ,other characteristic features are the 8,t0 which shows the direction of the wind to the returning pilot1 and of course the planes themselves1 standing on the ground4 ,but the field may be inactive1 and the planes merely canvas dummies1 so that to pierce the disguise1 all paths1 ruts1 and other indications of activity must be minutely studied4 ,overhead telegraph and telephone lines are revealed when new by a series of light points 7,fig4 #agd71 where the posts have been erected in the fresh turned earth4 ,later1 when the fields through which they pass are cultivated1 the post bases show as islands left unturned by the plow4 ,in winter the wires reveal their position by black lines in the snow caused by drippings4 ,buried cables are indicated while building by their trenches1 and for some time afterward by the comparatively straight line of disturbed earth4 ,illustration3 ,,fig4 #agb4--;,aviation field1 showing hangars1 planes1 landing 8,t0 and refuge trench4 ,just as the detective of classic story makes full use of freshly fallen snow to identify the footprints of the criminal1 so does the aerial photographer utilize a snowfall to pierce the enemy's attempts at deception4 ,tracks in the snow show which trenches or batteries are in actual use4 ,melting of the snow in certain places may mean fires in dugouts beneath4 ,black smudges in front of trench walls show where guns are active4 ,guns1 wire and other objects1 however carefully painted to match the gray-green earth1 stand out in violent contrast to this new white background 7,fig4 #agc74 ,illustration3 ,,fig4 #agc4--,trenches and barbed wire in the snow of an ,alpine ridge4 ,italian ,air ,service photograph4 ,illustration3 ,,fig4 #agd4--;,a fully interpreted aerial photograph4 ,after the aerial photograph has been interpreted the results of the interpretation must be made available to the artilleryman or the attacking infantryman4 ,this may be done by legends marked directly on the photograph4 ,another method is to mount over the photograph a thin tissue paper or oilskin leaf1 with the interpretation marked on it4 ,a yet more elegant method consists in outlining all the chief features of the photograph in ink1 writing in the points of importance in interpretation1 and then bleaching out the photograph with potassium permanganate solution4 ,photographic copies of the resultant line drawing are then mounted side by side with the original photograph4 ,fig4 #agd1 which shows a fully interpreted photograph1 is an example of this kind of mounting4 ,,chapter ,,xxx ,,naval ,,aerial ,,photography ,the problems of naval aerial photography are quite different from those of military aerial work1 and on the whole they are more simple4 ,at the same time1 photography has played a considerably less important part in naval aerial warfare than in land operations4 ,photography as a necessary preparation for attack has not figured in naval practice nearly so much as have the record and instruction aspects4 ,to some extent this is due to the nature of the naval operations in the ,great ,war1 to some extent to the limitations of ceiling and cruising radius of the naval aircraft4 ,a photographic reconnaissance1 preceding and following a bombardment of shore batteries2 a photographic record of the ships at anchor1 as at ,santiago2 a photograph of the forts defending a channel1 as at ,manila2 photographs1 quickly developed and printed1 of an approaching fleet--all these are possibilities of great usefulness in naval warfare between contestants both of whom 8come out0 and carry the struggle to the enemy's gates4 ,but in the recent war the use of the submarine1 operations under cover of fog1 the striving for 8low visibility10 and the considerable distances to be traversed to reach the enemy lairs1 have conspired to limit the development of photography as a major aid to naval combat4 ,probably when the whole history of the conflict is told we shall learn that the ,zeppelins which cruised over the ,north ,sea1 keeping the ,allied fleet under observation1 had a regular routine of photographic work4 ,in the ,italian zone1 where much of the enemy territory and several important naval centers lay at only short distances over the ,adriatic1 the naval photographic service more nearly rivalled that of the army than in the ,english1 ,french and ,american zone of activity in the ,channel and ,north ,sea4 ,illustration3 ,,fig4 #age4--;,a lighthouse1 as the naval flier sees it4 ,illustration3 ,,fig4 #agf4--;,a threatened submarine attack4 ,throwing out a smoke screen to protect a convoy4 ,british official photograph4 ,the majority of the photographs made in the ,british service were obliques1 taken by short focus 7#f to #aj inch7 hand-held cameras4 ,this type was employed partly because of difficulties to be noted presently1 in using other forms of cameras1 but more especially because such pictures sufficed for the kind of information desired4 ,a hand-held camera formed part of the outfit of each flying boat and dirigible1 but1 unlike land reconnaissance1 planes ascending primarily for picture taking were unknown in their naval service4 ,usually no photographic objective was predetermined--photographs were made only if objects of interest were come upon4 ,mapping also formed no part of the seaplane's work4 ,four plates would be carried1 instead of as many dozens in the land machine1 and often these would come back unexposed4 ,there were of course some photographic flights planned out beforehand1 for the purpose of photographing lighthouses and other landmarks whose appearance from the air should be known to the naval aviator 7,fig4 #age74 ,among the accidental and record types of photograph come convoys 7,fig4 #agf71 whose composition and arrangement were made a matter of record1 particularly if any ship was out of its assigned position4 ,photographs of oil spots on the sea surface1 or other results of bomb dropping1 were necessary evidence to establish the sinking of a submarine 7,fig4 #agi74 ,pictures of all types of ships friendly1 neutral1 and where possible1 enemy--were a much needed part of naval equipment1 in particular pictures of friendly destroyers and submarines1 which should not be bombed by mistake4 ,for safe navigation it was essential to have photographs of uncharted wrecks 7,fig4 #aha71 of buoys out of place and of ships failing to return signals or otherwise to comply with rules4 ,the great majority of the pictures were taken from altitudes of not more than #cjj meters4 ,illustration3 ,,fig4 #agg4--,submarine coming to the surface4 ,u4 ,s4 ,naval ,air ,service photograph4 ,illustration3 ,,fig4 #agh4--;,dropping depth bombs4 ,illustration3 ,,fig4 #agi4--,the submarine destroyed4 ,destroyer on tell-tale oil patch4 ,british official photographs4 ,illustration3 ,,fig4 #ahj4--;,a convoy at anchor in port4 ,hand-held cameras for naval work have practically the same design as those for land work4 ,in view of the smaller number of pictures taken on naval trips1 and the consequent absence of any need for great speed in changing plates1 the ordinary two-plate dark slide has been found satisfactory in the ,english service4 ,but these are much less convenient than the bag magazines used in the ,u4 ,s4 ,naval hand camera 7,fig4 #ca74 ,the sights on the naval hand camera are preferably of the rectangular1 field indicating type1 especially useful in photographing extended objects such as convoys4 ,as the flying boat travels comparatively slow1 it is easy for the observer to stand up to take pictures1 and the sight is conveniently placed on top4 ,but if held out over the side for verticals the sight must be on the bottom4 ,rectangular sights in both positions are provided in the ,english camera 7,fig4 #ahf74 ,naval cameras should be immune from moisture1 which means doing away with all wooden slides or grooves4 ,a praiseworthy practice is to carry the camera in a waterproof bag4 ,illustration3 ,,fig4 #aha4--;,airplane photography as an aid to salvaging4 ,position of wrecked merchantman twelve fathoms down revealed by photograph from the air4 ,photograph by ,british ,air ,service4 ,illustration3 ,,fig4 #ahb4--;,a sea plane4 ,illustration3 ,,fig4 #ahc4--;,a flying boat4 ,illustration3 ,,fig4 #ahd4--;,a dirigible or 8blimp0--possibly the photographic aircraft of the future4 ,illustration3 ,,fig4 #ahe4--;,english 8,type #ah0 hand camera on bracket for exposing through side window of flying boat4 ,british official photograph4 ,illustration3 ,,fig4 #ahf4--;,camera mounted in bracket from forward cockpit of flying boat4 ,british official photograph4 ,cameras other than of the hand-held form have been little used in sea planes1 owing to the difficulties of installation4 ,the hydro-airplane1 consisting of an ordinary airplane fuselage mounted on two pontoons 7,fig4 #ahb71 can carry the same kind of photographic equipment as the land machine4 ,but if it has a single central pontoon this is not feasible4 ,the hydro-airplane is1 however1 largely superseded by the flying boat 7,fig4 #ahc71 whose fuselage1 of boat form1 rests directly on the water4 ,in this type of sea plane1 views taken vertically downward are not easy to make4 ,in the larger flying boats the hull projects out horizontally a matter of several feet beyond the side of the cockpit4 ,an ordinary outboard mounting is therefore out of the question4 ,the camera must either be held out at arm's length or else mounted on a long bracket 7,fig4 #ahf74 ,the usual place for carrying the camera is in the front cockpit with its magnificent all-round view4 ,obliques can1 too1 be taken in great comfort from the side windows behind the wings1 as shown in ,fig4 #ahe4 ,the possibility of cutting a hole in the bottom of a flying boat to take care of a vertical camera is not entertained in ,british and ,american naval circles4 ,nevertheless it is the regular practice in the ,italian service1 with their small high ceilinged flying boats4 ,in them a round hole is cut in the floor1 stopped with a plug and rubber gasket4 ,after the boat rises into the air the hole is opened1 and the regulation ,italian camera is set securely in a frame on the floor over the hole 7,fig4 #ahg74 ,photographs are taken to the capacity of the camera1 and if it is desired another camera is put in its place1 till all its plates have been exposed1 and then even a third4 ,before coming down the hole must of course be closed again4 ,sliding doors have been designed to close this aperture1 but have not proved sufficiently water-tight1 although such a device could undoubtedly be worked out4 ,illustration3 ,,fig4 #ahg4--;,italian flying boat with camera mounted on the floor4 ,with its space for five or more passengers1 and with its low speed1 the modern flying boat affords an excellent craft for photographic work4 ,there is ample room for any size of camera1 and for any style of mounting1 if we assume that there is no objection to an opening in the bottom4 ,the low ceiling of these ships1 however1 prevents their use for certain forms of aerial photography which should be of the greatest importance4 ,operations against shore stations--harbors1 docks1 shipyards1 ships at anchor1 and fortifications--cannot be undertaken for fear of anti-aircraft guns and hostile land planes4 ,the solution of the problem of carrying and launching fast high flying planes from ships will immediately extend the usefulness of aerial photography to coastal work4 ,in the recent war1 such of this as was done1 along the ,belgian coast--the shore batteries1 and the results of naval operations at ,zeebrugge and ,ostend--was done by land planes from territory held by the ,allies4 ,the photographic equipment of sea planes of the type suggested will of course present special problems1 but the apparatus used will be apt to approximate closely to that of the land planes4 ,,vii ,,the ,,future ,,of ,,aerial ,,photography ,,chapter ,,xxxi ,,future ,,developments ,,in ,,apparatus ,,and ,,methods ,prophecy is an undertaking that always involves risk4 ,the prophet's guess of what the future will bring forth is based only on the tendencies of the past1 the most urgent needs of the present1 and the activity of his imagination4 ,he may easily--and he usually does--entirely overlook certain possibilities which may arise apparently from nowhere and which profoundly affect the whole trend of development4 ,conditions which dominate at the present time--such as military necessity--may happily drop into the background and free the science from some of its severest restrictions4 ,with this caution1 some future possibilities in apparatus and methods may be presented along the lines already used in discussing the present status of aerial photography4 ,lenses4--,from the military standpoint the next steps in lens design would be toward telephoto lenses on the one hand1 and on the other toward lenses of short focus and wide angle4 ,the telephoto lenses used for spotting would be of long equivalent focus--a meter and more--but of handy size1 that is1 not more than #ej centimeters over all working distance4 ,the wide angle short focus lenses would be designed for low flying reconnaissance or quick mapping work4 ,they would also be demanded for peace-time mapping projects1 where the largest possible amount of territory should be covered in a single flight4 ,both types of lens should be pushed to the extreme in aperture1 for short exposures and the maximum of working days will always be demanded4 ,cameras4--,peace-times will give the necessary opportunity to develop self-contained and therefore simply installed cameras4 ,they will at the same time be made very completely automatic but simple to operate in spite of their complexity4 ,such cameras have1 during the war1 been the ideal of all aerial photographers1 but the time has been too short since the necessary conditions have been understood for that lengthy development work and those complete service tests which are so necessary to develop all automatic apparatus4 ,several designs which are now being perfected may be counted on to take us a long way toward this ideal4 ,on the other hand1 that military ideal which leaves the camera operator the greatest possible freedom for other activities1 is apt to be entirely reversed in peace4 ,the camera operator can now be required to be an expert1 who will be free to change plates or filters and to estimate exposures1 instead of giving his best efforts to the problem of defence4 ,for him a simple and reliable hand-operated or semi-automatic camera is entirely satisfactory1 and the great expense of complicated automatic apparatus has no longer its former justification4 ,camera ,suspension4--,perhaps the most pleasing prospect before the aerial photographer as he turns from war to peace work is that of having planes built for and dedicated primarily to photography4 ,instead of his camera being relegated to an inaccessible position1 picked after the plane design has been officially 8locked20 instead of yielding first place to controls1 machine gun and ammunition2 instead of being jealously criticised for the space and weight it takes up1 the camera can now claim space1 weight1 and location suitable for any likely aerial photographic need4 ,high speed no longer will be vital1 and slower planes1 permitting longer exposures in inverse ratio to their speed1 will be chosen for photographic purposes4 ,a development which is sure to intrigue many investigators is the gyroscopically controlled camera4 ,this has its chief raison d'tre in precision mapping1 whose possibilities from the air will undoubtedly be intensively studied at once4 ,with the automatically leveled camera will come renewed attention to indicators of time1 altitude1 and direction1 with the ultimate goal of producing aerial negatives that show upon their face the exact printing and arranging directions necessary to put together an accurate map4 ,sensitive ,materials4--,manufacturers of plates and films will direct efforts toward producing emulsions of good contrast1 high color sensitiveness and high effective speed1 especially when used in conjunction with the filters necessary for haze penetration4 ,exposure data will be accumulated and exposure meters appropriate for aerial work will be developed4 ,color ,photography4--,color photography from the air by any of the screen-plate or film-pack methods is probably out of the question because of the long exposures required4 ,the screen-plates are unsuitable also because of the relatively large size of their grain compared to the detail of the aerial photograph4 ,ordinary three-color photography1 using three separate negatives1 is always subject on the earth's surface to the difficulty that the three negatives must be exposed from the same point of view1 either in succession or by means of some optical arrangement which is costly from the standpoint of light4 ,in photographing from the air this difficulty of securing a single point of view for the three photographs is absent4 ,three matched cameras1 side by side in the fuselage1 have identical points of view as far as objects on the earth below are in question4 ,consequently1 three-color negatives are entirely possible1 and indeed will be simple to make as soon as plates of adequate color sensitiveness and speed are available4 ,probably the new ,ilford panchromatic plate has the necessary qualities4 ,night ,photography4--,the searching eye of photography was so omnipresent in the later stages of the ,great ,war that extensive troop movements and other preparations had to be carried out either in photographically impossible weather or else at night4 ,the natural reply to the utilization of the cover of night is to 8turn night into day0 by proper artificial illumination4 ,at first thought it might well appear that the task of illuminating a whole landscape adequately for airplane photography is well-nigh hopeless by any artificial means4 ,on one hand we have the very short exposures alone permissible2 on the other the fact that the intensity of daylight illumination is overwhelmingly greater than those common in the most extravagant forms of artificial illumination4 ,toward the close of the war1 however1 actual experiments made with instantaneous flashes of several million candlepower showed that if proper means were provided to insure the flash going off near the ground1 and if its duration were made no longer than about #a_/ej second1 interpretable photographs were obtainable on the fastest plates4 ,it appears1 therefore1 merely a matter of manufacturers perfecting the technique of flash production1 and of inventors providing the launching and igniting devices to push this kind of photography to the practical stage4 ,the achievement of night photography cannot fail to have an enormous effect on future tactics4 ,the technique of night photography may take either of two directions4 ,on one hand we may develop flashes of the requisite intensity to give all their light in #a_/ajj second2 on the other hand1 it may prove more feasible to use flashes of longer duration and to arrange for the camera shutter 7of the between-the-lens type7 to be exposed synchronously with the middle of the flash4 ,one way1 frequently suggested1 to use these longer flashes would be to trail the charge on a long wire1 through which the ignition is effected electrically4 ,this is not likely to be satisfactory1 however1 for the resistance of a wire is so great that when the plane flies at any practical height1 the trailed flash1 if it reaches near the ground1 will be forced to a very great distance behind4 ,probably the best solution will involve accurate synchronizing of the fuse in the freely dropped sack of flash powder with the exposing mechanism in the camera4 ,,chapter ,,xxxii ,,pictorial ,,and ,,technical ,,uses ,aside from their element of novelty1 aerial photographs have undoubted qualities of beauty and utility4 ,the 8bird's-eye view0 has always been a favorite for revealing to the best advantage the entire form and location of buildings and of other large objects4 ,heretofore such views have usually had to be drawn by an imaginative artist4 ,illustration3 ,,fig4 #ahh4--,rheims ,cathedral4 ,aerial oblique views possess the virtues both of pictures and of plans4 ,they are destined to be extensively used in the study of architecture 7,fig4 #ahh74 ,cathedrals1 castles1 town halls1 particularly those still in their cramped medieval surroundings where they can never be seen in their entirety from the ground1 come forth in all their beautiful or quaint proportions from the airman's point of vantage4 ,stereoscopic aerial views are destined to occupy a valuable position also4 ,stereo prints of the famous buildings of ,europe1 taken from the air1 will give to the prospective traveler or the arm-chair tourist a many fold more accurate idea of their construction than will any number of mere surface views4 ,illustration3 ,,fig4 #ahi4--;,a portion of ,vienna seen from the air1 during a 8propaganda raid40 ,italian official photograph4 ,illustration3 ,,fig4 #aij4--,the ,rialto bridge1 ,venice4 ,italian ,air ,service photograph4 ,illustration3 ,,fig4 #aia4--;,a partly developed suburb4 ,a vertical aerial photograph is most closely akin to a map1 but has advantages over any ordinary surveyor's product4 ,as a guide it is infinitely superior to the best draftsman's diagram1 for it provides a wealth of detail whereby the traveller may definitely locate himself4 ,at a single glance he notes the objects of interest within his radius of easy travel4 ,the guide-book of the future will therefore be incomplete without numerous aerial views1 both vertical and oblique4 ,as an illustration of the peculiar merit of the view from the air1 consider the photograph of ,vienna made during d',annunzio's 8propaganda0 bombardment4 ,or the picture of the ,rialto bridge 7,fig4 #aij74 ,no ordinary photograph from land or water suggests the central roadway and no map shows the beauty of its elevation4 ,both are shown here1 as well as an intimate view of the arched and pillared courtyard of the ,fondaco de' ,tedeschi to the right4 ,illustration3 ,,fig4 #aib4--;,a sea-side resort4 ,illustration3 ,,fig4 #aic4--;,a bathing beach seen from the air4 ,airplane photographs will undoubtedly be widely used in certain fields of advertising4 ,architects and real estate agents may be expected to display their wares by the aid of aerial views4 ,a well-planned country estate or golf course1 or a suburban development 7,fig4 #aia71 can be shown with a completeness1 both as to environment and stage of progress which no other form of representation can approach4 ,a sea-side resort can now show the extent and grouping of its natural and artificial amusement features in a single picture 7,fig4 #aib74 ,even the extent of its bathing beach under water is revealed to the aerial photographer 7,fig4 #aic74 ,real estate agents can utilize aerial photographic maps of cities to great advantage4 ,on these their properties can be pointed out1 with the nature of their surroundings shown at a glance1 together with their relation to transportation1 schools1 churches1 shopping districts1 parks1 or factories4 ,the future purchaser of lots in a distant boom town will no longer be satisfied with a map outlining the streets with high-sounding names4 ,he will demand an authentic aerial photograph1 showing the actual number of houses under construction1 the streets1 gutters and sidewalks already laid1 the size and planting of trees4 ,illustration3 ,,fig4 #aid4--,mt4 ,vernon from the air4 ,illustration3 ,,fig4 #aie4--;,a contrast in roofs4 ,the ,capitol retains its individuality1 while the ,white ,house loses all character when seen from above4 ,the study of landscape gardening is another field for which the aerial photograph is peculiarly fitted4 ,a collection of oblique pictures of the chteaux and palaces of ,europe showing their approaches and grounds1 or of the historic estates of our own ,south1 7,fig4 #aid71 will be worth more to the prospective designer of a country estate than maps and ground pictures can ever be4 ,closely allied to landscape gardening is city planning1 for which the aerial map will be quite indispensable4 ,the appearance of a city from the air may indeed become a matter of pride to its inhabitants1 and not only the arrangement of streets and parks1 but even the character of the roofs of the buildings1 be the subject of study 7,fig4 #aie74 ,illustration3 ,,fig4 #aif4--;,an aviation field under construction2 early stage4 ,engineers and constructors will depend more and more on preliminary photographic surveys as a basis for locating their operations4 ,at the later stages of their work they will use aerial photographs for recording progress4 ,periodic photographs of buildings in process of construction1 such as are now made from the ground1 are much more illustrative when made from the air4 ,only from above is it possible to obtain in a single picture the progress of the complete project1 such as the construction of an aviation field 7,figs4 #aif and #aig7 or of a shipyard4 ,the building of large structures--bridges1 hotels1 ships on the stocks--particularly demands aerial views if the foreground is not to eclipse the center of real interest4 ,illustration3 ,,fig4 #aig4--;,an aviation field under construction2 later stage4 ,news events will soon call for an aerial photographic service4 ,already we are seeing newspapers and magazines featuring aerial photographs of the entry into conquered cities and the parades of returning fleets4 ,accidents1 fires1 floods and wrecks1 of either local or national interest1 can best be represented by this newest form of photography4 ,illustration3 ,,fig4 #aih4--,the crater of ,vesuvius4 ,photograph by ,royal ,italian ,air ,service4 ,the photographing of wrecks1 fires and floods suggests the importance of aerial views to insurance underwriters1 who require the most minute information on the characteristics of buildings in every neighborhood1 and on the extent and nature of damage done4 ,marine insurance companies might with profit use the airplane camera to help estimate the chances of salvage of a stranded ship or a vessel foundered in shallow waters 7,fig4 #aha74 ,illustration3 ,,fig4 #aii4--,waves set up by a ship--of interest to the naval architect4 ,numerous scientific uses for aerial views seem likely4 ,prominent among these is their use in geology1 for the study of the various forms of earth sculpture4 ,pictures from the air of extinct volcanoes will give information as to their configuration that would otherwise require months of painstaking survey to obtain4 ,aerial photographs of active volcanoes 7,fig4 #aih71 showing the results of a succession of outbursts--one obliterating the other--would prove of the greatest value1 especially when studied in conjunction with other scientific data1 the whole making a record unobtainable by any other means4 ,in earthquake regions--notably ,southern ,italy and ,japan--the changing coast lines1 shallows and safe harbors1 could be promptly ascertained after the subsidence of each fresh shock1 with a consequent keeping open of trade routes and often the saving of life4 ,river courses1 glacier formations1 caons1 and all the larger natural formations which man usually sees only in minute sections1 and which he must build up in his mind's eye or by models1 are today quickly and accurately recorded by the camera in the air4 ,such formations as coral reefs1 whose configurations can now be accurately learned only by laborious surveys of a limited number1 could be studied in quantity and with heretofore unknown satisfaction as the result of a single expedition with a ship-carrying seaplane and aerial camera4 ,another scientific field--probably one of many similar ones--lies in the study of the waves set up by ships 7,fig4 #aii74 ,these are of extreme importance in the realm of naval architecture1 but before the day of the airplane could never be easily studied in full scale4 ,,chapter ,,xxxiii ,,exploration ,,and ,,mapping ,aerial photographic mapping in war-time has been almost entirely confined to inserting new details in old maps4 ,for such work some distortion or a lack of complete information on altitude and directions is not a serious matter1 because the known permanent outlines serve as a basis4 ,furthermore1 in so far as outline maps are concerned1 as distinguished from pictorial maps1 these have been drawn on the ordinary scales1 and with the ordinary conventions of engineering map practice4 ,aerial photography may be used in the future in practically the same way1 as an aid to the quick recording of those minute details which would ordinarily consume an enormous amount of labor to survey directly4 ,the region shown in ,fig4 #bjj affords a good illustration4 ,a discouraging amount of time and effort would be required to map this section of ,virginia by the usual methods1 while the smallest curve of creek and shore is instantly and completely recorded on a single photographic plate4 ,but there are other possibilities1 diverging from this application both toward greater and lesser requirements for precision4 ,pictorial maps1 in which the actual photographs figure1 promise to be an essential part of the airman's equipment1 whether he be pilot or passenger1 mail carrier or sportsman4 ,without any pretention to detailed accuracy of location1 these maps will show1 in strip or mosaic form1 the general appearance of the country to be traversed1 with particular reference to good landing fields and other points of interest to the aviator4 ,vertical pictorial maps may be supplemented by obliques giving the view ahead1 whereby the pilot may direct his ship4 ,thus the ,washington monument as seen by the pilot from ,baltimore is a truer guide than is the country beneath him4 ,the crossing of mountain ranges is another case where the oblique picture will be more useful than the vertical 7,fig4 #bja74 ,illustration3 ,,fig4 #bjj4--;,an aerial photographic survey of ground difficult to cover by ordinary surveying methods4 ,illustration3 ,,fig4 #bja4--,seeking out mountain passes4 ,contrasted with the merely pictorial maps will be precision surveys4 ,whether it will prove practical to make these entirely from the air is still an open question4 ,it is to be assumed that cameras can be constructed with lenses having negligible distortion of field1 with between-the-lens shutters to obviate the distortions due to the focal-plane type1 with auxiliary devices for indicating compass direction1 altitude1 and inclination1 or with gyroscopic mounting so that an inclination indicator is unnecessary4 ,the application of aerial photography to precision mapping will depend upon the perfection which such cameras attain1 as estimated by the permissible errors in this form of mapping4 ,entire dependence on photography1 as in uncharted regions1 is likely to be worked up to slowly1 beginning with a stage of rather complete triangulation of natural or artificial points--say three in each constituent picture--then through several stages each successively employing fewer and fewer well determined points4 ,the photographic mapping of some of our ,western ,states will be greatly facilitated by the #ajj-yard squares into which the land is divided and already marked in a manner which shows clearly in aerial photographs4 ,a theoretical possibility is the plotting of contours from stereo-aerial pictures4 ,given two elements of a stereoscopic pair1 taken from points whose separation is known1 the position of any point in space shown in the stereoscopic view can be determined by the use of the stereo-comparator4 ,this is an instrument already employed in mountain photo-surveying1 which consists essentially of a compound stereoscope in whose eye-pieces are two points movable at will so that the relief image formed by their fusion can be made to coincide with any chosen part of the landscape4 ,the chief difficulty in the application of this idea to aerial work is to fix the base line4 ,this problem may be met in some cases by using stereo obliques1 and getting the base line by simultaneously made vertical photographs of well surveyed territory beneath4 ,possibly also methods can be developed by which photographs from two or more known altitudes may furnish the requisite data4 ,illustration3 ,,fig4 #bjb4--;,business section of ,hampton1 ,virginia4 ,a survey made by a single instantaneous exposure4 ,illustration3 ,,fig4 #bjc4--,mosaic map of the ,city of ,washington4 ,white rectangle shows portion included in next figure4 ,illustration3 ,,fig4 #bjd4--,portion of ,washington mosaic1 full size4 ,city mapping is a field for which aerial photography is peculiarly fitted 7,fig4 #bjb74 ,a complete map of a large city is a labor of years4 ,in fact1 a modern city is always dangerously near to growing faster than its maps4 ,an aerial map1 on the contrary1 can be produced in a few hours4 ,paris was mapped with #hjj plates in less than a day's actual flying4 ,washington was completely mapped in #b hours1 with less than #bjj exposures4 ,the entire map is shown1 on a greatly reduced scale1 in ,fig4 #bjc1 while ,fig4 #bjd shows a small portion of it in full size1 from which can be obtained an idea of the dimensions of the original4 ,these maps1 while not accurate enough for the recording of deeds and mortgages1 yet serve the majority of needs4 ,there is indeed no reason why with long focus cameras1 given several accurately marked points1 the photographic map of a piece of real estate should not be made with all the accuracy needed1 still leaving the whole process of partial surveying helped out by photography an enormously simpler one than the usual method4 ,rougher types of surveying1 in open country1 offer a most promising opportunity4 ,railway surveys1 showing the character of the country3 passes through mountain ranges3 the available timber and other materials of construction4 ,canal routes1 with the available sources of water supply1 and the best choice of course to avoid deep cuttings and aqueducts4 ,irrigation projects1 with the natural lakes1 river courses and valleys1 which may be dammed to form storage basins4 ,coast1 river and harbor surveys are possible by aerial means with a promptness and frequency which should entirely revolutionize the making of maps of waterways4 ,shifts in channels and shallows1 even of considerable depth1 stand out prominently in the aerial photograph4 ,the actual bottom1 if not more than three or four meters down--as in a bathing beach--shows in the aerial photograph 7,fig4 #aic71 while the varying surface tints caused by light reflected from the bottom at far greater depths are readily differentiated by the camera from the air4 ,an instantaneous photograph will thus perform the work now done by a week's soundings4 ,fig4 #bje1 taken near ,langley ,field1 shows how the aerial photograph may be used to chart natural channels1 while ,fig4 #bjf shows the dredged channels of the port of ,venice4 ,navigation of such a river as the ,mississippi with its shifting bars may come to be guided by monthly or even weekly aerial photo maps4 ,illustration3 ,,fig4 #bje4--,shallows and channels revealed by the aerial photograph4 ,illustration3 ,,fig4 #bjf4--,venice from the air1 showing dredged channels4 ,italian official photograph4 ,illustration3 ,,fig4 #bjg4--;,bengasi1 a ,north ,african town1 surveyed for the first time from the air4 ,italian official photograph4 ,illustration3 ,,fig4 #bjh4--,thurnberg on the ,rhine4 ,photograph by photographic section ,a4 ,e4 ,f4 ,among other uses for aerial photography will be the location of timber4 ,as one illustration1 may be taken the discovery of mahogany trees4 ,their foliage at certain times of the year is of characteristic color4 ,this may be recorded on color sensitive plates with a scientifically chosen filter1 and the cutting expedition sent out with the photograph as a guide4 ,in this as in other cases where rough or unexplored country is to be covered1 it is a question whether the airplane will after all be the most feasible craft1 on account of its necessarily rapid rate of travel1 and its need for known landing fields4 ,the dirigible of large cruising radius1 which can seek its landing field at leisure1 is probably indicated for this kind of work4 ,it may indeed1 as already hinted1 prove to be the chief photographic aircraft of the future4 ,archological surveys offer a fascinating opportunity for airplane or dirigible balloon photography to render scientific service4 ,buried in desert sands or overgrown with tropical vegetation the ancient cities of ,asia ,minor1 of ,burma1 and of ,yucatan evade discovery1 and even when found remain unmapped for decades4 ,discovery and mapping can now go hand-in-hand4 ,the topography of barbaric or colonial towns and villages1 whose importance could not warrant elaborate surveys1 but which should nevertheless be a matter of record1 will be quickly and easily plotted by photography 7,fig4 #bjg74 ,to this day who knows how the streets run in ,timbuctu1 and how1 save from the air1 can we ever map the teeming cities of ,china8 ,he who would follow in the footsteps of ,haroun-al-,raschid can even now explore the by-ways of ,bagdad by the aid of the ,royal ,air ,force photographic map6 ,,index ,aberrations1 lens1 #dg1 #ed ,aberration1 spherical1 #dg1 #ed chromatic1 #dh1 #ed ,acetylene light1 #bhd ,advertising1 use of aerial photography in1 #cib ,aero #a and ,aero #b filters1 #bda ,airplane1 as camera platform1 #bj types of1 #bd ,air speed1 #bg1 #cjh indicator1 #cc1 #cd ,alcohol1 use of in plate drying1 #bge use of in print drying1 #bhf ,altimeter1 #cc1 #agd reading recorded on film1 #ace1 #aga ,altitudes of flight for photography1 #fa1 #bbd ,anastigmatic type of lens1 #dg ,aperture1 lens1 #ci1 #dd1 #ef1 #eg ,archological surveying1 #dac ,astigmatism1 #di1 #ej1 #ed ,auxiliaries1 camera1 #afc installation of1 #bad ,bagley camera1 #bhf1 #cad ,balance1 of camera1 #if of plane1 #aeg1 #bjh1 #bag ,balloons1 #ae1 #af1 #ah ,banking1 #bg1 #cbd ,batteries1 storage1 #aej1 #aef ,bay1 camera1 #abj1 #baj ,bellows1 camera1 #fe1 #ie ,biplane1 #bd ,blast marks in front of guns1 #cfc ,bleaching out print to leave interpretation marks1 #cfg ,boat1 flying1 #be1 #cgd ,bowden wire1 #ajc1 #ajf1 #ajh1 #aaa1 #aad1 #aaf1 #aah1 #abf1 #abi1 #acf1 #afc ,brightness1 range of1 #bba1 #bbe ,burchell photographic slide rule1 #cjc1 #cci ,camera1 airplane1 #ci1 #chd automatic1 #ah1 #dc1 #ij1 #aaf1 #abd1 #abe1 #caa ,b4 ,m41 #abj1 #bjc ,bagley1 #bhf1 #cad ,brock automatic plate1 #abf ,c type1 #dc1 #hg1 #ajc1 #aji classification of1 #dc de,maria1 #hf1 #hi1 #ajc de,ram1 #hb1 #ic1 #aba1 #abi1 #aeg1 #bje1 #bad1 #cbf ,e type1 #dc1 #hg1 #ajc1 #aji elements of1 #db film1 see ,film cameras ,folmer automatic plate1 #abf hand held1 #ie1 #cba1 #cfi ,english1 #ii ,german1 #ii ,u4 ,s4 ,air ,service1 #ajj ,ica1 #ajc ,lamperti 7,italian71 #aab1 #baa ,l4 ,b41 #abj long focus1 #ajc1 #cja1 #cbd ,l type1 #dc1 #hb1 #id1 #ajb1 #aag1 #afb1 #baj ,m41 #ajf1 #bjc non-automatic1 #dc1 #ajb ,piserini and ,mondini1 #aaa semi-automatic1 #dc1 #aaf1 #adi stereoscopic1 #cda ,camouflage1 filters for the detection of1 #bbe1 #bdc stereoscopic views and1 #cbi1 #ceh ,ceiling of plane1 #bg1 #acj ,channels1 detection of by photography1 #djh ,chemicals1 photographic1 #beg ,chlorhydrochinon developer1 #bfa ,chromatism1 lateral1 #di1 #ed ,city planning1 use of aerial photography in1 #cif ,clock-work for driving cameras1 #adi1 #aee ,clouds1 #bbd1 #bdb ,collimator1 #ff ,color1 coefficient or index of negative1 #bei filters1 see filters photography1 #che sensitive plates1 #ae1 #agd1 #bcc1 #bcg sensitiveness of film1 #aca sensitizing1 methods of1 #bce ,coma1 #dg1 #dh ,communication1 means of on plane1 #bif ,compass1 #cc1 #agc reading recorded on film1 #ace1 #aga ,cone1 camera lens1 #db1 #aad interchangeable1 #ajh1 #abj ,construction operations1 aerial photographic records of1 #cif ,contact1 imperfect in printing1 #bgi1 #bhd prints1 #de1 #bgi ,contacts1 electric1 on plane1 #afc ,contours by stereo aerial photography1 #djd ,control1 distance1 of camera1 #aaj1 #afc speed1 of camera1 #acf1 #add1 #aeg ,controls1 duplicate1 #ai1 #be1 #aie1 #bji1 #bad of plane1 #ba1 #bf ,convergence point1 #ccg ,cord for adjusting shutter aperture1 #hb ,core rack development1 #bga ,contrast1 in brightness on earth's surface1 #bba in photographic emulsions1 #ae1 #bcj1 #bcf1 #beh ,counter1 exposure1 on magazine1 #hh on release1 #afd ,covering power of lens1 #dd1 #di1 #ej1 #eh ,crabbing1 #bg1 #cjh ,cradles1 camera1 #aie ,cross wires1 #bc insertion of camera through1 #db1 #baj ,curtain1 auxiliary shutter1 #hj1 #hd1 #ajf speed of travel of shutter1 #gd uniformity of1 #gf1 #hb1 #hd1 #hf1 #cae ,cylinders1 relation between vibration and number of1 #ahe ,dark slides1 double1 #hg1 #ii ,daylight1 intensity of1 #bbb ,definition1 lens1 #dd ,density1 of air1 effect of on propeller1 #aed of photographic image1 #bbh ,developers1 for plates and films1 #beg1 #bfj for papers1 #bfb ,developing machines1 film1 #acc1 #bgc ,ansco1 #bgc ,brock1 #bgd ,eastman1 #bgd ,g4,e4,m41 #bgc ,development1 core rack method of1 #bga factor1 #bbh film1 #bgb methods of1 #bfg1 #bfi of prints1 #bhf speed of1 #bcf1 #bfg tank1 #bgj time1 #bfi ,,dh-#d plane1 #baj1 #bag1 #bif photographic1 #bac ,,dh-#i plane1 #bif ,diaframs1 to equalize illumination of plate1 #gh lens1 #dh1 #eh ,dilution coefficient of developer1 #beh ,dirigibles1 #ae1 #af1 #dac ,distortion1 absent with between-the-lens shutter1 #gd barrel1 #ea due to camera tilting1 #bjf1 #bhf1 #cje1 #cae in aerial maps1 #cag lens1 #ci1 #dd1 #ea1 #ed1 #ef1 #fb pin-cushion1 #ea produced by film shrinkage1 #bcg produced by focal plane shutter1 #gd produced by glass plate in front of film1 #aca with wide angle lenses1 #fc 8,dodging0 in printing1 #bgi1 #cae ,doors in plane for camera to work through1 #bad ,drying of films1 #bfg1 #bgf of plates1 #bfg1 #bge of prints1 #bhf ,earth1 appearance of from plane1 #cj ,eastman apron film developing machine1 #bgd twin reel film developing machine1 #bgd ,efficiency1 propeller1 #aee shutter1 #gj1 #gb1 #gf ,english aerial photographic practice1 #de1 #df1 #bhc1 #bia1 #cdj ,,ek filters1 #bda ,electric1 drive for cameras1 #aaf1 #aai1 #abc1 #ade1 #adi generator operated by motor1 #adf motor1 characteristics of1 #aea1 #aef motor1 service1 #afc ,elevation possible to detect in stereoscopic views1 #cdj ,emulsions1 photographic1 characteristics of1 #bbg ,enlarging1 #de1 #bgi1 #bhc camera1 #bhc ,enlarging versus contact printing1 #ei ,exhaustion of developer1 #bei ,exploration1 use of aerial photography in1 #dja ,exposure1 data charts1 #bej distance between for mosaic maps1 #cjg distance between for stereos1 #ccf estimation of1 #bdh limitations to1 #bdg meters1 #bea meter1 ,wynne1 #bea of aerial negatives1 #bdg relation between motion of plane and1 #fh1 #ahe under1 period1 #bcj ,field1 angular1 of lens1 #eg1 #cjb flatness of lens1 #ef ,field laboratory1 mobile1 #bfh ,film cameras1 #dc1 #acj1 #acd ,brock1 #ach ,duchatellier1 #aca1 #acf ,f type1 #acd1 #aga ,g4,e4,m41 #ach ,german1 #fd1 #gf1 #aci1 #cag ,k type1 #adb1 #bjc1 #bad ,film1 celluloid1 #acj backed1 #acc changing in the air1 #acg1 #add color sensitiveness of1 #bcg cut1 #bge1 #bgh development of1 #acj1 #bgb drying1 #aca1 #bgf means for holding flat1 #acj1 #aca relative performance of compared to plates1 #bcg satisfactory kinds for aerial work1 #bch shrinkage1 #bcg ,filters1 #ae1 #eh1 #ajf1 #agd1 #bbd1 #bcc1 #bci1 #bda1 #bdc effects secured by use of1 #bda ,filters1 gelatin1 #bdd glass1 #bde holders for1 #fg1 #ajf1 #bbd ratio1 #bdj ,fixing bath1 #bfb ,flaps1 auxiliary to shutter1 #hj1 #hd1 #he1 #ih1 #ii1 #aja1 #aai to protect lens1 #bad ,flash lights1 #ah1 #chf ,flying boat1 #cgd ,focal length1 relationship of lens characteristics to1 #ee1 #eg requirements for1 #ci1 #db1 #eh1 #fa1 #ajc ,focal plane1 #ej shutter1 #gj1 #ga distortion by1 #gd performance of1 #hf types and representative1 #hj ,focus1 depth of1 #dd effect of temperature on1 #da1 #fe fixed1 in aerial cameras1 #dj ground and air1 #fe ,focussing1 automatic1 #bhd by parallax1 #fe1 #ff ,fog1 atmospheric1 #bbd photographic1 #bcg1 #beh ,french aerial photographic practice1 #de1 #df1 #bhc ,friction disc speed control1 #acf1 #aei1 #afj ,fuselage1 #ba1 #hg shape and size depending on type of engine1 #bc ,future of aerial photography1 #chc ,german aerial photographic practice1 #fc1 #ajc1 #bji ,glass1 optical1 used in lenses1 #dd ,gloves1 handling apparatus through1 #da1 #hi ,governor for camera speed1 #aei ,graphite1 use of to prevent static discharge1 #acd ,gravity1 action of in aerial cameras1 #da1 #aab1 #aae1 #aai center of1 should not change in cameras1 #abe1 #bjg change of center of1 in magazines1 #ib1 #aab depended on in magazines1 #hh handles best at center of1 #if pseudo1 in moving vehicle1 #ahh support at center of1 #ahb1 #bjc ,ground speed1 #bg1 #cjg indicator1 #caa ,guide-books1 illustration of1 #ae1 #cib ,guides for inserting magazines1 #da ,gyroscope1 #ahi ,gyroscopically controlled instruments1 #bi1 #agd1 #aib1 #cab ,gyroscopic erector1 #ahh mounting of camera1 #ahg stabilizer1 ,gray1 #aij ,hardener1 acid1 #bfb ,hand operation of de,ram camera1 #aba1 #abe1 #abi ,haze1 #ae1 #cj1 #bbc1 #bcc1 #bci ,head resistance and weight1 equivalent1 #aef ,heat1 effect of on plate sensitiveness1 #age1 #bcb ,heater1 electric1 in camera1 #adb1 #agd1 #bcb ,horizon1 photography of to indicate inclination1 #agd position of1 #cj ,hurter and ,driffield sensitometric curve1 #bbg ,illumination of field by lens1 #ej1 #ed1 #ef1 #eg ,image of point source1 size of1 #di1 #ed1 #ef size of in relation to focal length1 #ei ,incidence1 angle of assumed by plane at high altitudes1 #bjf ,inclinometer1 #cc1 #ce1 #aga1 #agc ,indicators1 distance1 #afc1 #bie ,inertia1 #bbi1 #beg ,installation1 camera1 #bd1 #bjh ,instructions1 operating1 to be placed on apparatus1 #db ,instruments1 airplane1 #cj ,instrument board1 #cj photographing1 #agj ,intensification of aerial negatives1 #bfb ,intensity of daylight1 #bbb ,interpretation of aerial photographs1 #ag1 #dj1 #cea ,interval between exposures1 #dj1 #aeh1 #cje1 #cjg for stereoscopic pictures1 #ccd1 #cch methods of regulating1 #abd ,isochromatic plates1 #bcc ,italian photographic practice1 #abb1 #cgg ,jamming of cameras in operation1 #aai1 #abj ,k#a and ,k#b filters1 #bci ,keeping power of developer1 #bei ,kites1 #ae1 #af1 #ah ,laboratory1 mobile photographic1 #bfi ,landscape gardening1 use of aerial photography in1 #cie ,latitude of plates1 #bbi ,lens1 #db1 #ci1 #dd1 #chc aperture1 #ci characteristics1 #df mounts1 #fe suitable for aerial photography1 #fb symmetrical1 #eb telephoto1 #fa testing and tolerances1 #eb unsymmetrical1 #eb wide angle1 #fb ,levels1 spirit1 on camera1 #ie ,light1 distribution of in aerial view1 #bba trail method of testing camera mountings1 #ahc ,loop1 centrifugal force in1 #bi ,machine gun ring as camera mount1 #cba ,magazines1 #hg bag1 #hh1 #aja ,bellieni1 #ib ,chassel1 #ib de,maria1 #hi1 #ih ,ernemann1 #hi ,folmer1 #ij ,fournieux1 #ib ,jacquelin1 #ib ,piserini and ,mondini1 #ij ,ruttan1 #ib ,magazine racks1 #id installation of1 #bag ,mapping1 #fd1 #ace1 #ahe1 #ahf1 #cjd1 #dja precision1 #cag1 #djd ,maps1 mosaic1 #ag1 #ci1 #fd1 #cad sketch1 #cad ,marking of negatives1 #bgh ,metol-hydrochinon developer1 #bfa ,mirrors for oblique photography1 #cbd1 #cbf ,monoplane1 #bd ,motions of camera1 #agi ,motive power for aerial cameras1 #ade ,mounting1 camera1 #ajb1 #ajc1 #agi1 #ahc #chd bell-crank1 #abj1 #aih1 #bjc ,brock1 #aci1 #bjg center of gravity1 #bje floor1 #aie ,g4 ,e4 ,m41 #ach1 #bjg ,italian1 #bjg outboard1 #aid parallel motion1 #aih pendular1 #ahe tennis ball1 #aif ,mounting1 camera1 turret1 #cab of prints1 #caf of stereograms1 #cdf ,movement1 of film during exposure1 #ge1 #adb of image1 permissible1 #fh ,moving pictures from plane1 #cej ,mud splashing on camera1 #bad ,naval aerial photography1 #cfh ,negative lens sight1 #afh1 #bii1 #cje ,night photography1 #chf ,numbering devices in cameras1 #afi ,oblique views1 #ci1 #cbj angles at which taken1 #dj1 #cba exposures for1 #fi filters for1 #bdb use of hand cameras for1 #ie ,observer1 function of in aerial photography1 #bia1 #bie1 #cjd ,oil spray from motor1 #bad ,opacity1 #bbh ,opening for camera1 #baa ,opposite directions1 shutter to move alternately in1 #gf1 #aci ,orthochromatic plates1 #bcc ,overlaps1 for mapping1 #cjg for stereoscopic views1 #dj on a turn1 #fd1 #aci ,panchromatic emulsions1 #bcc plates1 #di1 #bch ,panoramic views1 #cba ,parallax method of focussing1 #fe1 #ff ,parallel1 flying in1 #cjh ,photographic planes1 special1 #bac ,pilot1 function of1 in aerial photography1 #bia1 #bie ,pinpoints1 #ci1 #bia ,pistol grip for hand cameras1 #ie1 #if ,plate holders1 #hg ,plates1 bathed1 #bce behavior of compared to film1 #bcg color sensitive1 #bcc iso- and ortho-chromatic1 #bcc panchromatic1 #bcc satisfactory kinds for aerial work1 #bch self screening1 #bde shape of1 #fc size of1 #dc1 #fb ,plumb line1 behavior in banking plane1 #bh ,ply-wood veneer construction1 #bc1 #baa ,positype paper1 #bch ,potassium carbonate for drying plates1 #bgf ,power required to drive cameras1 #abe ,pressure1 of shutter curtain1 #aca plate1 for holding film flat1 #aca1 #ada ,printing1 #bgi contact1 #bgi machines1 #bgi media1 #beb ,prints1 paper1 #bec development of1 #bhf ,prisms for oblique photography1 #cbd1 #cbf ,propeller1 characteristics1 #aeb constant speed1 #abi1 #aei drive for cameras1 #ajb1 #aaf1 #aai1 #ace1 #acf1 #add1 #aeg1 #aeh position of1 #abj variable speed1 #aei ,pump1 for producing suction on film1 #acb ,punch marks on film1 #add1 #bgb ,pyro developer1 #bfa ,racks1 negative1 #bga ,real estate1 aerial mapping of1 #cic ,rectifying1 #bhf1 #cje camera1 #cad ,reduction of aerial negatives1 #bfb ,release1 shutter1 #if1 #ii duplicate1 for pilot1 #afd ,release1 shutter1 time controlled1 #abd ,relief1 criterion for correct1 #cce1 #cdd exaggerated1 #ccg impression of1 produced by motion1 #cdi ,resolving power of plates1 #ei1 #bce1 #bfj ,richard stereo printing frame1 #bhc1 #cdh ,rinsing of plates1 #bei ,rubber1 sponge1 use of in camera mountings1 #aie1 #bjc ,safe lights1 photographic1 #bfi ,safety catch on camera mounting1 #bjc device on camera driving mechanism1 #afb ,salvaging of ships1 aerial photography and1 #cii ,seaplane1 #be1 #cgd ,self screening plates1 #bde ,semaphore signalling in plane1 #big ,semperfocal enlarging camera1 #bhd ,sensitized materials1 requirements for1 #bbe ,sensitometry1 #bbg of papers1 #bec ,shadows1 compass directions from1 #cef proper direction for1 in examining prints1 #ceb ,shaft1 flexible1 #aai1 #abc1 #adb1 #afa ,sheaths1 plate1 #hg1 #hh1 #ic1 #abf1 #agj ,shrinkage1 film1 #bcg1 #cag paper1 #bhe1 #cae1 #cag ,shutter1 #db1 #fh between-the-lens1 #eh1 #gj1 #aab1 #aae1 #caf1 #chg efficiency1 #gj1 #gb1 #gf focal plane1 #gj1 #ga1 #gc focal plane1 double for stereo work1 #cda1 #cde focal plane1 moving alternately in opposite directions1 #gf focal plane1 types of1 #hj ,shutter1 ,folmer1 #hj ,ica1 #ha ,klopcic1 #gh1 #hd1 #ih release1 #if1 #ii1 #abd speed1 #ci1 #dj1 #eh1 #gj1 #bdi testing1 #gf testing apparatus1 #gg ,sights1 #afd1 #aff1 #bif1 #cja1 #cbg adjustable for angle of incidence of plane1 #afi attached to plane1 #afg negative lens1 #afh1 #bii1 #cje rectangular1 #ih1 #afg1 #cgc stereoscopic1 #cch to indicate size of field1 #aff1 #cgc tube1 #aja1 #aff ,single-seaters1 carrying cameras in1 #aad1 #baa ,slide rule1 photographic1 #cjc1 #cci ,solenoid1 #aea1 #afc ,sound1 not to be used for indication in plane1 #afd ,spacing of pictures in film camera1 #add ,speaking tubes on plane1 #bif ,speed1 of development1 #bcf1 #bfg of plates1 #bbh1 #bcf criteria of1 #bcj effect of temperature on1 #age1 #bcb variable1 control of camera1 #add1 #aea ,spotting1 #fd1 #abe1 #bia ,spring motors1 #adi1 #aee ,springs1 use of in aerial cameras1 #da in magazines1 #hh ,stabilized camera1 #ie1 #ahg ,static electric charges on film1 #aca1 #acc ,stereo-comparator1 #djd ,stereo-oblique views1 #aae1 #cba1 #cdc ,stereoscopes1 #cca ,stereo printing1 #bhc ,stereoscopic cameras1 #cda1 #cdd effect1 absence of at flying heights1 #cj1 #ccd ,stereoscopic photography1 #cbi mounting1 #cdf pictures1 fusion of without instruments1 #ccj1 #cdc sights1 #cch views1 #ci1 #dj1 #fd uses for1 #cdh vision1 principles of1 #cbi ,stop-watch attached to shutter release1 #caj ,strap1 on hand camera1 to go around observer's neck1 #ii on plate magazine1 #hg to go over hand1 on hand camera1 #ajj ,stream lines1 #bf lined hood1 #bad ,suction for holding film flat1 #aca1 #acb1 #adb advantages of continuous and intermittent1 #acb ,surveying by aerial photography1 #dja ,tank development1 #bgj ,tearing edges of prints1 #cag ,telephones on planes1 #bif ,telephoto lens1 #fa ,temperature1 coefficient of development1 #beh effect of on focus1 #da effect of on mechanical functioning1 #da1 #ajb1 #abe effect of on plate speed1 #bcb limits of development1 #beh ,test chart1 lens1 #eb ,threshold value1 #bcj ,thrust1 propeller1 #aeb ,timber1 location of by aerial photography1 #dac ,tone rendering1 correct1 #bbf1 #bcj1 #bci ,touch1 sense of1 not dependable in plane1 #da1 #abe1 #afd ,trailer1 photographic truck and1 #bfh ,transmission of power to camera1 #afa ,transparencies1 #beb1 #ccj ,transparency1 #bbg ,trays1 camera1 #aie ,tri-color ratio1 #bce ,triplane1 #bd ,tuning fork1 used in shutter tester1 #gi ,turbine1 wind1 for driving camera1 #aaf1 #abg1 #add1 #adg1 #aeh ,uniformity of curtain speed in focal plane shutter1 #gf1 #hb1 #hd1 #hf1 #cae ,unit system of camera construction1 #db ,uprights1 camera1 in plane1 #bji ,uses for aerial photography1 #chh ,velocity constant1 #beh ,veneer construction1 #bc1 #bji1 #baa ,venturi tube1 #acb1 #adb1 #add ,vibration1 #af1 #ah1 #bf1 #dj1 #da1 #eh1 #ajb1 #agi ,volcanoes1 photography of1 #ae1 #cii ,water for mixing chemicals1 #bfb ,watkins factor1 #beh ,weight and head resistance1 equivalence1 #aef of film compared to plates1 #aja1 #acj of de,ram camera1 #abc of hand cameras1 #if of ,k type camera1 #add of ,k film roll1 #add of ,l type camera1 #aag of ,m type camera1 #aji of storage batteries1 #aeg ,wind1 flying against1 #fh1 #cjh motor1 #adf ,windows in side of plane1 #bad1 #cbh ,wire1 barbed1 appearance of in aerial photograph1 #cfj 99 99 99 99 99 99 ,transcriber's note3 #a4 ,silently corrected typographical errors4 #b4 ,retained anachronistic and non-standard spellings as printed4