,,optical ,,projection ,a ,,treatise ,,on ,,the ,,use ,,of ,,the ,,lantern ,,in ,,exhibition ,,and ,,scientific ,,demonstration ,,by ,,lewis ,,wright ,,author ,,of ',,light3 ,a ,,course ,,of ,,experimental ,,optics' #e,,th ,,edition ,,re-,,written ,,and ,,brought ,,up-,,to-,,date ,,by ,,russell ,s4 ,,wright1 ,m4,i4,e4,e4 ,,in ,,two ,,parts ,,part ,i ,,the ,,projection ,,of ,,lantern ,,slides ,,with ,,illustrations ,,longmans1 ,,green1 ,,and ,,co4 #ci ,,paternoster ,,row1 ,,london1 ,e4,c4 ,,fourth ,,avenue #cj,,th ,,street1 ,,new ,,york ,,bombay1 ,,calcutta1 ,,and ,,madras #aibj 7,all rights reserved7 99 99 99 99 99 v ,,preface ,,to ,,the ,,fifth ,,edition ,the first edition of this work was written by my father1 the late ,mr4 ,lewis ,wright1 and was published in #ahij4 ,the reception that it received testified to the fact that it met a long-felt want1 and successive editions were published in #ahie1 #aija1 and #aijf4 ,my father1 unfortunately1 met his death in a railway accident in #aije1 and the corrections and additions to the last edition1 which had been to a certain extent prepared by him1 were completed and written by myself1 and the work as published then was again reprinted in #aiaa4 ,as the original text is now thirty years old1 it has seemed better entirely to re-write the whole book rather than make fresh revisions1 the more so as the last ten years have seen great advances in the science of ,lantern ,projection1 and especially in the developments of ,acetylene and ,electric ,lighting4 ,it has also seemed best at the present juncture to issue the book in two parts1 the first dealing with the ,projection of ,lantern ,slides only1 and the second with the ,demonstration of ,opaque and ,microscopic ,objects1 ,scientific ,phenomena and accessory apparatus1 including ,cinematograph ,projection4 ,it must of necessity be many months before this second volume can be produced1 for the simple reason that ,optical vi ,instrument ,makers have as yet hardly had time to turn round after the war and produce their new models1 and therefore any such book written now could do little more than describe apparatus that was on the market prior to #aiad4 ,the present work1 therefore1 deals solely with the exhibition of ,lantern ,slides in the ,optical ,lantern1 and as such ,i trust will be found of value to ,schoolmasters1 ,social ,workers1 ,lecturers1 and1 in fact1 to all who use the lantern as a means of illustration4 ,,russell ,s4 ,,wright4 ,january #aibj4 99 99 99 99 99 vii ,,contents ,,chapter ,,page ,i4 ,,introductory #a ,,ii4 ,,the ,,illuminant #c ,,iii4 ,,paraffin-,,oil ,,lamps1 ,,incandescent ,,gas ,,and ,,spirit ,,burners #f ,,iv4 ,,the ,,acetylene ,,light #aa ,v4 ,,limelight ,,and ,,the ,,acetylene ,,blast #af ,,vi4 ,,the ,,electric ,,light #ci ,,vii4 ,,the ,,optical ,,system ,,of ,a ,,lantern #eg ,,viii4 ,,the ,,body ,,of ,,the ,,lantern #gj ,,ix4 ,,lantern ,,boxes1 ,,stands1 ,,reading ,,lamps1 ,,etc4 #gf ,x4 ,,screens ,,and ,,screen ,,stands #gi ,,xi4 ,,the ,,practical ,,manipulation ,,of ,a ,,lantern #hb viii ,,illustrations ,,fig4 ,,page #a4 ,oil ,lamp #f #b4 ,inverted ,incandescent ,lamp #h #c4 ,methylated ,spirit ,burner #i #d4 ,luna ,lamp #aj #e4 ,the ,moss ,generator #ab #f4 ,the ,a4,l4 or ',popular' ,model #ad #g4 ,acetylene ,jet #ae #h4 ,oxygen ,cylinder in hemp cover #ag #i4 ,double ,lever ,key #ah #aj4 ,fine ,adjustment ,valve #ai #aa4 ,construction of ,beard's ,regulator #bj #ab4 ,beard's ,regulator #ba #ac4 ,regulator and ,gauge #bb #ad4 ,gas-bags #bd #ae4 ',blow-through' ,nozzles #be #af4 ',blow-through' ,jet #be #ag4 ,mixed ,jet #bg #ah4 ,mixed ,jet1 ,gwyer pattern #bg #ai4 ,mixing ,chamber of ,jet #bh #bj4 ',injector' ,jet #cj #ba4 ',gridiron' ,saturator #cb #bb4 ',pendant' ,saturator #cc #bc4 ,fallot ,air ,blast #cg #bd4 ,fallot ,air ,blast1 and ,cylinder #cg #be4 ,lime-tongs #ci #bf4 ,universal ,hand-fed ,arc ,lamp #de #bg4 #df #bh4 ,resistance #di #bi4 ',scissors' ,arc ,lamp #ea #cj4 ',right-angled' ,arc ,lamp #eb #ca4 ',westminster' ,arc ,lamp #ec #cb4 ,arc ,lamp with ,induction ,ring #ef #cc4 ,the ,optical ,system of a ,lantern facing p4 #eg #cc;,a4 ,optical ,system of ,lantern #eg #cd4 ,optical ,system without ,condenser #ei #ce4 ,action of ,condenser #ei #cf4 ,forms of ,condensers #fj #cg4 ,double ,sliding ,carrier #fb #ch4 ,beard's ,dissolving ,carrier #fc #ci4 ,focussing ,action of ,lens #fd #dj4 ,achromatic ,lens #fe #da4 ,petzval ,combination #ff #db4 ,hughes' ,short-,range ,lantern #ga #dc4 ,long-,range ,lantern #gb #dd4 ,connections for a ,bi-unial ,lantern #gc #de4 ,beard's ,circulating ,water ,tank #ge #df4 ,quadruple ,lantern ,stand #gh #dg4 ,reading ,lamp #gi #dh4 ,roller ,screen #hj #di4 ,portable ,screen ,stand #ha #ej4 ,adjustment of the ,light #hd 99 99 99 99 99 #a ,,optical ,,projection ,a ,,treatise ,,on ,,the ,,use ,,of ,,the ,,optical ,,lantern ,,chapter ,i ,,introductory ,lantern ,projection1 as commonly understood1 may be broadly subdivided into two branches3 7,a7 ,the ,projection of ,lantern ,slides1 and 7,b7 ,the ,projection of ,scientific ,phenomena1 ,opaque ,objects1 ,microscopic ,specimens1 c41 usually referred to broadly under the heading of ',scientific ,demonstration4' ,to these two classes may perhaps now be added a third1 viz4 ,the ,projection of ,so-called ,living ,pictures1 or1 in other words1 the ,cinematograph4 ,in the earlier editions of this work both ,a and ,b were dealt with in the same volume1 but1 as there are thousands who require to use a lantern for the demonstration of lantern slides only1 and who have no interest or concern with ,science ,projection1 it has seemed to the writer that the work might1 with advantage1 be divided into two portions1 ,vol4 ,i4 dealing with slides only1 and ,vol4 ,,ii4 with the various adaptations of the science lantern4 ,this present book therefore only deals with the exhibition of lantern slides1 and as such it will1 ,i trust1 be found to be of real assistance to the ordinary user of the optical lantern1 including clergymen1 schoolmasters1 army and cadet officers1 and others #b who require advice and instruction in the purchase or use of a lantern4 ,the essential parts of a lantern are3 7a7 ,a slide-holder or carrier to hold the slide2 7b7 a lens to 'focus' it on the screen2 7c7 a condenser to converge the light upon slide and lens2 7d7 a source of light or radiant to provide the necessary illumination2 and 7e7 a body or framework to hold the whole together4 ,all possible variations in choice of a suitable lantern relate to one or another of the above parts1 and will be treated of in turn2 but1 fortunately1 we have this all-important simplification that every ordinary ,english lantern slide is the same standard size1 viz4 #c inches square4 ,some ,continental and ,american slides differ in one dimension from the above1 but not enough to cause any serious difficulty1 and the convenient ,english standard is being gradually adopted throughout the world4 ,the varieties of slide-holders or carriers are therefore comparatively few and are chiefly concerned with the question of rapidly and easily changing the slides4 ,the choice of a focussing lens or objective is mainly a matter of the size of picture required1 and the most convenient distance from the screen for the lantern to be placed4 ,variations in condensers1 which are comparatively small1 are usually only a matter of conforming these with the size or type of objective to be used1 and should be left to the manufacturer's judgment4 ,the question of a suitable radiant is partly a matter of the amount of illumination required1 and partly that of the practical possibilities2 for example1 if electric current is available some form of electric light is usually the most convenient1 as well as the least expensive1 but where this is not the case1 paraffin-oil1 methylated spirit1 incandescent gas1 acetylene1 limelight1 c41 are alternatives which all have their uses and must be considered on their own merits4 ,sometimes1 as for example in the case of a travelling lecturer1 a lantern is required fitted with a range of lenses for #c halls of different size1 and also with a variety of illuminants1 and this in most lanterns can be easily provided for4 ,the body is usually a matter of taste and price only1 and may range from a simple but efficient shell of ,russian iron to an elaborate mahogany instrument with a brass front1 screw tilting arrangements and other adornments2 but of late years there has been a wholesome reaction against unnecessary finish1 and a simple metal body of some description is now chiefly the order of the day4 ,in the foregoing remarks the various parts of a lantern have been mentioned in what ,i should consider the correct order1 starting from the slide and slide-holder1 and so to speak building up the rest of the instrument round these items2 but ,i now propose somewhat to vary the procedure and for convenience deal in detail first with the ,radiant1 or ,illuminant4 99 99 99 99 99 ,,chapter ,,ii ,,the ,,illuminant ,the first necessity for lantern projection is a strong light1 and this can be obtained from a variety of sources1 the principal means in common use being approximately in order of excellence as follows3 paraffin-oil1 incandescent spirit1 incandescent gas1 acetylene1 acetylene air blast1 oxyhydrogen 7limelight71 oxyether1 and electric light in its various forms4 ,the ideal characteristics to be sought for are 7#a7 great intrinsic brilliancy2 7#b7 minimum size of luminous spot2 7#c7 freedom from flicker2 7#d7 freedom from smell2 7#e7 absence of any preponderating colour2 7#f7 cheapness2 and 7#g7 convenience4 ,there is no question whatever as to which of the available sources of light most perfectly combines all the above if it is available1 viz4 the electric arc4 ,if a current supply is in #d the building1 this form of lighting easily excels all others1 except possibly in the matter of flicker1 and even in this respect there is very little fault to be found with it4 ,from all other points of view it is wellnigh perfect1 inasmuch as it provides an extremely concentrated and intensely luminous spot1 of almost perfect whiteness 7if anything slightly bluish71 no smell1 comparatively little heat1 convenient and inexpensive4 ,so great is the advantage of the electric arc that attempts have been made to use it from accumulators in places where a current supply is not available1 but this cannot be seriously recommended1 except in special cases4 ,where an electric supply is1 however1 available there can be no real choice1 whether the lantern is required for use in a large hall or a small class-room4 ,the advantages of using the arc are so great that no other method need be seriously considered4 ,the one real objection that ,i have heard urged against it is due1 curiously enough1 to its very perfection1 and that is1 that it lends itself to such exceedingly sharp definition that any slight imperfection in the slide is too faithfully reproduced on the screen1 for which reason it is sometimes recommended that the operator shall work with the objective the least fraction out of focus2 but this is a matter for individual taste and judgment4 ,if1 however1 there is no possibility of using the electric current1 one of the other sources of illumination must perforce be adopted1 and for a large hall this can only be limelight in one of its many forms1 viz4 oxyhydrogen1 oxyether1 oxyacetylene1 c4 ,as regards results on the screen1 this light compares well even against the electric arc1 but it involves the expense and trouble of compressed gas cylinders1 or the infinitely worse recourse to the now obsolete method of filling gas-bags4 ,limelight is therefore now but little used in this country1 as the majority of large halls are equipped with the electric #e current1 and for smaller buildings it is deemed unnecessary and too expensive4 ,,acetylene is undoubtedly the illuminant most in favour next to electric light1 as the light is brilliant enough to illuminate a picture #ab feet in diameter at a distance up to1 say1 #cj feet from the screen1 and this suffices in a large majority of cases1 and acetylene is comparatively cheap1 and reasonably simple to work4 ,,incandescent-,,gas is often employed for small class-rooms and is fairly effective for a picture not exceeding #i or #aj feet in diameter1 and the same can be said of the same type of burner heated by methylated spirit4 ,,paraffin-,,oil is the poorest of all present-day forms of lantern illuminants4 ,the flame is large1 impairing the definition1 yellow in colour1 uneven in illumination1 liable to smoke and smell1 and barely equal to incandescent gas in illuminating power4 ,it is therefore going gradually out of use in this country1 but in out-of-the-way places1 especially abroad1 it is sometimes the only practicable light1 and is therefore still employed from the best of all reasons1 necessity4 ,it is not the intention of the author to give precise working instruction for all and every variety of the above illuminants as manufactured by different firms4 ,for such the reader must be referred to the directions usually issued by the makers themselves1 but a general description of the various types offered for choice will not be out of place1 and it will be more convenient to begin with the poorest1 viz4 paraffin-oil1 and finish with the most perfect1 the electric arc4 99 99 99 99 99 #f ,,chapter ,,iii ,,paraffin-,,oil ,,lamps1 ,,incandescent ,,gas ,,and ,,spirit ,,burners ,illustration3 ,,fig4 #a4--,oil ,lamp4 ,there are several varieties of oil lamps on the market1 but in practically every case they take the same general form1 a metal reservoir sliding in grooves in the lantern body and holding approximately a pint of oil with 7usually7 four wicks nearly parallel1 but slightly converging from rear to front1 these enclosed in a flame chamber of ,russian iron1 with loose well-annealed ends of sheet glass and an adjustable reflector at the back1 or sometimes the reflector itself forms the rear end of the flame chamber4 ,the chimney must be tall and is now usually made adjustable1 though ,i have never been able to trace any real advantage from this complication #g 7,fig4 #a74 ,the whole secret of obtaining the best results from these lamps may be summed up--good oil and perfect cleanliness2 and it is wonderful what can be done when these points are properly attended to4 ,care should be taken in trimming the wicks to see that no charred parts fall down between the wick holders1 but it makes little difference whether the trimming is done with scissors or by rubbing with the finger4 ,special lamp scissors are sold by all makers with a large flat on one side to catch the portions cut off4 ,these lamps should be well rubbed over the last thing before use1 as paraffin-oil is apt to 'creep1' and the operator does not want to be told that his apparatus is suggestive of a fried fish shop4 ,in working with these lamps it is difficult to avoid a dark streak down the centre of the sheet1 representing the space between the two centre wicks2 to a certain extent this can be obviated by adjusting the reflector1 and in any case is not very obvious when the slide is in place4 ,lamps constructed with either three or five wicks are better in this respect1 but the former are usually considered to be too poor in illuminating power1 and the latter are apt to crack the sheet-glass ends by excessive heat4 ,,incandescent ,,gas4--,incandescent gas burners do not need much description1 as they are practically similar to those in general use for house lighting4 ,they may be either of the erect or inverted forms1 the latter being preferable owing to the light being more concentrated1 and a reflector is provided to increase the illumination 7,fig4 #b74 ,these reflectors should be spherical and so adjusted that the radiant is in the centre of curvature1 thus ensuring that the light from the reflector passes again through the original source4 ,if this point is not attended to1 we shall be dealing with essentially two sources of light instead of one1 to the detriment of the definition4 ,the same remark applies to every lantern illuminant #h which is supplemented by a reflector1 and it is extraordinary how often it is neglected by the manufacturer4 ,of course the opacity of the illuminant destroys much of the efficiency of the reflector1 and hence in the case of incandescent gas mantles there is not much real gain in making use of them1 but with these comparatively weak illuminants every fraction tells1 and the reflector does not add much to the cost4 ,illustration3 ,,fig4 #b4--;,inverted ,incandescent ,lamp4 ,in light the inverted gas burner is very little superior to oil1 but it is whiter1 slightly more concentrated1 and freer from smell1 and therefore to be regarded as preferable if a supply of gas is available4 ,,methylated ,,spirit ,,burners4--,incandescent mantles heated by methylated spirit are also largely used1 and provide a light decidedly superior to gas and nearly equal to acetylene4 ,some arrangement must be made for volatilising the spirit and driving the vapour out under pressure1 and the most usual contrivance is somewhat as illustrated in ,fig4 #c4 ,in this apparatus the spirit is contained in a metal reservoir at the rear and air pressure is provided by a pair of rubber balls and valves after the manner of a medical spray4 ,sufficient #i pressure having been obtained1 the liquid spirit is forced into a vaporising chamber immediately behind the mantle1 and a kind of miniature pitchfork1 with its prongs wrapped in asbestos wool1 is soaked in spirit1 and pushed over the brass fitting of the burner in such a way that when lighted the flame heats the chamber and volatilises the spirit4 ,the burner can now be lit1 and although the fork burns out in the course of a minute or so1 the heat from the mantle itself is thereafter sufficient to vaporise the spirit as rapidly as required4 ,this lamp works exceedingly well in practice1 but has one drawback1 viz4 that it is possible to obtain too much pressure and squirt liquid spirit through the burner1 when it naturally catches fire and may even run on to the floor4 ,illustration3 ,,fig4 #c4--,methylated ,spirit ,burner4 ,an accident of this sort is rare and usually harmless even if it does occur1 but an audience is easily frightened1 and hence this burner should only be used by an operator with experience4 ,an altogether better arrangement is that made by ,messrs4 ,hughes of ,kingsland and known as the ',luna' ,lamp 7,fig4 #d74 ,in this burner there is no pump and no volatilising chamber2 #aj the spirit is contained as before in a metal reservoir and a separate burner underneath is used to keep this sufficiently hot to both vaporise the spirit and provide the necessary pressure4 ,the heat can be regulated by means of an adjustable sheath to the burner1 and a simple safety valve provides against an excess of vapour4 ,i do not say that an accident of the sort previously referred to is impossible even with this burner1 but ,i have never heard of it happening1 and the lamp is certainly the best apparatus of its kind that ,i am acquainted with4 ,illustration3 ,,fig4 #d4--,luna ,lamp4 ,,incandescent ,,electric ,,lamps4--,incandescent electric lamps of the ordinary metal or carbon filament type are also frequently used in small class-rooms1 and should be mentioned here1 as they provide approximately the same illumination as a gas mantle1 or in some cases rather better4 ,it will1 however1 be more convenient to deal with the question of electric lighting as a whole in the chapter devoted to it4 ,it will suffice here to say that lamps are made for the purpose with a special filament arranged to provide a concentrated light1 the ordinary type being almost useless in this respect1 and that small battery lamps1 worked by a suitable accumulator1 can also be used1 but except under very special circumstances are hardly worth the trouble of keeping the batteries charged4 99 99 99 99 99 #aa ,,chapter ,,iv ,,acetylene ,there is no doubt that at present acetylene holds second place to electric light in popularity for optical lantern work4 ,the light is good2 not1 it is true1 so good as limelight or the electric arc1 but still sufficient for a picture up to #ab feet in diameter at a working distance from the screen of not more than #cj feet1 and this suffices for the large majority of halls4 ,it has great advantages over limelight in convenience and cheapness1 although on both these points it must yield place to the electric arc1 always providing that current is available1 and therefore it is chiefly used in country districts and in gas-lit halls in large towns4 ,acetylene gas is formed1 as is well known1 by the action of water upon carbide of calcium1 and the generators constructed for lantern work are essentially the same in construction as for other purposes4 ,the alterations introduced are chiefly directed towards obtaining a light as steady as possible from a comparatively small generator1 and1 secondly1 towards the entire elimination of smell1 which obviously is far more serious in a lecture hall than1 for instance1 on a motor car4 ,the generators in most common use may be divided into two classes1 i4e4 those on the gasometer principle in which the carbide is gradually lowered into the water1 and those in which the water is allowed slowly to gain access to the carbide4 ,a good example of the former is perhaps that made by ,messrs4 ,moss of ,birmingham1 though there are several others equally good1 and clear and explicit directions for working should be supplied by the makers4 ,the ,moss ,generator 7,fig4 #e7 consists of a tall iron vessel ,a fitted with a gas tap at bottom1 this communicating #ab with a vertical iron tube within the vessel4 ,into this container fits the inner bell or container ,b1 divided internally into two concentric portions entirely separated from each other1 but connected by the pipe ,p ,p and the tap ,t4 ,a guide inside the bell encircles the iron tube in the outer tank and prevents rotation4 ,into the inner portion fits again the carbide-container 7shown separately on the left71 which is locked when in place by giving it a half turn1 when a hook inside the bell engages with the lower edge of the carbide container and prevents it from falling4 ,the carbide container is fitted with a series of shelves1 and the contents of a #b lb4 tin of carbide should be roughly divided among them2 there is no need to make any accurate division4 ,the carbide used should be that known as inch mesh1 and should be pure4 ,that described as 'chemically' treated is apt to give trouble by over-generation in these gasometers and should be scrupulously avoided4 ,illustration3 ,,fig4 #e4--,the ,moss ,generator4 ,the carbide having been placed in the receptacles1 these should be closed by means of the loose flap and the whole pushed into the bell and secured4 ,water should be poured into the outer vessel up to a mark on the iron tube1 and the bell placed in position4 ,the lower tap being then turned on and the upper one closed1 air from the outer portion of the bell can gradually escape by means of the iron tube and lower tap1 and the bell gradually sinks by its own weight until it is on the bottom1 but still #ac no water can reach the carbide1 the air imprisoned in the inner portion of the bell effectually excluding it4 ,the lower tap should now be connected by means of india-rubber or flexible metallic tubing to the burner in the lantern 7of which more anon71 and the upper tap on the generator turned on1 the tap or taps on the burner being likewise opened4 ,the air from the inner portion of the bell can now escape by the pipe ,p ,p into the outer part1 and thence through the iron tube1 and out through tubing and jet1 and as it does so water will rise in the interior and attack the carbide4 ,in a few moments the burner can be lit2 but the gas1 being generated far in excess of requirements1 and filling both the inner and outer portions of the bell faster than it can escape1 lifts the latter until the carbide is entirely out of the water1 when in a few minutes generation ceases4 ,if the jet is left burning the bell will gradually sink again as the gas is used up1 and should thereafter maintain an automatic balance without attention4 ,it can be turned off at any moment by simply closing the taps at the jet or1 better1 the lower tap at the generator1 when the bell rises sufficiently to take the carbide out of the water2 but if it is required to leave the generator unlit for a considerable time1 it is better to turn off the tap on the top first4 ,this causes the inner portion of the bell to fill with gas which cannot escape1 and as that in the outer part burns out1 the bell sinks to the bottom and remains there1 the gas itself imprisoned in the inner chamber excluding the water from the carbide4 ,the exact arrangement varies in different patterns of generator1 but the above may be taken as roughly indicating the action1 and further information may always be obtained from the maker or dealer4 ,emptying should always be done out of doors1 as the odour of acetylene gas is most objectionable1 and for the same reason rubber tubes1 c41 should be securely tied on1 so that the slightest escape may be avoided4 #ad ,if the exhibition has been a short one it will often be found that the upper cells have not been affected by the water1 in which case they may be put back in the tin and used again1 but it is not generally advisable to put in less than the full charge to begin with as the weight of the carbide plays a definite part in securing the smooth action of the apparatus4 ,the sludge should be thrown away 7it forms a good manure for the garden7 and the entire generator thoroughly dried1 otherwise rust will quickly appear4 ,illustration3 ,,fig4 #f4--,the ,a4,l4 or ',popular' ,model4 ,theoretically one of these generators may be filled and left standing indefinitely1 but in practice it is not advisable1 as the damp in the atmosphere is apt to produce a very slow generation of gas1 sufficient often to cause a decided smell4 ,of generators which act by admitting water to the carbide perhaps the best known is the ,a4,l4 or ',popular' ,model 7,fig4 #f71 this being1 in fact1 a pattern designed for motor-car head-lights1 but which answers well for lantern work4 ,its exact operation need hardly be described here in full detail4 ,it will suffice to say that the water gains access to the carbide by 'creeping' up between two concentric copper cones1 and in the event of over-generation the pressure of the gas automatically checks the flow4 ,this generator is smaller than the gasometer pattern1 and hence can be recommended for portability2 but in my experience the light is not quite so steady1 and the control rather less delicate1 thereby causing on occasions a perceptible smell1 especially if left standing for a considerable time4 ,there are other types of generators1 such as the ',water #ae dropping' variety1 in which the water drips on to the carbide1 and the reverse1 in which fine granulated carbide drops a little at a time into water2 but these types are not very frequently met with and need hardly be described4 ,it should never be forgotten that acetylene is an explosive gas and should be treated as such4 ,searching for a leak with a lighted match1 though perhaps permissible when the operator knows his business1 may be a dangerous proceeding when the contrary is the case4 ,illustration3 ,,fig4 #g4--;,acetylene ,jet4 ,acetylene burners are generally of the ',batswing' type1 and are as a rule four in number1 mounted in a row with a reflector behind1 each burner being separately controlled by its own tap 7,fig4 #g74 ,an acetylene flame is very smoky1 and care must be taken that the burners are not turned too high4 ,a nipple cleaner1 consisting of a fine wire in a short handle1 can usually be obtained from any dealer1 and is very handy4 ,acetylene gas can also be used for lantern illumination in quite another way1 viz4 by a blast from a blowpipe1 in combination with either air or oxygen1 on to a special ',pastille' provided for the purpose1 or an ordinary limelight jet can be used4 ,these methods entail the use of acetylene under pressure1 and are so analogous to limelight that ,i shall for convenience deal with them in the chapter devoted to that illuminant4 99 99 99 99 99 #af ,,chapter ,v ,,limelight ,,and ,,the ,,acetylene ,,blast ,the illumination possible with this light is almost unlimited1 and for really large halls it is1 as remarked before1 the only substitute for the electric arc4 ,it consists essentially of a blowpipe flame1 composed of oxyhydrogen1 oxyether1 oxyspirit1 oxy-acetylene1 c41 or acetylene air blast1 heating to incandescence a block of lime1 or other refractory material1 and the essential feature is that one at least of these gases must be under pressure4 ,thirty years ago this was usually achieved by storing the gas in rubber bags1 and obtaining the requisite pressure by means of heavy weights2 but except in a very few outlying districts this method has now been completely superseded by the use of compressed gas cylinders4 ,the earlier editions of this work contained very full directions for manufacturing gas for storage in bags1 but it is so exceptional now to find an operator who uses this method that it seems hardly necessary to devote much space to it1 and the same may be said of automatic oxygen 'generators4' ,the present work will therefore deal chiefly with compressed gas cylinders4 ,most elaborate precautions are now enforced by the ,board of ,trade to ensure the absolute safety of these1 and any doubt existing from occasional accidents of years ago may be promptly dismissed4 ,humanly speaking1 an accident nowadays cannot happen1 except by such wilful negligence on the part of the maker or filler as would almost render the culprit subject to criminal proceedings4 ,compressed gas cylinders are painted a distinctive colour1 oxygen for example being black and coal gas or hydrogen red2 the screw connections to the pumps1 and all nozzle #ag and regulator fittings1 are made with a totally different screw and therefore cannot be interchanged2 the cylinders themselves are bound by law to be reannealed and retested under hydraulic pressure at regular intervals2 the steel itself has to be of a guaranteed quality2 and1 in fact1 every possible risk is guarded against4 ,the most usual sizes of cylinders supplied for lantern exhibitions are those containing #f1 #ab1 #bj1 or #dj cubic feet1 and are usually sent out in wooden or hemp cases4 ,illustration3 ,,fig4 #h4--,oxygen ,cylinder in hemp cover4 ,fig4 #h shows a #ab-;foot cylinder in its hemp case1 the approximate size without case being #bb in4 by #d in4 ,this size cylinder will supply an average limelight jet for just over two hours4 ,the extra powerful jets as used for cinematograph work or for illuminating a very large screen take a good deal more1 but for the usual apparatus as supplied for ordinary lantern purposes this is a pretty safe figure4 ,a #ab-;foot cylinder is therefore the favourite size for a lantern exhibition lasting from an hour to one and a half hours1 as it leaves a fair margin for gas used in adjusting the instrument1 c41 and a #bj-;foot cylinder will usually suffice for two such exhibitions4 ,the price of gas per cubic foot varies with the size of the cylinder1 being less for large cylinders than for small ones1 and the cost of transit is also less in proportion--hence it is frequently an economy to hire a large cylinder and retain it for several exhibitions4 ,on the other hand most suppliers charge a small rent if a cylinder is retained beyond a definite #ah time1 so this is a question to be decided by each user on its own merits4 ,alternatively1 of course1 cylinders can be purchased1 and the question of rent does not then come in2 also gas is supplied a little cheaper in a customer's own cylinder than if sent on hire4 ,if purchase is decided on it is frequently an economy to buy two1 or two of each gas1 if coal gas cylinders are required as well4 ,illustration3 ,,fig4 #i4--;,double ,lever ,key4 ,the whole contents of the cylinders can then be used up without waste1 as if a cylinder should become exhausted during the course of a lecture1 it is only a matter of a minute or two to change over to the spare one1 whereas the compressors are required by law to empty out every cylinder returned to them for refilling1 and any remaining gas is thereby wasted4 ,it is extremely tantalising1 to say the least of it1 to find the pressure gauge indicating that there is1 say1 #h feet of gas remaining in a cylinder1 and to be compelled to waste this or else risk running short for the next exhibition1 and duplicate cylinders are the only way of avoiding the loss4 ,the cylinders are filled to a pressure of #abj atmospheres1 or #ahjj lb4 per square inch1 and are closed by strong screw nozzles4 ,the keys for opening or closing these are of three types1 viz4 the ',t' pattern1 ',spanner' pattern1 and that known as the ',double ,lever' type4 ,this latter is so made that in closing the valve it shuts up to half its length and #ai opens out to double the leverage when being used to open the cylinder 7,fig4 #i74 ,the idea is to avoid the possibility1 which has been known to occur1 of the cylinder valve being screwed down by a powerful wrist and defying the efforts of the despairing lanternist to open it4 ,illustration3 ,,fig4 #aj4--;,fine ,adjustment ,valve4 ,cylinder nozzles are unfortunately not yet standardised1 but those most frequently met with in this country are those adopted by the ,british ,oxygen ,company1 both oxygen and coal gas cylinders being fitted with corresponding internal screws #g_/h inch diameter1 those for oxygen being right-handed1 and those for coal gas left-handed1 and in each case terminated at the bottom by a hollow metal cone4 ,as such an internal screw cannot obviously be connected to a piece of rubber tubing1 some type of screw connector must be employed1 and this may take one of three forms3 7#a7 ,a simple connecting nozzle1 7#b7 a fine adjustment valve1 or 7#c7 a regulator4 ,the first is seldom used in practice for lantern work1 for the reason that the direct pressure of a full cylinder 7#abj atmospheres7 cannot be checked or controlled by a tap on the jet1 as the intervening rubber tubing would either burst or blow off1 and must therefore be regulated at the cylinder nozzle itself1 and gradually readjusted as the pressure diminishes4 ,to achieve this regulation with the ordinary cylinder key is difficult1 though possible to a careful operator1 but for a slight extra expense a combined nozzle and fine adjustment valve 7,fig4 #aj7 can be obtained1 and regulation with this is #bj infinitely easier4 ,the best plan of all1 however1 is to use an automatic regulator1 which not only reduces the pressure so as to permit of the required adjustments being made at the jet-taps1 but also maintains a practically steady supply as the cylinder empties1 thereby obviating continual readjustments4 ,regulators are now so inexpensive that they have come into almost universal use1 and are generally reckoned an indispensable part of a limelight lantern equipment4 ,the form of regulator in most common use is that usually known as ',beard's1' having been originally designed and patented by ,messrs4 ,r4 ,beard ,sons1 though as the patent has now expired it is open to any firm to make the same article if they desire4 ,illustration3 ,,fig4 #aa4--;,construction of ,beard's ,regulator4 ,the construction of ,beard's ,regulator is shown in ,fig4 #aa4 ,the gas enters from below into a rubber bag1 ,c1 from which it can emerge through the nozzle4 ,any accumulation of gas raises the bellows against the pressure of a spiral spring pressing it down1 and this brings into action an arrangement of so-called ',lazy ,levers1' which in turn presses down a small conical valve and closes the supply from the cylinder1 this valve re-opening immediately the pressure diminishes4 ,the outward form of this regulator is shown in ,fig4 #ab1 #ba which incidentally also illustrates the usual form of connection to the cylinder1 referred to later on4 ,in ,beard's ,regulator the pressure at which the gas can be delivered is determined by the strength of the spiral spring1 and can only be altered by changing this spring4 ,illustration3 ,,fig4 #ab4--;,beard's ,regulator4 ,in practice ,beard's ,regulators are supplied set to a low pressure for ordinary mixed or 'blow-through' jets and for a higher pressure 7#ad-af inches7 for 'injector' jets4 ,at this latter pressure the rubber tubing used must be fairly thick and strong and well tied on1 and even so the taps of the jet should not be turned entirely off unless the gas at the cylinder is likewise turned off immediately afterwards4 ,the ,british ,oxygen ,company make a regulator which can be set to any desired pressure1 but it is not quite so delicate in its action as ,beard's1 and ,messrs4 ,clarkson also make a pattern regulator which is widely used and well spoken of4 ,the attachment of any of these fittings to the cylinder is a somewhat peculiar one1 as will be seen on reference to ,fig4 #aj or ,fig4 #ab4 ,the regulator or nozzle ends at its lower extremity in a screw and cone1 the latter being intended to make a gas-tight connection with the internal cone on the cylinder1 and over this screws a loose wing piece with another outer screw1 this latter fitting the thread in the cylinder4 ,in making the connection care must be taken that the wing piece is not screwed too far down the inner screw1 or the cone will not reach down and make a tight fit on its #bb seating2 in its correct position the wing piece when clamped down should leave a turn or two of its thread exposed1 in order to ensure that the cone does bed properly4 ,illustration3 ,,fig4 #ac4--,regulator and ,gauge4 ,care should be taken that the nozzle of the cylinder is free from dust before attaching any of these fittings3 the best plan is first to blow into it1 and finally wipe it round with the finger4 ,most professional operators hammer the wing piece home with a spanner or other convenient implement a barbarous method and really unnecessary if the cones are in good condition1 but1 nevertheless1 almost always adopted in practice4 ,,pressure ,,gauges4--,these are useful in determining the amount of gas remaining in a cylinder and are of a very usual type2 they may either be screwed on to the cylinder before commencing to work and taken off again to screw on the regulator1 or they can be supplied fitted to the regulator itself1 in which case they can be observed during the course of the exhibition 7,fig4 #ac74 ,as the same gauge may be used for cylinders of different sizes 7though never for those containing #bc different gases71 they simply register in atmospheres1 and knowing that a full cylinder shows a pressure of #abj atmospheres1 the requisite calculation must be made to determine how many cubic feet are unused4 ,in the case of oxygen cylinders an approximate idea of the amount of gas remaining can be got by weighing it carefully when known to be either absolutely full or absolutely empty1 and re-weighing it when information is required4 ,oxygen weighs approximately #a4d oz4 per cubic foot1 and this is easily detected by an average scale4 ,coal gas is too light to be gauged in this way4 ,,gas-,,bags ,,and ,,generators4--,it has already been remarked that there are two alternative methods of obtaining gas under pressure for limelight purposes1 viz4 gas-bags and generators 7the latter for oxygen alone3 there is no good hydrogen generator that ,i know of74 ,in both these cases the oxygen is generated by heating a mixture of chlorate of potash and manganese black oxide4 ,in the case of gas-bags the gas is prepared beforehand and passed through suitable purifiers into a rubber gas-bag4 ,with a generator the oxygen is evolved during the exhibition itself2 but this method has never come into very general use4 ,coal gas or hydrogen is very seldom home generated2 a gas-bag can1 if necessary1 be filled a few miles away and brought full to the place of exhibition1 or filled on the spot if gas is laid on2 or1 failing this1 acetylene or ether1 or even methylated spirit may be utilised instead4 ,the bags in use are placed between double pressure boards 7if both gases are required under pressure7 and weights sufficiently heavy placed on the top 7,fig4 #ad71 or with a 'blow-through' jet only the oxygen need be stored in a bag and the coal gas used from the supply main4 ,cylinders have1 however1 so universally superseded these appliances1 that space is hardly warranted in fully describing them1 especially as any operator wishing to adopt #bd the process can obtain full directions from any responsible dealer4 ,,limelight ,,jets4--,these are of three general types1 viz4 the ',blow-through1' the ',mixed1' and the ',injector4' ,of these the ',blow-through' is now very little made1 having been largely superseded by the ',injector' pattern1 but1 as there are hundreds in common use in this country1 they cannot yet be regarded as a thing of the past4 ,illustration3 ,,fig4 #ad4--;,gas-bags4 ,the exact design of this jet varies considerably1 but all are alike in this1 that a jet of coal gas is burned at the orifice of a more or less open nozzle1 and a stream of oxygen blown through it on to a cylinder of lime which it thereby renders incandescent4 ,fig4 #ae represents the various designs chiefly adopted for this jet1 that marked ,a being perhaps the most usual1 though ,c is also frequently met with4 ,in light-giving power there is not much to choose between the various types2 probably ,d on the whole is the best in this respect1 but so much depends upon the exact position of the two nozzles1 and the smoothness or otherwise of that #be provided for the oxygen blast1 that exact comparisons are difficult4 ,illustration3 ,,fig4 #ae4--';,blow-through' ,nozzles4 ',blow-through' jets are the weakest form of limelight as used at the present day1 and may be taken roughly as some #ej per cent4 better than acetylene1 or in other words1 sufficient to illuminate a #ab-;foot picture at a distance of some #dj to #ej feet2 but their advantage is1 or was1 that they only required one gas 7oxygen7 under pressure1 the coal gas supply being obtained from the ordinary house main4 ,illustration3 ,,fig4 #af4--';,blow-through' ,jet4 ,this advantage is now shared by the more recently introduced ',injector' jets1 which give a far better light1 and have therefore rendered the ',blow-through' type nearly extinct4 ,the general construction of a ',blow-through' jet is shown in ,fig4 #af1 and it will be seen that a short vertical spindle is #bf provided to carry the lime cylinder1 and that this can be rotated from the back by means of bevelled gear wheels1 which at the same time screw the spindle up and down4 ,a lime cylinder of the usual pattern being placed on this spindle can be rotated from time to time to expose a fresh surface1 as that in use gradually becomes 'pitted' by the blast1 while the screw provides sufficient vertical movement to ensure that a complete rotation does not bring round the same position again4 ,some arrangement is also generally provided by which the distance between the lime spindle and the jet can be adjusted4 ,the exact position of this does not matter within a reasonable margin1 but limes vary in size1 and ',pastilles1' and other substitutes for limes1 which will be referred to later1 vary still more1 at any rate as regards this adjustment4 ,the average distance which gives the best result is usually about half an inch1 and once set need not be altered with that particular jet unless a lime of different size is employed2 minor variations due to limes being drilled slightly out of centre1 c41 do not seriously matter4 ,there is no accepted rule for colouring jet-taps in accordance with the cylinders1 and although jets are sometimes met with painted in this way1 i4e4 red for coal gas and black for oxygen1 it is more usual to find coal gas taps black and oxygen bright1 or sometimes both black or both bright4 ,care must therefore be taken that the right cylinder is connected to the right tap on the jet1 but there should be no difficulty in telling which is which1 and fortunately any mistake1 even if it be made1 is quite harmless4 ,,the ,,mixed-,,gas ,,or ,,double-,,pressure ,,jet4--,this jet is fundamentally different from the 'blow-through' form1 inasmuch as the two gases are combined in one mixing chamber before combustion1 and burn in their correct proportions at one nipple4 ,it is usually stated that this jet necessitates both gases being under equal or approximately equal pressure1 but this #bg is not literally accurate1 and ,i have given many a lantern exhibition with one of these jets1 using coal gas from the ordinary supply1 and oxygen from a cylinder4 ,to use a mixed jet in this way needs care1 as a very slight excess of oxygen puts the light out with a 'pop' which1 although not dangerous1 is disconcerting1 while the light obtained under these conditions is very little better than with a 'blow-through' jet1 and far inferior to the ',injector' jets to be described next4 ,illustration3 ,,fig4 #ag4--,mixed ,jet4 ,illustration3 ,,fig4 #ah4--,mixed ,jet1 ,gwyer pattern4 ,the mixed-gas jet is intended then to be used with both gases under pressure1 and is the only jet to be seriously #bh considered in cases where a really powerful light is required4 ,the power of this jet is indeed almost unlimited1 and those made with large bores1 such for example as used for cinematograph work1 provide a light amounting often to some two or three thousand candles1 and consume an enormous amount of gas2 but the ordinary pattern1 with a nipple of one-twentieth to one-sixteenth of an inch bore1 and using some #e feet of each gas per hour1 or perhaps slightly more for the coal gas1 will suffice for all ordinary work4 ,illustration3 ,,fig4 #ai4--,mixing ,chamber of ,jet4 ,the mixed-gas jet1 like the 'blow-through1' is made in many forms1 but these may be roughly divided into two main types1 viz4 those with small mixing chambers immediately below the nipple 7,fig4 #ag71 and those with larger chambers in the horizontal part of the jet as in the ',gwyer' pattern 7,fig4 #ah74 ,the construction of the mixing chamber itself varies also1 but that advocated by my father1 the original author of this work1 is generally followed1 the chamber being packed with alternate discs pierced as in ,fig4 #ai1 which ensures a thorough mixture of the gases4 ,a layer or two of gauze is often introduced as well by way of further improvement4 ,the distance between the lime and nipple is much less than with the 'blow-through' jet1 and the adjustment has to be more exactly made4 ,about #a_/h inch is approximately correct for a jet of moderate power1 and rather more for a bigger bore2 also care must be taken to turn the lime frequently1 as the latter 'pits' pretty quickly with these jets1 and if it is neglected the jet may spurt back out of the hole1 which is gradually formed1 and crack the condenser4 #bi ,there is still an erroneous opinion extant that these jets are dangerous1 and if the operator is working with the now obsolete gas-bags it is certainly a fact that an accident in careless hands is possible2 but with cylinders there is1 so far as ,i know1 no possibility even of an accident under ordinary conditions4 ,it is true that if too much oxygen is turned on the jet may suddenly go out with a loud snap or pop1 and this is in reality a miniature explosion in the mixing chamber2 but it can in any case hardly be serious enough to matter1 though ,i have found after such a snap that the gauze packing1 inside the chamber above referred to1 has been pierced right through1 and1 when first lit afterwards1 the jet has for a few minutes burnt with a characteristic green flame1 denoting the presence in the gas of fine copper or brass particles4 ,to obtain a good light with these jets1 and in fact with all jets1 great care must be taken that the nipple is absolutely smooth1 otherwise the flame is bound to hiss4 ,the simplest plan is to slightly roughen a suitable sized needle with emery paper and to burnish the inside of the nipple from time to time with this4 ,especially if there has been one of the 'snaps' referred to is it desirable to see that the inside of the nipple is thoroughly smooth and polished4 ,,manipulation ,,of ,,the ,,mixed-,,gas ,,jet4--,on this point there is not much to be said4 ,a good hard stone lime must be used--'soft' limes are useless for this jet--and the coal gas flame should be lit first1 and the lime thoroughly heated with this before the oxygen is slowly turned on4 ,as the oxygen increases the flame will gradually disappear and the light increase1 until it is at a maximum for that particular amount of coal gas4 ,this latter can then be turned on a little more1 and more oxygen passed to balance it until the jet begins to 'roar1' when we are getting the maximum light for that particular sized nipple4 ,when the two gases are1 however1 in the proper proportion to give the best light1 there will always be a slight excess of coal gas flame visible playing about the lime4 #cj ,,the ,,injector ,,jet4--,this is essentially a mixed jet1 and in outward appearance differs but little from one of the ordinary type 7,fig4 #bj71 but is so constructed that the pressure of oxygen 'sucks' coal gas into the mixing chamber1 and so obviates all necessity for the latter being under pressure4 ,with this jet there is little or no danger of the jet 'snapping' out through a surplus of oxygen1 as the greater the flow of this gas1 the greater the suction on the coal gas side4 ,the light is not quite equal to a good mixed jet1 but very nearly so1 and therefore this jet is deservedly gaining in favour every day4 ,illustration3 ,,fig4 #bj4--';,injector' ,jet4 ,one point must be noted3 the oxygen itself must be under greater pressure than with the ordinary mixed jet if the best light is to be obtained1 and therefore a special regulator must be used1 or one of ordinary type modified 7which can easily be done by the maker71 and rubber connections must be securely tied both on to jet and regulator1 as the pressure required to work this jet to advantage1 while not enough to burst a rubber tube1 is enough to blow it off an easy fitting connection4 ,,the ,,oxyether ,,light4--,this is practically similar to the oxyhydrogen1 except that ether vapour is used in place of the hydrogen or coal gas4 ,the method adopted consists essentially of passing a stream of oxygen through a vessel packed with some porous material 7such as cotton wool or cotton gauze7 which is saturated with ether4 ,the oxygen #ca becomes saturated with ether vapour1 and the mixture is then used in place of the coal gas supply in a double-pressure jet1 an additional supply of free oxygen being still required through the ordinary oxygen tap4 ,the arrangement is cheap1 as it dispenses with the necessity for a coal gas cylinder1 and effective1 as the light is little1 if at all inferior to the oxyhydrogen1 but differs from the latter in this1 that with careless handling an accident is possible4 ,in competent hands there is no danger1 and ,i have used ether saturators myself scores of times without one single contretemps2 but it should not be entrusted to any chance amateur4 ,the use of the ether light has a curious history4 ,in the earlier days before the proper construction of ether saturators was understood1 and gas-bags were still in vogue1 it was largely condemned on the score of danger4 ,modern improvements in apparatus rendered it perfectly safe against anything but gross carelessness or bungling1 and the ,london ,county ,council and other similar bodies immediately supplied it broadcast to elementary schools 7in disregard of warnings offered by myself and others71 where it was often entrusted to incompetent operators or even senior boys4 ,so far as ,i know no serious accident ever resulted1 a pretty conclusive proof that the light is really safe1 but in time the ,london ,county ,council realised that the universal adoption of this illuminant was not advisable1 and ,i believe now prohibit it altogether in halls licensed by them for entertainments4 ,in time1 no doubt1 they will learn to adopt a sane policy between the two extremes1 but at present the official attitude in many localities has placed ether saturators out of the running1 and before purchasing one the would-be operator should ascertain that he will be allowed to use it4 ,ether saturators as made at the present day may be divided into two principal patterns1 viz4 those in which saturator and jet are combined in one piece of apparatus #cb which fits bodily into the lantern1 and saturators which are used outside and connected by means of tubing to any ordinary oxyhydrogen double-pressure jet4 ,both forms have their advantages and disadvantages2 the first pattern tends to become too warm from its position in the lantern and generates ether vapour too quickly1 while the second has the fault of becoming too cold 7owing to evaporation of the ether7 and therefore not vaporising quickly enough4 ,illustration3 ,,fig4 #ba4--';,gridiron' ,saturator4 ,writing at the present date1 when manufacturers are slowly beginning to resume their normal occupations after the stress of war work1 it is impossible to say exactly what models will or will not be made1 but ,i will mention one typical example of each pattern as made in pre-war days4 ,the first of these is the ',gridiron' 7,fig4 #ba71 adopted largely by the ,london ,county ,council in the days ,i have referred to1 and certainly one of the best designed saturators ever put on the market4 ,in the ',gridiron' saturator there are three taps3 two at the #cc rear and one in front1 between the saturator and the mixing chamber4 ,between the rear taps is the inlet for the oxygen1 which divides into two channels1 that on the left passing upwards through the ,u tube shown in the illustration 7the corresponding tube on the right is merely a dummy71 and thence through the saturator and out through the horizontal tube and tap into the mixing chamber1 whence the saturated stream of oxygen finally passes to the nipple1 and the combination burns with a whitish flame closely resembling that produced by coal gas4 ,illustration3 ,,fig4 #bb4--',pendant' ,saturator4 ,the other channel for the oxygen is to the right1 down the vertical tube shown there 7the lower vertical tube on the left is also a dummy71 underneath the saturator1 and finally coming up into the mixing chamber from below1 transforming the white flame into an intensely hot blowpipe exactly as it does with a coal gas jet4 ,the front tap controls the supply of saturated ether to the mixing chamber1 and whereas at first a good stream of oxygen is needed to pick up enough ether1 by degrees as the instrument warms in the lantern1 the oxygen passing through the saturator can be cut off entirely1 and even then the front tap must be gradually closed down to prevent the hot ether coming off too fast4 ,there is a disagreeable feeling of 'sitting on the safety-valve' in doing this1 but in reality the pressure is never likely to become great enough to cause danger4 ,of saturators for use outside the lantern the best-known is probably the ',pendant' 7,fig4 #bb74 ,with this instrument the oxygen supply is connected to the inlet marked ,a2 ,b goes #cd direct to the oxygen tap of any ordinary mixed-gas jet2 while ,c1 from whence issues the saturated stream1 is connected to the coal gas tap of the jet4 ,whichever pattern is used1 the essential thing is to keep a good supply of oxygen well saturated4 ,if the lime becomes incandescent without any free oxygen1 or it is found that this requires gradually turning off1 it indicates that the saturation is becoming defective1 and to continue is to risk the jet snapping out4 ,in the case of an outside saturator such as the ',pendant1' this may even blow off the connecting tubes with a loud report1 though no worse accident is likely to happen1 and for this reason an outside saturator should be placed as close to the jet as possible1 so that the rubber tube may be kept short1 and incidentally this keeps the saturator warm and accelerates vaporisation4 ,as ether vapour usually contains a certain amount of moisture which does not vaporise to any great extent1 this gradually accumulates and the capacity of the instrument becomes reduced4 ,it is therefore usually necessary to return a saturator to the makers every now and again for repacking4 ,the only real danger with a modern saturator is not in using but in filling4 ,this should be done if possible in the open air1 and at any rate never near a light4 ,ordinary sulphuric ether of specific gravity #gbj-gcj is usually considered the best1 and a quarter of a pint will keep an ordinary small-bore jet going for nearly two hours4 ,more precise directions are usually sent out by the makers1 and as the various patterns of saturator in use are pretty numerous1 it would be useless here to attempt more detailed instructions for working4 ,,oxy-,,acetylene ,,jets4--,any good mixed gas jet may be used with acetylene instead of coal gas1 provided that it is under pressure more or less corresponding to that from an oxygen cylinder1 and at the present day there is no difficulty in obtaining this1 in civilised countries at all events1 by #ce means of compressed or1 to speak more correctly1 'dissolved' acetylene cylinders1 referred to later on4 ,with an ',injector' jet there is no need for the acetylene gas to be under pressure at all1 and a simple generator such as described on page #ab will answer perfectly1 though in practice very seldom used4 ,with such a generator the pressure is so low that in many cases the jet will not even burn until some oxygen is turned on2 but this introduces no real difficulty1 as with a good ',injector' a snap is practically impossible1 provided the generator is large enough to evolve sufficient acetylene4 ,it is far better in every way1 however1 to use the acetylene from a cylinder1 just as with coal gas4 ,only in this case the cylinder is completely filled with a porous material1 and this again filled with liquid acetone or other suitable fluid1 in which the acetylene is dissolved as rapidly as it is pumped into the cylinder4 ,to compress acetylene in the ordinary way is neither safe nor practicable2 but these 'dissolved' cylinders are now used extensively for both oxy-acetylene welding and motor car lighting1 and may be entirely relied upon4 ,the ,d4,a4 7,dissolved ,acetylene7 ,company were the pioneers in this country of the industry1 and their methods of business are peculiar and ingenious4 ,the user is requested in the first place to purchase a cylinder1 and he then becomes the owner of a cylinder1 but not of one particular cylinder4 ,a list is supplied to him of various depots in the country where the ,company's cylinders are stored1 and when empty he can1 on payment of a fixed sum1 exchange his empty cylinder for a full one1 which then becomes his cylinder pro tem4 ,this saves the delay and expense of returning a cylinder to ,london1 and incidentally clears the customer of any question of deterioration1 this being obviously covered by degrees with each individual exchange4 ,the system was first introduced in connection with the lighting of cars and only applies to the standard size for this purpose1 viz4 #bj cubic feet capacity1 #cf but as this is1 on the whole1 the most convenient size for lantern work also1 the limitation is not a disadvantage4 ,the arrangement is also in vogue to a less extent with cylinders of #f feet capacity 7a size sometimes used for motor cycles71 but the depots of exchange are at present far fewer for this size4 ,the oxy-acetylene blast is much hotter than the ordinary oxyhydrogen1 and therefore produces a more intense light4 ,i have therefore used it with success on occasions when even the ordinary limelight would fail1 and the choice has lain between an oxyhydrogen jet of enormous bore 7and1 of course1 corresponding consumption of gas71 and the oxy-acetylene4 ,for this very reason great care must be taken only to use the hardest limes1 and even then to use the lime-turning movement frequently1 or the lime will pit or crack and a broken condenser follow4 ,,the ,,fallot ,,acetylene ,,light4--,this light consists of a jet of acetylene under pressure1 without oxygen1 but producing its own air blast from the atmosphere by suction1 much as the ',injector' jet does1 but the reverse way round4 ,the light is better than with an ordinary acetylene jet1 though not quite so good as with a 'blow-through' jet2 but as it only requires a cylinder of dissolved acetylene1 or even a ',pressure' generator1 it is fast coming into favour4 ,the peculiarity of the ,fallot apparatus is that1 instead of providing a direct beam of light in the direction of the screen1 it projects the beam backwards on to a concave mirror1 and it is the reflected light from this that is used 7,fig4 #bc74 ,instead of a lime is used a spherical ',pastille' of peculiar composition1 and before use each pastille must be burnt off exactly like an incandescent gas mantle1 after which it is extremely fragile and difficult to handle4 ,to use this illuminant one lens of the condenser must be removed1 the curvature of the mirror taking its place1 and it will be seen at once that the pastille itself will get in its own #cg light and throw a shadow1 which actually happens1 but it is hardly perceptible unless specially looked for4 ,illustration3 ,,fig4 #bc4--;,fallot ,air ,blast4 ,a complete ,fallot ,air ,blast ,outfit1 with cylinder1 fine adjustment valve1 pressure gauge and burner1 with two spare pastilles1 is shown in ,fig4 #bd1 but if preferred a regulator1 such as previously described for oxygen1 can be used instead of the fine adjustment valve4 ,illustration3 ,,fig4 #bd4--;,fallot ,air ,blast1 and ,cylinder4 #ch ,,fallot ,,oxy-,,acetylene ,,blast4--,this is similar to the foregoing1 utilising oxygen from a cylinder instead of air1 and the light is equal to a powerful limelight1 and may be considered as an efficient substitute1 though for long range work the shadow before alluded to becomes more noticeable 7for optical reasons which need not be here discussed74 ,the ,fallot ,company also make a special ',pressure ,generator' which can be used instead of a ,d4,a4 ,cylinder2 but my experience of this so far is that1 although perfectly safe1 the blast from it is a little unsteady as compared with a cylinder4 ,,limes ,,and ,,accessories4--,limes for ,optical ,lantern work are usually supplied in the form of cylinders1 the 'ordinary' size being #g_/h inch in diameter and about #a inches long1 with a hole drilled longitudinally to take the lime pin4 ,extra large limes up to #b inches in diameter are supplied for more powerful jets4 ,so-called 'soft' limes used to be recommended for 'blow-through' jets as giving a better light than 'hard' limes1 but the advantage1 if any1 is very little1 and these limes are now very seldom heard of1 possibly because 'blow-through' jets themselves are becoming less and less used1 and 'soft' limes will not stand the heat of a mixed or ',injector' jet for long4 ',hard' limes are turned out of the hardest stone lime1 and must be kept in sealed tins until used1 as they rapidly disintegrate when exposed to the air4 ,there are one or two quarries known to provide the best lime for lantern purposes1 and the various good brands on the market practically have the same origin as regards raw material1 though called by different trade names2 and the ',hardazion' 7hard as iron7 limes1 placed on the market some years ago by a well-known wholesale firm1 to be countered shortly after by the ',hardastil' 7harder still7 brand1 are1 ,i take it1 legitimate though amusing instances of phonetic advertisement4 ,even the best of limes is liable to crack under the heat #ci of a powerful jet1 and so a pair of lime-tongs should always be provided1 and there is nothing better than the simple form shown in ,fig4 #be1 and which is1 or should be1 sold by all dealers4 ,illustration3 ,,fig4 #be4--,lime-tongs4 ,,substitutes ,,for ,,limes4--,a good substitute for lime1 that will give the same light1 stand heat equally as well1 and not deteriorate if exposed to the atmosphere1 has long been sought for1 and some of the more recently discovered refractory materials are more or less satisfactory4 ',mabor' limes1 for example1 belong to this class1 and so do some of the 'pastilles1' which before the war came chiefly from ,france and to a less extent from ,germany4 99 99 99 99 99 ,,chapter ,,vi ,,the ,,electric ,,light ,the electric current provides the light for an optical lantern1 though it may take various forms1 such as the incandescent glow-lamp in some shape or other1 the comparatively new ,ediswan ',pointolite' lamp1 the enclosed arc1 and the open arc4 ,this little book is not a treatise on electricity1 but a few elementary notes may not be out of place1 and may be of assistance to the non-technical lanternist4 ,the first point then to be considered in adopting the electric light for the purpose of lantern projection is the character of the supply1 and the information required may be summed up thus3 7#a7 ,e4,m4,f41 voltage1 or tension 7these three expressions having exactly the same meaning72 7#b7 amprage or amount of current available2 7#c7 whether current is 7a7 continuous1 constant1 or direct 7again three words meaning #dj the same thing71 or 7b7 alternating4 ,the ,e4,m4,f4 or tension corresponds to pressure1 to use the mechanical analogy of a water pipe1 and the amprage to volume1 and the voltage of the supply currents in this country are usually between #ajj and #bej volts4 ,private lighting sets are frequently as low as #ej1 and current derived from accumulators may be anything from a few volts and upwards4 ,power currents1 such as commonly employed for tramways1 c41 are usually about #ejj volts1 but the use of these currents for lighting purposes1 though practicable1 is not to be advocated4 ,ampres and volts are convertible terms in a sense2 that is to say1 a current of #aj ampres at #ajj volts requires the same horse-power to generate it as one of #e ampres at #bjj volts1 or #bj ampres at #ej volts1 but they are by no means convertible as regards their efficient use for our purpose4 ,the ampres used multiplied by the number of volts give the total power consumed in watts1 and #ajjj watts used for one hour represent #a unit as charged for on our dreaded lighting bills4 ,the current available from a public supply may be said to be unlimited so far as our purpose is concerned1 and the amount actually used depends only on the total electrical resistance of our circuit1 and this is measured in ohms1 the three factors1 viz4 volts1 ampres1 and resistance1 being connected by the well-known and simple equation ,c ,e _/ ,r1 ,c representing the current in ampres1 ,e the tension or ,e4,m4,f4 7electro-motive force7 in volts1 and ,r the resistance in ohms4 ,the total current we can use1 however1 is limited by the size of the cable laid on in the building1 and this is automatically safeguarded 7or should be7 by the fuses1 which consist1 as is generally known1 of thin wires or strips of tin or lead fixed on a fuse board in an easily accessible place1 and which melt directly the current exceeds a safe amount in ampres4 ,whatever method of lighting we use therefore1 enough resistance must always be kept in the circuit to ensure #da that no more current can pass than has been provided for1 and in the case of an arc lamp this usually means a resistance or rheostat being retained in the circuit in addition to the arc itself1 through which the current is passed and absolutely wasted1 though fortunately the waste in money is negligible1 and for reasons to be discussed later such a resistance is necessary with an optical lantern arc lamp in any case4 ,in the case of a glow-lamp1 the entire resistance is provided by the filament of the lamp itself1 and that is why an ordinary metal or carbon filament lamp1 for say #bjj volts1 has to be manufactured with an extremely long and slender1 and therefore fragile1 filament1 while with an ordinary pocket-torch1 which is usually supplied with current from a dry battery of some #c or #d volts only1 the filament can be short and thick4 ,speaking generally1 glow-lamps on a low voltage current can be made more efficient than on a high one1 and are also longer lived for very obvious reasons2 but1 on the other hand1 the transmission of current over long distances is cheaper the higher the tension1 as for a given number of watts the amprage is less1 and therefore smaller cables can be employed4 ,on the whole1 then1 currents of #bjj to #bej volts have during recent years become more common than #ajj1 in spite of greater difficulties in making the lamps2 but occasionally one finds a hall where two or more lamps are wired in series1 two #ajj-volt lamps for example being wired together in series on a #bjj-volt circuit4 ,if we are using current for our lantern from an ordinary lamp socket1 this is a possibility that must be borne in mind4 ,the same considerations1 viz4 the economy of transmitting power at high tension and of using it at a lower one1 have been mainly responsible for the rapidly increasing number of alternating current circuits now met with1 especially in sparsely populated districts4 ,an alternating current main is one in which the current reverses its direction1 usually in this country #ej1 but sometimes #fj1 #hj1 #ij1 or even #ajj times #db per second 7there being unfortunately in ,great ,britain no standard ',periodicity' or number of cycles per second71 and for technical reasons which need not be entered upon here1 with these alternating currents the tension and amprage can be mutually converted by means of transformers1 so that current can be transmitted at so high a tension1 for instance1 as #aj1jjj volts1 and used at a voltage of #ej or #ajj or whatever is required1 the amprage available being increased in inverse ratio as the tension is decreased4 ,the same ready power of transformation unfortunately does not apply to the continuous current1 or alternating currents would probably never have been heard of1 but as it is they are very common4 ,for glow-lamps it is immaterial which current is available1 but for arc lamps the continuous is much to be preferred1 though both can be utilised4 ,with these initial remarks1 ,i will now take in order of illumination the various methods of utilising the electric current for optical lantern work4 ,,the ,,electric ,,glow-,,lamp4--,the ordinary metal filament lamp is not very suitable for lantern work1 the light not being sufficiently concentrated1 but from what has already been said1 it will be evident that this method of lighting is more suitable where currents of low voltage are available4 ,an extremely good and intense light can be obtained from a comparatively small battery of accumulators1 which can easily be carried in the hand1 and a short and thick metal filament lamp1 similar to those supplied with a powerful electric torch2 and this arrangement is actually used to some extent by travelling lecturers1 but the mess and trouble of keeping the accumulators in order have prevented the method being generally adopted4 ,when alternating current is available such a lamp will work well with a transformer to step down the voltage to the required degree1 and the arrangement is simple1 cheap1 and efficient1 and produces a light at least equal to that from #dc acetylene4 ,in comparatively small halls1 where the current is alternating1 this is undoubtedly the best method of working1 as it is simpler than the arc and amply brilliant enough for all practical purposes4 ,with the continuous current the problem is not so simple1 as transformation of voltage is not an easy matter1 and a glow-lamp on2 say1 a #bjj-volt circuit involves a long and fragile filament1 which it is difficult to arrange in a small space4 ,many years ago the ,ediswan ,company produced a ',focus' lamp for the purpose1 with the filament arranged in the form of a square grid1 and this lamp gave a light of about #ajj candles1 and was fairly successful for a small room4 ,more recently the ,osram ,company introduced a similar lamp with a metal filament arranged somewhat in the form of a cone1 and this lamp also sufficed for a small class-room4 ,it was1 ,i believe1 made in ,germany and was practically unobtainable during the war4 ,i understand the ,osram ,company are at present arranging to manufacture it in this country1 but up to the time of writing it has not made its appearance4 ,none of these lamps worked direct on a public lighting circuit can be regarded as really satisfactory1 as it has been found impossible so far to get a concentrated light2 the #ajj-volt lamps have1 of course1 been superior to those made for #bjj or #bej1 but they are all for lantern purposes far behind low voltage lamps1 which are really good when a suitable current can be obtained4 ,,the ,,pointolite ,,lamp4--,this lamp produced by the ,ediswan ,company is in reality a miniature arc with tungsten electrodes in a highly exhausted vacuum bulb4 ,to attempt a technical description would be beyond the scope of this book2 it will suffice to say that the action depends upon the same principle as the various wireless vacuum valves or the ,coolidge ,x-ray tube4 ,this lamp requires a peculiar starting device which is supplied with it1 and gives a good1 intense1 and concentrated #dd light1 not equal to the ordinary arc or to limelight1 but comparing well with any other form of illuminant4 ,it can only be used with the continuous current4 ,,the ,,nernst ,,lamp4--,this lamp at the present moment is practically non-existent in this country1 having been made exclusively in ,germany4 ,also as recent improvements in metal filament lamps have rendered it almost obsolete for ordinary lighting purposes1 it is1 ,i think1 very doubtful whether it is still manufactured even in that country1 and hence ,i do not propose to waste space in an extensive description4 ,it will suffice to say that the lamp consists of one or more straight rods or filaments of a refractory material1 which are semi-conducting to the electric current when hot1 but non-conducting when cold4 ,to commence with the filament must be heated1 and in the lamps as supplied for lantern work this is usually done by means of a spirit lamp1 which can be removed immediately the current begins to pass1 as the filament is thereafter maintained at a white heat automatically4 ,a three-filament ,nernst lamp gives as much as #ajjj candles1 but it is extremely hot1 and the light rather diffuse4 ,the filaments are also very fragile1 so on the whole the lamp was never very much in favour2 but on the other hand it consumed very little current1 and could be worked from any ordinary house lighting main1 points which led to its adoption in certain cases4 ,,the ,,electric ,,arc4--,we now come to the light for optical lantern work1 the brightest1 the most concentrated1 the cheapest1 the easiest to work1 in fact1 the illuminant which combines all the virtues and but few drawbacks1 but of course requires one indispensable condition1 viz4 electric current laid on4 ,this current may be of any voltage from #gj-bej1 or even higher2 it may be continuous or alternating1 though the former is to be preferred2 and it requires a cable for at least #e ampres1 and for a large hall #aj or #ab ampres4 ,the simplest form of arc lamp for lantern purposes is the #de hand-fed type as illustrated in ,fig4 #bf4 ,the essential feature is the pair of carbon rods1 the remainder of the apparatus consisting of mechanical adjustments to 'feed' these as they burn away1 and to accurately maintain them in their proper positions and in the optical centre of the lantern4 ,just because the electric arc provides so small and concentrated a light1 it is of extreme importance that the centring should be exact2 and hence mechanical movements are usually provided for this which are unnecessary with other illuminants4 ,illustration3 ,,fig4 #bf4--;,hand-fed ,arc ,lamp4 ,the whole question of optical adjustments has1 however1 been left over for a future chapter1 as it more or less applies to whatever illuminant is used4 ,the illustration shows a lamp arranged for continuous current1 the upper carbon1 which must be connected to the positive wire1 being larger than the lower 7the negative71 and very slightly behind it4 ,the light from a continuous current arc lamp comes chiefly from this upper or positive carbon1 #df which 'craters' as it is used1 and this arrangement has the effect of radiating the light in the direction required 7,fig4 #bg74 ,the positive carbon is usually of the 'cored' type1 that is provided with a core of softer carbon1 as this assists the 'cratering' action1 while the negative is generally used 'solid1' that is homogeneous right through4 ,the arc has to be 'struck' in the first place by touching the carbons together for a moment by the mechanical means provided1 and then separating them to the working distance1 which is approximately #a_/h inch4 ,they must then be maintained at that distance by 'feeding' as they slowly burn away1 and this 'feeding' in arc lamps for lantern work is usually done by hand1 as in the lamp illustrated in ,fig4 #bf1 but may be done by an automatic arrangement1 as will be described later4 ,illustration3 ,,fig4 #bg4 ,the current is really carried across the arc by convection1 or in other words conducted by a bridge of white hot carbon particles1 which continually stream across from the positive carbon to the negative1 and this bridge1 while conducting the current1 interposes a very considerable resistance 7otherwise it would not of course become hot74 ,a certain potential or tension is therefore necessary if a given current is to be maintained1 and this potential has to be greater the longer the arc and also 7though not in direct proportion7 the smaller the carbons4 ,when1 however1 everything is in the best proportion1 i4e4 length of arc1 size of carbons1 and current passing1 the potential at the arc lamp terminals required is approximately #de volts1 and this may be taken as a fixed figure for any current4 ,the length of arc to give the best results may also be taken #dg as approximately fixed at #a_/h inch1 and the variable factor for different currents as required is provided by altering the sizes of carbons employed4 ,the error must not be made1 however1 of assuming that an ,e4,m4,f4 of #de volts is sufficient to work an arc lamp1 as the minimum in practice is at least #fe volts1 and #ajj or even #bjj volts are advantageous4 ,i have come across more than one private generating installation where the innocent owner has put in a dynamo for #de or #ej volts1 depending upon some carelessly written statement that this is sufficient4 ,why a higher ,e4,m4,f4 is required can be simply explained4 ,take for instance an average hand-fed arc lamp as used for lantern work and consuming1 say1 #aj ampres4 ,take also1 as a fact1 the statement given above that the necessary ,e4,m4,f4 at the actual terminals of the arc lamp may be accepted as a constant at #de volts1 and reverting to the equation given on page #dj1 ,c ,e _/ ,r1 and substituting these figures we get-- ,current 7#aj ampres7 ,e 7#de volts7 _/ ,r 7,resistance of ,arc74 ,it is therefore obvious that under these exact conditions the resistance or back ,e4,m4,f4 of the arc1 as it is termed1 must equal #d4e ohms4 ,now suppose the lamp left for a few seconds unattended1 while the carbons are burning away and the arc is lengthening2 in a very few moments the resistance will have increased1 owing to the greater distance between the carbons1 and we will suppose it to have become #e ohms instead of #d4e4 ,the current passing will now be #de _/ #e #i ampres only4 ,in other words1 a very slight lengthening of the arc has reduced the current1 and therefore the light1 by #aj per cent4 ,not only so1 but #de volts being needed to maintain an arc of #dh normal length1 it is insufficient to maintain a longer one1 and in practice the effect of leaving an arc under these conditions to itself for even a few seconds is that it goes out1 to the annoyance of the lecturer and the confusion of the operator4 ,it is just possible to work an arc lamp with a total ,e4,m4,f4 of #de volts by giving one's whole attention to it and never taking the hand off the feeding handle2 but in practice no one with any experience would attempt it4 ,the arc would almost certainly go out several times during the exhibition4 ,now1 take an example of a similar arc lamp consuming #aj ampres but worked from a supply of #bjj volts4 ,our equation ,c ,e _/ ,r must then obviously become ,c 7#aj ampres7 ,e 7#bjj volts7 _/ ,total ,resistance 7#bj ohms74 ,the resistance of the arc itself being the same as before1 viz4 #d4e ohms1 it is obviously necessary to put an extra fixed resistance equal to #ae4e ohms in series with it in order to make up the total of #bj ohms4 ,now leave the arc unattended until the resistance of #d4e ohms has again become #e ohms2 the only effect is that our current1 instead of remaining at #aj ampres1 has become #bjj _/ #bj4e or #i4h nearly1 a difference which is imperceptible4 ,this is not all1 for it is an elementary rule in electrical science that the total ,e4,m4,f4 of any circuit distributes itself along that circuit in proportion to the distribution of resistance4 ,in other words1 our original ,e4,m4,f4 of #bjj volts will so distribute itself as to reserve1 so to speak1 an ,e4,m4,f4 of #de volts for the arc1 while the resistance of this remains at #d4e ohms1 but directly this resistance increases1 the ,e4,m4,f4 at the arc lamp terminals automatically rises1 and therefore the actual diminution in current is even less than the figures above quoted4 #di ,should the arc tend to 'break' or go out1 the resistance across it automatically becomes infinite and the whole #bjj volts is at that moment available to prevent the occurrence4 ,under these conditions1 therefore1 the operator can safely leave the arc for many minutes at a time4 ,in carrying out experimental work ,i have often left the lantern1 walked up to the screen1 discussed results with a friend1 and walked back1 and the arc has shown no signs of misbehaviour whatever4 ,illustration3 ,fig4 #bh4--,resistance4 ,in practice any current from #ajj volts to #bej volts may be considered as satisfactory for lantern work with a suitable resistance4 ,less than this involves feeding the arc rather frequently1 and more may give a nasty shock1 should the operator inadvertently touch a live wire1 though ,i have worked an arc lamp on a current of as much as #ejj volts4 ,the resistance usually consists of a suitable length of wire of high resistance 7,iron1 ,german ,silver1 or those alloys known as ,platinoid1 ,eureka1 ,manganin1 ,beacon1 c41 are most commonly used7 wound in spirals on a frame1 and is generally supplied adjustable 7,fig4 #bh71 so that more or less current may be used as desired4 ,these resistances get pretty hot in use1 and care must be taken that they are placed where they cannot scorch woodwork1 c41 and in cases where the lantern is a fixture it is a good plan to have the resistance bolted up against a wall once and for all4 ,the resistance may be placed anywhere in the circuit1 so long as the current passes through it1 then through the arc lamp 7or vice vers71 and back to the other #ej pole of the supply main2 it does not matter in the least whereabouts it comes4 ,in cases1 however1 where one pole of the supply main is earthed1 it is a good thing to place the resistance in the 'live' side1 as this keeps the arc lamp within #de volts of earth potential while it is working1 to the comfort of the operator should he touch a terminal or wire1 though with an ordinary lighting main there is no real fear of a dangerous shock in any case4 ,the amount of current required depends of course on the size of the sheet1 length of the hall1 and density or otherwise of the slides2 but it is usually accepted in practice that the efficient light from a continuous current arc lamp equals #ajj candles per ampre1 and therefore a #aj-;ampre arc will give #ajjj candles4 ,this is sufficient for all ordinary halls and slides1 but where these latter are very dense1 as for example with the ,lumire three-colour process1 as much as #bj or #be ampres may be required4 ,in these cases some special precautions must be taken for keeping the slides cool1 or the result may be disastrous1 but this is a question that will be referred to in a later chapter4 ,a current of #aj ampres is pretty safe for all ordinary slides1 and may be taken as the normal current used in large halls1 though in arranging for the wiring it is as well to stipulate for at least #ab or even #ae ampres1 especially as there must necessarily be a momentary increase of current at the instant the arc is 'struck4' ,,varieties ,,of ,,hand-,,fed ,,arc ,,lamps4--,the pattern of hand-fed arc lamp illustrated in ,fig4 #bf is only typical of many of the same general design1 and there are others in which the design itself is fundamentally different4 ,of these the ',scissors' arc lamp made by several firms deserves mention on account of its simplicity and cheapness4 ,as its name implies1 the mechanism resembles a pair of scissors1 the carbons being attached to the ends of a pair of levers hinged together #ea 7,fig4 #bi74 ,in this lamp centring movements are usually dispensed with1 the arc being clamped on to a tray pin as in the case of a limelight jet4 ,this is not1 of course1 so convenient1 and a further disadvantage of this pattern arc lamp is that the feeding process gradually alters the position and angle of the carbons4 ,in fact1 the one great merit of the lamp is cheapness1 and where expense is an object1 it should certainly be considered4 ,illustration3 ,,fig4 #bi4--',scissors' ,arc ,lamp4 ,yet another arc lamp deserving of mention is the ',parallel1' a name again very aptly chosen1 as the two carbons are either exactly parallel to each other or very slightly inclined4 ,in the former case the arc has to be 'struck' by touching the ends of the carbon rods with a piece of metal or carbon4 ,of the actual manipulation of this lamp ,i have had very little practical experience1 but ,i have heard it well spoken of1 though ,i believe it has so far only been made for currents of #e ampres or so4 ,yet another type which must not be ignored is the ',right-angled' pattern 7,fig4 #cj71 a name again self-descriptive4 ,the horizontal carbon is the positive1 and the vertical the #eb negative1 and this lamp again is made by several manufacturers in slightly different forms4 ,this pattern lamp is in my experience the best of all for small currents1 say1 of #e ampres or so1 but inferior to ,fig4 #bf for currents of #aj ampres or more4 ,this last remark perhaps hardly applies to alternating currents1 which1 however1 ,i have not yet discussed4 ,i cannot conclude this brief category of arc lamps without referring to the enclosed pattern1 of which the ',westminster' is perhaps the best-known and most popular 7,fig4 #ca74 ,illustration3 ,,fig4 #cj4--',right-angled' ,arc ,lamp4 ,this is a lamp of the right-angled type1 but the arc burns in a cylindrical glass chamber1 not air-tight1 but partially so4 ,after burning a few minutes the oxygen in this chamber becomes used up and its place is taken by carbonic-acid gas and other products of combustion1 after which the carbons burn away very much more slowly1 and therefore require feeding at much greater intervals4 ,this lamp again is chiefly made for small currents not exceeding #e ampres 7and can therefore be used from any ordinary lamp socket71 and for a moderate-sized hall is on the #ec whole as cheap1 efficient and simple a lamp as any ,i am acquainted with4 ,it can be supplied with or without mechanical centring movements as required1 and is usually sent out with its own resistance for the particular current on which it is to be used1 so that it only requires connecting up to the nearest lamp socket1 and is ready for use4 ,illustration3 ,,fig4 #ca4--',westminster' ,arc ,lamp4 ,it is not sufficient for anything larger than a #ab-;foot sheet or for working at a greater distance than1 say1 #dj feet1 but within these limits the lamp1 and in fact any good #e-;ampre arc lamp1 will be found quite satisfactory and saves the expense of putting in a special cable4 ,,automatic ,,arc ,,lamps4--,arc lamps for lantern work in which the feeding is done automatically are also made4 ,like hand-fed lamps1 they vary in exact design1 but all1 or practically all1 are so designed that the carbons are brought together by means of springs or weights1 and some form of 'brake' controlled by a system of electro-magnets checks the #ed movement4 ,as the carbons burn away the arc lengthens1 the current weakens1 the electro-magnets lose their grip1 and the carbons move together until the increasing current puts on the brake again4 ,some of these lamps are 'semi-automatic' only1 that is to say1 the arc has to be struck by hand1 while others perform this operation automatically as well1 usually by an additional magnet which draws back the carbons by the correct amount after the arc is struck4 ,my frank advice to intending lanternists is to leave these lamps alone4 ,some of them are satisfactory up to a point1 but they are all apt to be 'jumpy1' and on the whole the hand-fed type is in my opinion to be preferred4 ,,arc ,,lamps ,,on ,,alternating ,,currents4--,the alternating current is not so good as the continuous for lantern work with arc lamps3 the light per ampre is not so great1 the light has an irritating habit of travelling round the carbons and there is always a slight 'hum4' ,the sum total of these drawbacks is nothing very serious1 provided that proper arrangements are adopted1 and ,i have frequently manipulated arc lamps on alternating circuits with such good results that professional lecturers have at first refused to believe that the circuit really was alternating4 ,as it is frequently stated that to obtain a steady light with an alternating current is impossible1 ,i can understand their surprise1 and ,i can also understand the statement in question1 as the problem is usually tackled on entirely wrong lines4 ,it is almost always stated that arc lamps for alternating currents should be arranged with the carbons vertical1 and many makers actually so construct their lamps as to allow of this4 ,to obtain a steady light under these conditions is impossible and ,i pity anyone who attempts it2 but the statement that this is the best method of working has been repeated so often that it seems to have been taken for granted4 ,the best arrangement 7in my hands at any rate7 is to #ee slant the carbons as for the continuous current1 and also to have the upper carbon cored and the lower one solid1 but to use a rather larger lower carbon than would be correct if the main were continuous4 ,also the upper carbon should not be quite so far back as with ,d4,c42 to have the front edges of the two carbons practically in line is about correct1 but the exact position should be carefully adjusted to obtain the steadiest light1 and it will be found that a slight alteration makes a considerable difference4 ,it is also a great help to have a weak electro-magnet1 or its equivalent1 so arranged that it tends by its influence to keep the arc to the front4 ,on some lamps this is provided for1 as even with a continuous current it is quite harmless and1 if anything1 beneficial2 but1 if not1 any competent mechanic can easily fit an ',induction ,ring1' consisting of a single turn of stout copper wire1 which has sufficient magnetic influence to do all that is required 7,fig4 #cb74 ,this ring must be wired in series with the arc itself1 and as the current passing in it automatically reverses in synchronism with the arc1 its effect is always to deflect the arc in the same direction1 and care must of course be taken that it is so wired that the deflection is forward and not backward4 ,this is the exact arrangement ,i have myself adopted1 and ,i never experience any difficulty on the score of the arc wandering4 ,right-angled arc lamps1 as described on pages #eb and #ec1 are also very efficient on ,a4,c4 mains1 and frequently these lamps are already equipped with electro-magnets for the purpose required4 ,the 'hum' of an alternating current cannot be altogether eliminated1 but can be reduced to a minimum by reducing the voltage as far as possible4 ,as has been already said1 the ,a4,c4 lends itself readily to transformation of voltage1 and ,i find in practice #ij-ajj to #ef be ideal4 ,more than this is inclined to be noisy1 and less is apt to result in an unsteady arc4 ,the arrangement1 therefore1 which ,i recommend from long experience is to employ a transformer to reduce the ,e4,m4,f4 to #ajj volts or thereabouts1 and then work with a resistance in the usual way 7if the original current is #ajj volts1 of course no transformer is required7 with a properly constructed arc lamp fitted with an induction ring or electro-magnet4 ,no difficulty should then be experienced in obtaining a good1 steady1 and fairly quiet light4 ,illustration3 ,,fig4 #cb4--;,arc ,lamp with ,induction ,ring4 ,any little 'hum' remaining can be silenced to a very considerable extent by placing the entire lantern on a thick block of saddlers' felt1 but in practice ,i have never found this necessary with ordinary currents1 though a few abnormal circuits where the 'periodicity' is very high are noisier than others4 ,illustration3 ,,fig4 #cc4--,,the ,,optical ,,system ,,of ,a ,,lantern4 #eg ,the following table gives the sizes and particulars of carbons for various currents that ,i have found best in actual practice3 ,,continuous ,,current ,ampres4 ,carbon ,cored4 - ,carbon ,solid4 #g-aj #ab mm4 #g mm4 #aj-ae #ac 11 #h 11 #ae-bj #af 11 #aj 11 ,,alternating ,,current ,ampres4 ,upper ,carbon ,cored4 ,lower ,carbon ,solid4 #g-aj #ab mm4 #aj mm4 #aj-ae #ac 11 #aa 11 #ae-bj #af 11 #ac 11 99 99 99 99 99 ,,chapter ,,vii ,,the ,,optical ,,system ,,of ,a ,,lantern ,illustration3 ,,fig4 #cc;,a4--,optical ,system of ,lantern4 ,as previously noted1 the essential parts of an ,optical ,lantern are1 in order from rear to front3 7#a7 ,the illuminant2 7#b7 the condenser2 7#c7 the slide and slide stage2 7#d7 the objective1 to which must be added1 7#e7 the body or framework which holds the whole together4 ,fig4 #cc is a diagrammatic representation of the entire optical system and ,fig4 #cc;,a shows all the various parts in situ3 ,a being the illuminant1 shown in ,fig4 #cc #eh as an arc lamp1 ,b the condenser1 ,c the slide stage1 and ,d the objective4 ,the foundation1 so to speak1 of the whole instrument is of course the slide1 which1 as made in this country1 consists of a square of glass #c inches diameter1 the slide itself being somewhat less than this on account of the binding1 c42 in making calculations it is usually taken as a #c-;inch circle4 ,slides are usually made by binding together with strips of paper or cloth two such squares1 on one of which is the photographic film or painting forming the picture1 the other being simply a plain cover glass placed over the slide surface to protect it1 and between the two being placed a paper mask with an aperture of whatever size or shape is required1 that of the aforesaid #c-;inch circle being usually taken as the standard or normal dimension for this aperture4 ,the slide being illuminated by one of the various methods discussed in the previous chapters1 is focussed on the screen by the objective1 which must be selected according to the size of picture required and the distance between lantern and screen4 ,these points will be gone into later1 and also details as to various types of objectives and their respective advantages2 but it may be said here that a lantern objective consists usually of a combination of lenses of #b inches or #b inches diameter mounted in a rackwork focussing system at a distance from the slide of #f inches to #ah inches1 according to the length of its 'focus4' ,as our slide is from #c to #c inches diameter1 it is evident that all the light radiating from this cannot possibly get through the objective unless it is converged upon it1 and to do this is the function of the condenser4 ,the following two diagrams1 ,figs4 #cd and #ce1 will make the matter clear4 ,s represents our glass slide of #c inches clear diameter1 ,r the radiant or illuminant1 and ,l our objective1 shown here for the sake of simplicity as a single lens4 ,the slide is well illuminated by the light emanating from #ei ,r1 but it is obvious that the bulk of this light will never pass through the lens1 and1 in fact1 only the very centre of the slide will under these circumstances appear upon the screen at all4 ,illustration3 ,,fig4 #cd4--,optical ,system without ,condenser4 ,illustration3 ,,fig4 #ce4--;,action of ,condenser4 ,what is evidently wanted is to converge these outer rays1 or in other words to bend them in so that they also pass through the objective1 and this is the function of the condenser as illustrated in ,fig4 #ce4 ,the condenser is here represented also by a single lens1 but in practice it also is invariably constructed of two or even three lenses1 for both optical and mechanical reasons4 ,it is evident from the above diagrams that the condenser must be somewhat larger in diameter than the slide itself1 and condensers for ordinary lantern work are usually #d inches to #d inches diameter4 ,the former size #fj will suffice if the condenser is placed very close to the slide1 but it is often advisable to leave a little intervening space1 especially if the illuminant is a powerful one1 in order to allow any condensation of moisture readily to evaporate and escape4 ,hence lanterns for long range work 7which involve1 of course1 good illumination7 are usually made with condensers of #d inches diameter4 ,lantern condensers of to-day usually take one of the two forms shown in ,fig4 #cf1 but the exact curve must be left to the manufacturer1 as the focus of the condenser must have a definite relation to that of the objective4 ,taking1 however1 the design of ,e1 the most common of all1 the two lenses should not be exactly similar unless the objective is pretty short in focus1 or1 in other words1 unless the distance of the illuminant on the one hand and that of the objective on the other are approximately equal4 ,if the lantern is intended for long range work1 that is equipped with a long focus objective1 the front component of the condenser should also be constructed longer in focus 7that is to say1 with a shallower curve7 than the rear one1 and it is amazing how careless manufacturers are in this respect4 ,if1 as is often the case1 the lantern is fitted with several objectives of different foci1 it is usually necessary to supply alternative condensers also1 or at least to supply an interchangeable front component4 ,illustration3 ,,fig4 #cf4--;,forms of ,condensers4 ,if the entire condenser is too long in focus1 light is lost2 if too short1 it is impossible to obtain an even disc1 as there is invariably a dark patch either in the centre or round the edges4 ,the mounting of the condenser also varies with different makers2 but it must be remembered in any case that it gets #fa extremely hot1 especially the back component1 and hence the glass must be mounted loose in its cell1 otherwise there is great danger of it cracking4 ,also the space between the components should be well ventilated1 in order to provide for the escape of moisture which usually at the start of a lantern exhibition is deposited upon the glass1 and should be got rid of before the actual lecture commences4 ,even with all care1 the back component of a condenser will sometimes crack1 though such an accident should be a rare occurrence2 and hence a professional operator will usually provide himself with a spare lens1 and the condenser should be so constructed that it can readily be changed1 and with as little delay as possible4 ,condenser lenses as made in this country are usually ground from the glass known as ',english ,crown1' and comparatively rarely crack2 but they are very slightly green in colour4 ,french condensers1 on the other hand1 are whiter1 but the glass is more brittle1 and a fracture a more common occurrence4 ,the ,french variety are 7or were before the war7 cheaper and generally met with in cheaper instruments4 ,more expensive lanterns are usually fitted with ,english condensers1 as the tinge of green is almost imperceptible1 and the advantage as regards greater security pretty considerable4 ,,the ,,slide ,,carrier ,,and ,,slide ,,stage4--,taking still the optical system of the lantern in order from back to front1 we now come to the slide1 slide carrier1 and slide stage4 ,the slide itself has already been described1 and the carrier is simply a mechanical contrivance1 usually of wood1 designed for the purpose of readily changing the pictures and which in its turn fits into the stage of the lantern4 ,it may be asked why1 if slides are now always made to a standard size1 the slide carrier should not itself be built into the lantern and form the stage2 but the answer is1 in the first place1 that slides of a different size1 i4e4 ,american or ,continental1 may be met with1 #fb and also that there are various mechanical slides on the market--for example1 chromotropes or scientific models1 such for instance as are made to illustrate the movements of the planetary bodies--and these slides are permanently mounted in wooden frames which could not be put into a carrier4 ,the commonest form of carrier is that known as the ',double ,sliding' pattern 7,fig4 #cg71 which consists of a frame with two apertures for the slide1 and an outer frame through which this itself slides and which fits the stage of the lantern4 ,illustration3 ,,fig4 #cg4--;,double ,sliding ,carrier4 ,this carrier1 as will be seen1 allows the next picture to be placed in position in the second aperture while the former one is being projected1 and at a signal from the lecturer1 the inner frame slides smoothly through the outer1 and the slides are thereby changed4 ,this carrier is simple1 cheap1 and quiet in its action2 its one disadvantage is that each alternate slide has to be inserted from opposite sides of the lantern1 and unless the operator is fairly tall this almost necessitates an assistant4 ,nevertheless1 the carrier is the most popular of any1 its other advantages1 especially as regards price1 being so great4 ,it is usually constructed in such a way that the slide1 as it moves out from the central position1 automatically rises in its groove in order to facilitate quick removal4 ,another pattern deservedly popular is that known as #fc ',beard's ,dissolving ,carrier' and is shown in ,fig4 #ch4 ,in this ingenious carrier all the slides are inserted from the same side1 the oncoming slide being pushed in front of its predecessor1 and being therefore somewhat out of focus it produces a blur on the screen4 ,the movement is performed by pushing in a projecting handle1 and on withdrawing this the slide which is finished with comes with it1 and the finish of the movement presses the new slide back until it is in its proper position and in focus4 ,illustration3 ,,fig4 #ch4--;,beard's ,dissolving ,carrier4 ,the entire action is simpler than it sounds1 and the temporary blurring of the image on the screen during the process of changing is supposed to give somewhat the effect of ',dissolving ,views1' and hence the name ',dissolving ,carrier4' ,this appliance is three times the price of the ',double ,sliding' pattern1 but the fact that it is worked from one side only is a decided advantage1 though on the other hand it is not 7unless great care is used7 quite so silent in its action as the ',double ,sliding' type4 ,a further modification of this carrier adapts it to take any of the recognised 'foreign' sizes of slides1 so that if a few ,american ones1 for instance1 are met with among a collection of ,english manufacture1 there is no need to change the carrier4 #fd ,there are other varieties of carriers on the market which there is no need particularly to describe1 such as1 for example1 carriers fitted with roller curtains to give the effect of a curtain rolling up1 magazine carriers to hold twenty-four or more slides and exhibit them in rotation1 and other patterns too numerous to mention4 ,of these the reader must be left to judge for himself1 but1 generally speaking1 simplicity in a carrier is the most important point to be looked for1 and complications1 however ingenious1 should be avoided4 ,illustration3 ,,fig4 #ci4--;,focussing ,action of ,lens4 ,the lantern stage must also receive consideration1 but it will be better to discuss it as part of the mechanical construction of the lantern4 ,,the ,,objective is really the most vital part of a lantern1 as the definition of the picture almost entirely depends upon the excellence or otherwise of this lens4 ,this will be obvious at once when it is realised that the objective has to project on to the distant screen a greatly magnified image of the comparatively small lantern slide1 and the intending purchaser is strongly advised to economise almost anywhere rather than on this item4 ,the action of a lens in focussing the image is perhaps best explained by a simple diagram 7,fig4 #ci71 from which it will be seen that all the rays proceeding from any one point on the object are re-converged 7when the lens is in focus7 to a definite point on the image1 and the perfection of the picture depends upon the lens performing this function accurately4 #fe ,the imperfections are chiefly two1 viz4 those known as chromatic and spherical aberration respectively4 ,chromatic aberration simply means that all the colours composing the original beam of1 say1 white light are not equally refracted or converged1 and therefore do not meet again at the same spot 7the well-known prism or lustre effect71 and reveals itself by coloured fringes round the edges of the various details in the picture4 ,illustration3 ,,fig4 #dj4--;,achromatic ,lens4 ,by spherical aberration we mean that the light falling upon the centre of a lens is not brought to a focus at exactly the same spot as the marginal rays1 and a general want of definition is the result1 usually accompanied also by a want of 'flatness' in the image1 that is to say the edges of the picture do not focus at the same time as the centre4 ,chromatic aberration is easily cured by using an achromatic or compound lens made by cementing together two lenses of crown and flint glass respectively1 as in ,fig4 #dj4 ,it will be seen that the flint glass component by itself is a concave lens and therefore neutralises in part1 or in whole1 the convex crown lens4 ,flint glass has both greater dispersive power and also greater refractive power than crown glass1 but fortunately not to the same degree2 hence a compound lens made in this way and with curves carefully worked out may have its chromatic effect entirely neutralised while retaining very considerable refractive or 'focussing' power1 and simple achromatic objectives of this type are quite inexpensive4 ,in lanterns intended for ,science demonstration1 as distinct from the mere projection of slides1 lenses of this pattern are very frequently used1 as they will project the latter when required reasonably well1 and for the demonstration of #ff experiments or of apparatus on the screen have advantages that need not be discussed here4 ,for very long focus lenses also they are sometimes employed1 as the trouble from spherical aberration is much less apparent with lenses of long focus than with short1 and the difference in expense is much more in the former case than in the latter4 ,for short focus lenses1 however1 as used in moderate-sized halls1 they are not good enough1 and the type of lens almost universally employed is that known as the ',petzval' combination 7,fig4 #da74 ,illustration3 ,,fig4 #da4--,petzval ,combination4 ,this lens really consists of two achromatic combinations1 the pair at the front being cemented together1 and that at the rear having an air space between4 ,the combination is so designed that the spherical aberration of the one pair neutralises that of the other1 and the result is or should be a lens corrected both for chromatic and spherical aberration4 ,these lenses1 however1 vary very much in the perfection of their results1 and as they are at present usually imported in bulk from ,france1 the customer does well to insist upon a demonstration of his own particular lantern before acceptance4 ,the magnifying power of a lens depends upon its 'focus' multiplied by its distance from the screen1 and the focus in the case of a simple lens is easily determined by the familiar 'burning-glass' experiment1 that is by focussing an image of the sun upon a piece of paper and measuring accurately #fg the distance the lens must be away to produce the most concentrated spot4 ,in practice it is sufficiently accurate to focus a distant window1 or other luminous object1 upon the paper1 any error obtained by this method being for ordinary purposes a negligible one4 ,with a compound lens1 such as a ',petzval' combination1 this method does not hold good1 as the optical centre of such a lens is not necessarily midway between its two components4 ,the actual focus can be got pretty approximately by focussing a window or other object as before and measuring the distance from one definite point 7say the front edge of one of the lens cells7 to the paper1 then turning it round and taking a second measurement from the same point1 the mean between the two measurements giving the actual focus4 ,in practice the 'simple equivalent focus1' as it is termed1 of a lantern lens is usually determined by measuring the magnification of the image thrown upon the screen1 when1 by knowing the original size of the slide 7a 'standard' slide of #c inches diameter is usually taken7 and the distance between lantern and screen1 we get the focus from the following very simple equation3 ,diameter of picture on screen ,distance between lens and screen 7in feet7 7in feet7 ----------------------------- --------------------------------- ,diameter of slide 7in inches7 ,focus of lens 7in inches7 or perhaps more simply still3 ,distance between lens ,diameter of slide and screen 7in feet7 7in inches7 ---------------------------------------- -- ,focus of lens in inches2 ,diameter of picture 7in feet7 or1 if we know the focus of the lens but want to know how far from the screen we must go to produce a given-sized picture1 the formula will be3 #fh ,diameter of picture ,focus of lens 7in feet7 7in inches7 ------------------------------------ ,distance required 7in feet74 ,diameter of slide 7in inches7 ,it is handy for the lanternist to remember that1 dealing with a standard #c-;inch slide1 a #f-;inch lens will always give a picture whose diameter is one-half the distance from lens to screen1 a #ab-;inch lens half this again or one-quarter1 and a #i-;inch lens half-way between the two4 ,bearing these simple figures in mind1 the approximate distance can usually be guessed sufficiently near for the first trial1 and then the lantern shifted a little nearer or the reverse as required4 ,the following table may1 however1 be useful1 as showing readily the magnification produced at different distances by lenses of given foci3 ---------------------------------------- ----------------------- ,disc ,focus ,focus ,focus ,focus ,focus ,focus ,focus wanted #d in4 #f in4 #h in4 #aj in4 #ab in4 #ae in4 #ah in4 ---------------------------------------- ----------------------- feet4 ft4 in4 ft4 in4 ft4 in4 ft4 in4 ft4 in4 ft4 in4 ft4 in4 #i #ac #f #ah #j #bd #j #cj #j #cf #j #de #j #ed #j #ab #ah #j #bd #j #cb #j #dj #j #dh #j #fj #j #gb #j #ae #bb #f #cj #j #dj #j #ej #j #fj #j #ge #j #ij #j #ah #bg #j #cf #j #dh #j #fj #j #gb #j #ij #j #ajh #j #bj #cj #j #dj #j #ec #d #ff #h #hj #j #ajj #j #abj #j #be #cg #f #ej #j #ff #h #hc #d #ajj #j #abe #j #aej #j #cj #de #j #fj #j #hj #j #ajj #j #abj #j #aej #j #ahj #j ---------------------------------------- ----------------------- ,,the ,,diameter ,,of ,,the ,,objective4--,the diameter of the objective must depend to a certain extent upon its focus in the case of a double combination such as a ,petzval4 ,these lenses consist1 as has already been said1 of two achromatic components some distance apart1 and for technical considerations1 which need not be discussed here1 the distance between these components is usually about two-thirds of the focal length4 ,this is not a universal rule1 as the lenses of different makers vary a good deal2 but it is generally a fact #fi that the longer the focus of the lens the greater is usually the separation between the two lens systems4 ,the entire lens therefore mounted in its tube resembles a tunnel of varying length according to its focus1 and through this tunnel a cone of light rays have to be passed4 ,it is plain1 therefore1 that a lens of long focus1 which in practice means a long tube length1 must be made also of large diameter1 or a portion of the cone will be cut off1 with a consequent loss of light4 ,in practice lenses up to #f inches focus are usually made of #b inches diameter1 and there is no advantage in a larger size4 ,with a lens of #h inches focus there is a slight gain in increasing the diameter to #b-c_/h 7the next 'standard' size71 and lenses of longer focus than this should certainly be #b-c_/h inches up to1 say1 #ab inches focus1 when a lens of #c inches diameter is preferable4 ,these large lenses are1 however1 very expensive1 both in themselves and also on account of the fact that their weight entails heavy and expensive brass mounting1 and hence lenses up to #ad or #ae inches focus are often supplied in the #b-c_/h size for reasons of economy4 ,to sum up1 short-range lanterns1 as they are called1 are usually fitted with lenses of #b inches diameter1 and long-range instruments either with #c-;inch lenses or the intermediate size of #b-c_/h inches4 ,if a lantern is purchased for either long or short-range work1 it is usually fitted with a brass front for a large lens1 and so arranged that a shorter focus lens of #b inches diameter can easily be interchanged1 utilising the same brass mounting4 ,lenses of variable focus have also been designed1 in which an additional lens can be added or subtracted to increase or decrease the focal length2 but nothing very practical has yet been achieved in this direction1 and therefore these ',omnifocal' lenses have never come into general favour4 ,objectives like condensers want cleaning at times1 and care must be taken not to scratch the glass1 as the concave lens of each component is of flint glass1 and very soft4 ,a #gj clean chamois leather is the best thing to use1 but a soft cloth1 or even a handkerchief1 may be employed with care4 ,it is very important that a lens be reassembled1 after cleaning1 the correct way1 as a single lens reversed would utterly spoil the definition4 ,the front component is usually balsamed together1 and therefore all that is needed is to see that the whole combination is not reversed4 ,in the ,petzval system this lens should have its convex constituent towards the screen 7,fig4 #da74 ,the back combination is usually loose1 and the two lenses are sometimes separated by a thin brass ring4 ,in the ,petzval lens the concave element should be inside1 with its concave surface outwards1 the deep curve of the other lens should fit into this concavity1 and the flatter curve face towards the condenser4 ,one or two makers1 however1 have introduced a modification of the ,petzval system in which the whole of this back combination is reversed1 and the exact arrangement should therefore be noted very carefully when taking the lens to pieces4 99 99 99 99 99 ,,chapter ,,viii ,,the ,,body ,,of ,,the ,,lantern ,we now come to the mechanical construction of the optical lantern1 and a great variety of design presents itself1 according to price1 type 7i4e4 short range or long range71 and the individual ideas of the various makers4 ,lantern bodies as a rule are now made of metal1 although up till quite recently the better class instruments were more usually made of polished mahogany lined internally with iron2 but there has of late been a consensus of opinion in favour of metal only4 ,in the cheaper lanterns this metal body is usually made either of ,russian iron or of sheet-iron tinned and japanned1 #ga there being little to choose either in price or quality between the two varieties1 and in all but the very cheapest instruments the front is usually of brass4 ,in better lanterns the body is more often made of enamelled steel1 the front as before being of brass2 but brass1 copper1 or aluminium are also used occasionally for the body of the lantern4 ,illustration3 ,,fig4 #db4--;,hughes' ,short-,range ,lantern4 ,in deciding upon the type of body to be purchased the main considerations to be borne in mind are3 7#a7 ,the type or types of illuminant to be used1 a powerful arc lamp for example requiring a larger body than is necessary for a weaker radiant2 7#b7 the size and position of the lens to be carried1 a ,petzval objective of say #c inches diameter which has to be supported at the end of a long brass mount for long-range work obviously demanding a body of greater strength and rigidity than is required with a #f-;inch focus lens of #b inches diameter2 7#c7 price4 ,fig4 #db shows an extremely good lantern body for short-range work made by ,messrs4 ,hughes1 the illustration depicting the instrument complete with a ',luna' methylated spirit lamp1 though1 of course1 any other illuminant suitable for a small lantern could be used instead4 #gb ,this lantern illustrates well one point that has already been emphasised as important1 viz4 the ventilation of the condenser4 ,it will be noticed that this is placed outside the body of the instrument instead of inside as is usual with larger bodies1 and that wide slots are cut in the condenser mount to allow free escape of steam4 ,other points of this excellent design are the screw adjustment to the slide stage 7facilitating the use of special slides1 such1 for example1 as those illustrating the movements of the planetary bodies which sometimes involve the use of extra thick frames7 and a simple but efficient tilting arrangement to the base4 ,illustration3 ,,fig4 #dc4--,long-,range ,lantern4 ,such a lantern is hardly suitable for a powerful arc lamp or limelight jet1 or for heavy long-range lenses1 but is a very good typical instrument for use in moderate-sized halls1 and a lantern of this general type is usually found in lantern catalogues1 though1 of course1 the exact designs vary according to the ideas of the manufacturer4 ,of lanterns for long-range work a good example is perhaps ,messrs4 ,newton ,co4's ',intermediate' pattern 7,fig4 #dc74 ,this again is only typical of many others by the various makers1 but the principal points are common to all4 ,these are3 7#a7 ,the large and well-ventilated body2 7#b7 the long #gc baseboard2 7#c7 the strong and massive brass front necessary to carry the large long-range lenses2 7#d7 the velvet curtain at the back to close in any stray light from a powerful arc lamp4 ,illustration3 ,,fig4 #dd4--;,connections for a ,bi-unial ,lantern4 ,the two foregoing designs are perhaps sufficiently typical of lantern bodies in general to make further detailed description of individual designs unnecessary2 but reference should be made to features which special requirements may render advisable4 ,under this heading mention must be made of ,bi-unials or ,double ,lanterns1 as used for the once famous ',dissolving ,views4' ,a bi-unial lantern consists essentially of two different instruments1 each complete with its limelight jet or other illuminant--front1 condensers1 objective1 c41 usually mounted on one body--and with some arrangement for 'dissolving' or turning the light in each lantern gradually on and off4 ,fig4 #dd shows the back view of such a lantern with two limelight jets and dissolving tap1 this piece of mechanism 7shown below in the illustration7 being so arranged that when the lever is horizontal both lanterns are on full1 but moving the lever either way cuts off the gas supply to one lantern4 ,in the case of limelight the tap should always operate by cutting off the oxygen supply in advance of the coal gas 7in order to avoid a 'snap'71 and the latter should never be cut off entirely1 but a small bead of flame left to keep the jet alight1 until the lantern is required for the next slide4 #gd ,this is usually arranged for by means of a bye-pass1 and a bye-pass is sometimes provided on the oxygen side as well1 but is usually discarded in practice4 ,a bi-unial lantern can be worked in the same way with acetylene gas1 but with the electric arc it is impossible to turn the light on and off gradually1 and in practice dissolving must be done by keeping both lanterns fully alight1 and using a dissolving shutter1 that is a movable shutter that covers each objective alternately4 ,the same arrangement must be used with other illuminants1 such as oil1 only in this case the lanterns must be mounted side by side1 on account of the tall chimneys4 ,with oil lamps the arrangement answers fairly well1 the dissolving fan1 as it is termed1 being made with serrated edges which give the gradual obliteration required2 but with the electric arc the extremely sharp definition becomes a serious difficulty1 and a good dissolver for this illuminant has never yet been found1 though1 in view of the fact that dissolving views are more or less a thing of the past1 the matter cannot be regarded as important4 ,the advantages claimed for a double lantern are two3 first1 a ',dissolving' effect by which one picture fades gradually into the next1 and which is supposed to be more pleasing than the movement of a carrier2 and second1 ',dissolving ,effects' can be shown1 such as exhibiting a landscape by day and changing it into a moonlight scene1 or bringing on the appearance of a snowstorm1 which can easily be done by means of a roller slide1 with minute perforations shown in motion by the second lantern while the landscape remains on the screen from the first4 ,in the days when dissolving views were all the vogue1 a third or even a fourth lantern has been added for more complicated effects1 and at the famous ,polytechnic demonstrations of years ago1 ,i believe that as many as six were sometimes employed4 ,in these days of the cinematograph it is doubtful how far interest in such effects could be revived1 and a lantern has #ge gradually come to be looked on more as an instrument for showing illustrations as required by the lecturer rather than as a pleasing exhibition in itself1 and as dissolving views have lost their attraction1 the double or triple lantern has been relegated to the limbo of antiquity4 ,illustration3 ,,fig4 #de4--;,beard's ,circulating ,water ,tank4 ,among other 'special' lanterns should be mentioned models made with water-cooled stages1 for use with very delicate slides4 ,this elaboration is not necessary with ordinary slides and illuminants of moderate power1 but where very delicate slides1 such as specimens of natural colour photography1 have to be shown1 it is an advisable precaution to pass the beam of light first through a tank of water in order to absorb the heat4 ,lanterns intended for this work are usually constructed with a kind of double stage1 a glass trough of water fitting into the rear aperture and the slide-carrier into the front one4 ,such an arrangement answers quite well for most purposes1 but for extreme cases lanterns are equipped with a trough connected to a large outside tank and complete circulatory system1 after the manner of the cooling tank of a gas engine4 ,such a lantern1 constructed by ,messrs4 ,beard1 is illustrated in ,fig4 #de1 and it will be seen that in this instrument the water trough is placed between the lenses of the condenser4 #gf ,this is a very good position1 as the beam of light at this point is1 or should be1 parallel1 whereas between the condenser and the slide it is convergent1 and therefore a condenser of a larger diameter than the slide must be employed in the latter case if the trough is of considerable width4 ,while dealing with ',special' lantern bodies1 we should perhaps just mention here the numerous pattern lanterns made for the demonstration both of lantern slides and of ,scientific ,phenomena1 such as the projection of insect life or other microscopic objects1 polarised light experiments1 electrical apparatus1 opaque objects1 c4 ,a detailed description of these lanterns and how to use them belongs to the second part of this work1 as also does the popular cinematograph2 but educational institutes1 and even boys' clubs1 when considering the purchase of a lantern1 might well reflect whether it would be advisable to spend a little more money in the acquisition of an instrument which can be utilised for a variety of purposes4 99 99 99 99 99 ,,chapter ,,ix ,,lantern ,,boxes1 ,,stands1 ,,reading ,,lamps1 ,,etc4 ,having now discussed all the essential parts of a lantern1 the next points to be considered are those of lantern boxes and stands4 ,it is best to take these together1 as more often than not a lantern is arranged to stand upon its box during use1 and the plan is both convenient and simple4 ,the whole question is one to be settled upon its own merits in each individual case4 ,sometimes neither box nor stand is wanted at all4 ,the lantern is put away into a locked-up cupboard or other safe place1 and used upon a permanent support or 7as is often the case in a church7 from a gallery at the back4 #gg ,in most cases1 however1 a box of some sort is desirable1 and the two main considerations are strength and simplicity4 ,all patent arrangements1 such1 for example1 as those in which the sides of the box fall down and provide trays for the slides1 are beautiful in theory1 but cannot be recommended in practice4 ,a good1 simple and substantial box is what is required1 preferably painted black1 and provided with strong handles4 ,one addition may be permitted1 viz4 a tilting top4 ,some means for tilting the lantern is always advisable1 as it is seldom convenient to raise the instrument to the level of the centre of the screen1 and a slight upward elevation does not appreciably distort the image4 ,this arrangement for tilting may be either embodied in the lantern itself1 as for instance in the instrument shown in ,fig4 #db1 or may be provided for on the box or on the stand1 if a stand is used4 ,it is1 perhaps1 an elaboration that may be regarded as not strictly necessary1 as a book or two or other article may be placed under the lantern base as required2 but a tilting arrangement is so convenient that it can be strongly recommended1 and the addition is not expensive4 ,for large1 long-range lanterns a strong deal box1 on which the lantern can stand1 is usually all that it is desirable to purchase in the way of a support4 ,a good solid table can usually be found1 which will do all the rest1 as it must be remembered that a slight tilt at a long range means a good deal of total elevation4 ,where this is not procurable a stand must be provided1 and this for a large lantern should be strong and rigid4 ,anything in the way of a collapsible tripod should be avoided1 but such an arrangement as ,fig4 #df is quite good and rigid enough for all practical purposes4 ,for a small lantern a tripod stand is quite suitable1 though care must be taken that one of the legs does not get kicked1 either by accident or design1 or the result may be a catastrophe4 #gh ,slide ,boxes4--,on this subject not much need be said4 ,the variety of patterns on the market is endless1 some being designed from the point of view of safe transit by post1 others for convenience of storage and classification4 ,it is essentially a case where each individual user must use his or her taste1 and in any case the question of the box is one for the owner of the slides rather than for the lanternist4 ,illustration3 ,,fig4 #df4--,quadruple ,lantern ,stand4 ,,reading-,,desks1 ,,lamps1 ,,and ,,signals4--,some form of reading lamp for the lecturer is generally considered to be part of a lanternist's equipment1 and the most usual pattern is fitted with a candle1 after the manner of a carriage lamp1 or else constructed to burn colza or other vegetable oil1 such as supplied for cycle lamps4 ,oil gives the brighter light1 but is apt to get spilled in transit1 hence a candle lamp is the more #gi convenient for a travelling lecturer1 while oil is to be preferred if transport is not a factor to be considered4 ,these lamps are usually constructed with a red flashing signal at the rear1 actuated by a simple lever1 by which the lecturer can communicate his wish for a change of slide1 c41 to the lanternist 7,fig4 #dg74 ,illustration3 ,,fig4 #dg4--,reading ,lamp4 ,there are various other devices used for the same purpose1 such as a castanet1 to be held in the lecturer's hand and clicked when necessary1 an electric bell to ring in the lantern box1 c4 ,if this latter is used it is usual to remove the gong1 the buzz of the hammer being sufficiently loud without it4 ,some lecturers again prefer to use no such apparatus at all1 but simply to say ',next slide' as required1 or to tap on the floor with a pointer1 and the choice of a suitable means of communication between lecturer and lanternist must be largely a matter of individual selection4 ,more elaborate reading-desks are also supplied by most makers1 but here again judgment must largely come into play in what is hardly a technical matter4 99 99 99 99 99 ,,chapter ,x ,,screens ,,and ,,screen ,,stands ,the best of all screens for lantern purposes is undoubtedly a smooth whitewashed wall1 and this is now provided in many halls where lantern exhibitions are usual4 ,in places where this is not practicable the next best substitute is a canvas #hj screen1 which rolls up and down 7,fig4 #dh74 ,this can be obtained from any good maker1 but again can only really be used as a fixture in the hall where the lantern is to be used4 ,it can1 however1 be fitted into a wooden box which can be painted or varnished to suit the other architecture1 and the provision of such a screen is to be strongly recommended whenever possible4 ,if portability is required1 a linen or calico sheet that can be folded up is necessary1 but this can never be hung absolutely flat1 and also loses a considerable amount of light by transmission4 ,illustration3 ,,fig4 #dh4--,roller ,screen4 ,a so-called 'transparent' sheet is made of very thin linen1 and intended to work with the lantern behind it1 showing the picture through the linen to the audience on the other side1 but this is seldom used except in the open air for religious or political meetings1 c4 ,an opaque sheet can be had in one piece up to #i feet square2 larger sizes than this must have at least one seam1 and most skilful sewing is necessary1 especially with large sheets consisting of several strips sewn together4 ,sheets such as these are usually supplied with either eyelet holes round the edges or else linen tapes sewn on1 and the exact method of hanging must be left to circumstances4 #ha ,in the case of a small sheet it will be sufficient to stretch it at the four corners1 and this can often be done by screwing into the walls or some convenient girder two screw eyes and similar eyes into the floor1 all four being considerably farther apart than the size of the sheet4 ,illustration3 ,,fig4 #di4--,portable ,screen ,stand4 ,a stout cord being then passed through the two upper eyes1 long enough for both ends to reach near the floor1 one end of each can be fastened to the two top corners of the sheet and the latter drawn up1 the two bottom corners being afterwards stretched and tied down tightly to the lower eyes4 ,in the case of large sheets this hardly suffices1 and it will be found necessary to fasten the sheet at intervals all round or it will exhibit awkward creases1 and this again is a matter where the lanternist must use his own initiative according to the possibilities4 ,in some halls the erection of a sheet in the way above described is a sheer impossibility1 and in such cases a frame must be made by nailing strips of wood together1 or better by utilising a portable screen stand 7,fig4 #di74 ,these stands are usually made of bamboo1 with short brass connecting tubes1 and the method of using them is so obvious that a description need hardly be given4 ,the screen frames are supplied by all the leading opticians1 but an intending purchaser would be well advised to see one erected before ordering4 ,i have actually seen a #ab-;foot screen frame offered for sale that was too weak to carry its own weight1 let alone the weight of the sheet6 99 99 99 99 99 #hb ,,chapter ,,xi ,,the ,,practical ,,manipulation ,,of ,a ,,lantern ,having now described the optical lantern in its various forms and the more important accessories1 we come to the question of practical manipulation4 ,in making arrangements for an exhibition the first thing to be seen to is to ensure that every accessory that will be required will be there1 and the best plan is to make a complete list of all sundries to be provided4 ,such items as string 7for the sheet71 lime tongs if limelight is used1 pliers for changing carbons if the arc is to be the illuminant1 screw-driver1 matches1 the key of the lantern box1 and other similar items1 are likely to be left behind unless such a list is made and carefully checked4 ,on arriving at the hall1 the first thing to be done as a rule is to get up the sheet1 after which the professional operator generally begins to feel happy again4 ,the next thing1 if it has not been done first1 is to determine the position of the lantern1 and this1 as has been explained in ,chapter ,,vii1 is a matter of the size of picture to be shown and the focus of the objective4 ,it is a mistake to show too large a picture2 a little 'white' round the edges is a good thing1 and it is better to have a small disc well illuminated than a large one less bright4 ,convenience1 however1 must also be considered1 and it is often justifiable to go back a few feet farther than other considerations would dictate in order to place the lantern in a gallery or other spot where it is out of the way4 ,having fixed the position of the lantern1 it should be got into place1 the cable or tubing connected or whatever else is necessary1 according to the illuminant to be used4 ,it should then be lit up1 the flasher of the lens opened1 and the light #hc centred sufficiently to produce some sort of disc upon the screen4 7,it is1 of course1 presumed that the lenses1 c41 have previously been cleaned47 ,a carrier should now be placed in the stage and a slide inserted into it1 and the method of doing so requires a little explanation4 ,the slide must be placed in the carrier upside down1 as will be obvious to anyone who has studied ,chapter ,,vii1 but in addition to this it must be turned the correct way1 otherwise the picture will be reversed from left to right4 ,this in the case of certain subjects1 such as a copy of a picture1 may not greatly matter2 but in slides depicting buildings or landscapes with which the audience may be familiar1 or worse still1 printing or writing1 is a serious blunder4 ,slides made by a commercial firm will usually be 'spotted1' that is to say1 will have two white spots on the face of the slide when the latter is viewed in its correct position1 and at the top4 ,the slides should be turned upside down and placed in the carrier with the spots1 of course1 now at the bottom and towards the condenser4 ,if a slide is not spotted it should be viewed as it is to appear on the screen1 and then placed in the carrier with the face that was towards the operator as he viewed it turned to the condenser1 and of course inverted4 ,the above remarks apply only in cases where the image is thrown on the screen2 in the comparatively rare instances where it is shown through the latter the slides must be turned round laterally1 but of course still inverted4 ,the slide having been placed in the stage it should be 'focussed' by racking the objective in or out1 and if necessary pulling out the draw tube as well until the image on the screen is sharply defined4 ,so far the light has only been roughly centred1 sufficiently so to enable the slide to be focussed1 and to complete the operation both slide and carrier should next be taken out of the lantern1 leaving a clear disc on the screen1 and this disc may resemble any of the appearances shown in ,fig4 #ej4 #hd ,if it resembles ,a the light must be moved to the left1 if like ,b to the right1 like ,c it must be lowered1 like ,d it must be raised1 always moving it to the side opposite to the dark shade until this is central on the disc4 ,if it now resembles ,e1 the light must be moved nearer the condenser2 if1 on the contrary1 the centre is dark1 it must be drawn back until finally the circle should be as nearly as possible clear and bright all over1 as at ,f4 ,illustration3 ,,fig4 #ej4--;,adjustment of the ,light4 ,it is important to note that this adjustment cannot be properly made while a slide is in position1 and neither can it be made until the lantern has been focussed1 so the above procedure is the only way to get a satisfactory result4 ,with some of the larger illuminants1 such as a paraffin-oil lamp1 there are no centering adjustments1 the size of the radiant rendering exact centering unnecessary1 and generally speaking the smaller the luminous point1 the more exact must the operation be4 ,in the case of such illuminants as acetylene or limelight #he care must be taken that they are turned fully on before centering1 otherwise turning on the fuller amount afterwards will raise the position of the luminous spot4 ,the centering achieved1 the slide carrier may be replaced1 the first slides placed in position1 the remainder arranged in their proper order1 the system of signalling with the lecturer determined1 and all is ready4 ,if there is still an interval before commencing1 the light may be switched off or turned out1 or in the case say of limelight1 turned down very low until wanted4 ,it is of extreme importance to see that all the slides are in their right order1 though the duty of seeing to this usually rests with the lecturer rather than with the operator4 ,i remember hearing of one lecture on the life of ,queen ,victoria1 when the lecturer announced1 ',the next picture will be a photograph of the ,royal ,prince who for many years shared the ,throne with our gracious ,sovereign4' ,at the words the operator brought on the next slide1 which proved to be a restored specimen of a prehistoric monster 7tableaux674 ,such mistakes 'bring down the house1' but in serious lectures1 and especially at religious services1 cannot be too carefully guarded against4 ,mention has already been made of the liability of moisture to condense on the surfaces of the condensers or slides1 and to avoid this1 so far as the condensers are concerned1 it is well to light up say ten minutes before the lantern is actually wanted1 or alternatively to take out the condensers and thoroughly warm them in front of a stove1 or to place them wrapped in a cloth on hot-water pipes4 ,the slides should in the same manner be warmed before using and should be finally held above the lantern or placed on the top1 if this is flat1 the last thing before being placed in the carrier4 ,if these precautions are omitted1 on a cold night the first surface of the condenser will become so covered with moisture as to almost obscure the slide1 and this will quickly disappear #hf with the heat of the lantern4 ,next1 the two inner surfaces of the condensers will behave in turn in the same way1 and will take considerably longer to clear1 especially if the ventilation of the condenser is poor2 then the fourth surface will take up the running1 and finally1 when the lanternist is congratulating himself that the trouble is over1 each successive slide will become affected in the same way4 ,with an operator who knows his business1 none of these troubles should occur4 ,,accidents4--,these will occur sometimes1 even in the best managed exhibition2 the rubber tubing feeding a limelight jet gets kinked or trodden on1 or a fuse melts if electric light is being used1 c41 and out goes the light4 ,in such cases a loud request such as1 ',would you mind turning up the light for a minute1 please1' accompanied by a good-humoured laugh1 usually allays the fears of 'nervy' people4 ,an operator must never get 'nervy' himself4 ,i have known of more than one fiasco because some little hitch occurred1 and two or three timid ladies crowded round and asked anxious questions1 till the lanternist lost his head4 ,in one such case the cautious superintendent at a children's entertainment decided that it would be safer not to have the exhibition at all1 simply because a regulator was not screwed tightly enough into a cylinder to prevent an escape of gas1 only the operator 7a somewhat youthful one7 had been driven to the verge of lunacy by continual questions of the standard type1 ',are you sure it is safe8' ',will it blow up8' ',are you certain you understand it8' c41 c4 ,more serious accidents1 such as the entire lantern getting upset1 ought never to occur1 and it is up to the lanternist to take whatever precautions he deems necessary to safeguard his instrument4 ,with a juvenile audience1 for example1 it is often a good thing to arrange a barricade of forms round the lantern and to see that no one comes within it4 ,finally1 'whatever is worth doing at all is worth doing well1' and this is as true of lantern exhibiting as of anything #hg else4 ,there are a deplorable number of lantern exhibitions given with the sheet hanging in creases1 dirty lenses1 light poorly adjusted and centred1 and occasionally slides shown upside down4 ,a conscientious lanternist should see to every detail2 slipshod methods1 as in everything else1 mean poor results4 ,printed by ,,spottiswoode1 ,,ballantyne ,,co4 ,,ltd4 ,colchester1 ,london ,eton1 ,england