,transcriber's notes3 ,the text of this book has been preserved in its original form apart from correction of two typographic errors3 embarrasment embarrassment1 ,cassegranian ,cassegrainian4 ,inconsistent hyphenation has not been altered4 ,a lengthy preliminary section concerning the ,smithsonian ,institution precedes the actual subject matter4 ,,smithsonian ,,contributions ,,to ,,knowledge4 ,,vol4 ,,xiv4 ,illustration ,,every ,,man ,,is ,a ,,valuable ,,member ,,of ,,society1 ,,who1 ,,by ,,his ,,observations1 ,,researches1 ,,and ,,experiments1 ,,procures ,,knowledge ,,for ,,men4--,,smithson4 ,,city ,,of ,,washington3 ,,published ,,by ,,the ,,smithsonian ,,institution4 ,,mdccclxv4 ,,advertisement4 ,this volume forms the fourteenth of a series1 composed of original memoirs on different branches of knowledge1 published at the expense1 and under the direction1 of the ,smithsonian ,institution4 ,the publication of this series forms part of a general plan adopted for carrying into effect the benevolent intentions of ,,james ,,smithson1 ,esq41 of ,england4 ,this gentleman left his property in trust to the ,united ,states of ,america1 to found1 at ,washington1 an institution which should bear his own name1 and have for its objects the 8increase and diffusion of knowledge among men40 ,this trust was accepted by the ,government of the ,united ,states1 and an ,act of ,congress was passed ,august #aj1 #ahdf1 constituting the ,president and the other principal executive officers of the general government1 the ,chief ,justice of the ,supreme ,court1 the ,mayor of ,washington1 and such other persons as they might elect honorary members1 an establishment under the name of the 8,,smithsonian ,,institution ,,for ,,the ,,increase ,,and ,,diffusion ,,of ,,knowledge ,,among ,,men40 ,the members and honorary members of this establishment are to hold stated and special meetings for the supervision of the affairs of the ,institution1 and for the advice and instruction of a ,board of ,regents1 to whom the financial and other affairs are intrusted4 ,the ,board of ,regents consists of three members ex officio of the establishment1 namely1 the ,vice-,president of the ,united ,states1 the ,chief ,justice of the ,supreme ,court1 and the ,mayor of ,washington1 together with twelve other members1 three of whom are appointed by the ,senate from its own body1 three by the ,house of ,representatives from its members1 and six persons appointed by a joint resolution of both houses4 ,to this ,board is given the power of electing a ,secretary and other officers1 for conducting the active operations of the ,institution4 ,to carry into effect the purposes of the testator1 the plan of organization should evidently embrace two objects3 one1 the increase of knowledge by the addition of new truths to the existing stock2 the other1 the diffusion of knowledge1 thus increased1 among men4 ,no restriction is made in favor of any kind of knowledge2 and1 hence1 each branch is entitled to1 and should receive1 a share of attention4 ,the ,act of ,congress1 establishing the ,institution1 directs1 as a part of the plan of organization1 the formation of a ,library1 a ,museum1 and a ,gallery of ,art1 together with provisions for physical research and popular lectures1 while it leaves to the ,regents the power of adopting such other parts of an organization as they may deem best suited to promote the objects of the bequest4 ,after much deliberation1 the ,regents resolved to divide the annual income into two parts--one part to be devoted to the increase and diffusion of knowledge by means of original research and publications--the other part of the income to be applied in accordance with the requirements of the ,act of ,congress1 to the gradual formation of a ,library1 a ,museum1 and a ,gallery of ,art4 ,the following are the details of the parts of the general plan of organization provisionally adopted at the meeting of the ,regents1 ,dec4 #h1 #ahdg4 ,,details ,,of ,,the ,,first ,,part ,,of ,,the ,,plan4 ,i4 ,,to ,,increase ,,knowledge4--,it is proposed to stimulate research1 by offering rewards for original memoirs on all subjects of investigation4 #a4 ,the memoirs thus obtained1 to be published in a series of volumes1 in a quarto form1 and entitled 8,smithsonian ,contributions to ,knowledge40 #b4 ,no memoir1 on subjects of physical science1 to be accepted for publication1 which does not furnish a positive addition to human knowledge1 resting on original research2 and all unverified speculations to be rejected4 #c4 ,each memoir presented to the ,institution1 to be submitted for examination to a commission of persons of reputation for learning in the branch to which the memoir pertains2 and to be accepted for publication only in case the report of this commission is favorable4 #d4 ,the commission to be chosen by the officers of the ,institution1 and the name of the author1 as far as practicable1 concealed1 unless a favorable decision be made4 #e4 ,the volumes of the memoirs to be exchanged for the ,transactions of literary and scientific societies1 and copies to be given to all the colleges1 and principal libraries1 in this country4 ,one part of the remaining copies may be offered for sale2 and the other carefully preserved1 to form complete sets of the work1 to supply the demand from new institutions4 #f4 ,an abstract1 or popular account1 of the contents of these memoirs to be given to the public1 through the annual report of the ,regents to ,congress4 ,,ii4 ,,to ,,increase ,,knowledge4--,it is also proposed to appropriate a portion of the income1 annually1 to special objects of research1 under the direction of suitable persons4 #a4 ,the objects1 and the amount appropriated1 to be recommended by counsellors of the ,institution4 #b4 ,appropriations in different years to different objects2 so that1 in course of time1 each branch of knowledge may receive a share4 #c4 ,the results obtained from these appropriations to be published1 with the memoirs before mentioned1 in the volumes of the ,smithsonian ,contributions to ,knowledge4 #d4 ,examples of objects for which appropriations may be made3-- 7#a47 ,system of extended meteorological observations for solving the problem of ,american storms4 7#b47 ,explorations in descriptive natural history1 and geological1 mathematical1 and topographical surveys1 to collect material for the formation of a ,physical ,atlas of the ,united ,states4 7#c47 ,solution of experimental problems1 such as a new determination of the weight of the earth1 of the velocity of electricity1 and of light2 chemical analyses of soils and plants2 collection and publication of articles of science1 accumulated in the offices of ,government4 7#d47 ,institution of statistical inquiries with reference to physical1 moral1 and political subjects4 7#e47 ,historical researches1 and accurate surveys of places celebrated in ,american history4 7#f47 ,ethnological researches1 particularly with reference to the different races of men in ,north ,america2 also explorations1 and accurate surveys1 of the mounds and other remains of the ancient people of our country4 ,i4 ,,to ,,diffuse ,,knowledge4--,it is proposed to publish a series of reports1 giving an account of the new discoveries in science1 and of the changes made from year to year in all branches of knowledge not strictly professional4 #a4 ,some of these reports may be published annually1 others at longer intervals1 as the income of the ,institution or the changes in the branches of knowledge may indicate4 #b4 ,the reports are to be prepared by collaborators1 eminent in the different branches of knowledge4 #c4 ,each collaborator to be furnished with the journals and publications1 domestic and foreign1 necessary to the compilation of his report2 to be paid a certain sum for his labors1 and to be named on the title-page of the report4 #d4 ,the reports to be published in separate parts1 so that persons interested in a particular branch1 can procure the parts relating to it1 without purchasing the whole4 #e4 ,these reports may be presented to ,congress1 for partial distribution1 the remaining copies to be given to literary and scientific institutions1 and sold to individuals for a moderate price4 ,the following are some of the subjects which may be embraced in the reports3-- ,i4 ,,physical ,,class4 #a4 ,physics1 including astronomy1 natural philosophy1 chemistry1 and meteorology4 #b4 ,natural history1 including botany1 zoology1 geology1 c4 #c4 ,agriculture4 #d4 ,application of science to arts4 ,,ii4 ,,moral ,,and ,,political ,,class4 #e4 ,ethnology1 including particular history1 comparative philology1 antiquities1 c4 #f4 ,statistics and political economy4 #g4 ,mental and moral philosophy4 #h4 ,a survey of the political events of the world2 penal reform1 c4 ,,iii4 ,,literature ,,and ,,the ,,fine ,,arts4 #i4 ,modern literature4 #aj4 ,the fine arts1 and their application to the useful arts4 #aa4 ,bibliography4 #ab4 ,obituary notices of distinguished individuals4 ,,ii4 ,,to ,,diffuse ,,knowledge4--,it is proposed to publish occasionally separate treatises on subjects of general interest4 #a4 ,these treatises may occasionally consist of valuable memoirs translated from foreign languages1 or of articles prepared under the direction of the ,institution1 or procured by offering premiums for the best exposition of a given subject4 #b4 ,the treatises to be submitted to a commission of competent judges1 previous to their publication4 ,,details ,,of ,,the ,,second ,,part ,,of ,,the ,,plan ,,of ,,organization4 ,this part contemplates the formation of a ,library1 a ,museum1 and a ,gallery of ,art4 #a4 ,to carry out the plan before described1 a library will be required1 consisting1 #ast1 of a complete collection of the transactions and proceedings of all the learned societies of the world2 #b;d1 of the more important current periodical publications1 and other works necessary in preparing the periodical reports4 #b4 ,the ,institution should make special collections1 particularly of objects to verify its own publications4 ,also a collection of instruments of research in all branches of experimental science4 #c4 ,with reference to the collection of books1 other than those mentioned above1 catalogues of all the different libraries in the ,united ,states should be procured1 in order that the valuable books first purchased may be such as are not to be found elsewhere in the ,united ,states4 #d4 ,also catalogues of memoirs1 and of books in foreign libraries1 and other materials1 should be collected1 for rendering the ,institution a centre of bibliographical knowledge1 whence the student may be directed to any work which he may require4 #e4 ,it is believed that the collections in natural history will increase by donation1 as rapidly as the income of the ,institution can make provision for their reception2 and1 therefore1 it will seldom be necessary to purchase any article of this kind4 #f4 ,attempts should be made to procure for the gallery of art1 casts of the most celebrated articles of ancient and modern sculpture4 #g4 ,the arts may be encouraged by providing a room1 free of expense1 for the exhibition of the objects of the ,art-,union1 and other similar societies4 #h4 ,a small appropriation should annually be made for models of antiquity1 such as those of the remains of ancient temples1 c4 #i4 ,the ,secretary and his assistants1 during the session of ,congress1 will be required to illustrate new discoveries in science1 and to exhibit new objects of art2 distinguished individuals should also be invited to give lectures on subjects of general interest4 99 99 99 99 99 ,in accordance with the rules adopted in the programme of organization1 each memoir in this volume has been favorably reported on by a ,commission appointed for its examination4 ,it is however impossible1 in most cases1 to verify the statements of an author2 and1 therefore1 neither the ,commission nor the ,institution can be responsible for more than the general character of a memoir4 99 99 99 99 99 ,the following rules have been adopted for the distribution of the quarto volumes of the ,smithsonian ,contributions3-- #a4 ,they are to be presented to all learned societies which publish ,transactions1 and give copies of these1 in exchange1 to the ,institution4 #b4 ,also1 to all foreign libraries of the first class1 provided they give in exchange their catalogues or other publications1 or an equivalent from their duplicate volumes4 #c4 ,to all the colleges in actual operation in this country1 provided they furnish1 in return1 meteorological observations1 catalogues of their libraries and of their students1 and all other publications issued by them relative to their organization and history4 #d4 ,to all ,states and ,territories1 provided there be given1 in return1 copies of all documents published under their authority4 #e4 ,to all incorporated public libraries in this county1 not included in any of the foregoing classes1 now containing more than #aj1jjj volumes2 and to smaller libraries1 where a whole ,state or large district would be otherwise unsupplied4 ,,officers ,,of ,,the ,,smithsonian ,,institution4 ,,the ,,president ,,of ,,the ,,united ,,states1 ,ex-officio ,,presiding ,,officer ,,of ,,the ,,institution4 ,,the ,,vice-,,president ,,of ,,the ,,united ,,states1 ,ex officio ,,second ,,presiding ,,officer4 ,,salmon ,p4 ,,chase1 ,,chancellor ,,of ,,the ,,institution4 ,,joseph ,,henry1 ,,secretary ,,of ,,the ,,institution4 ,,spencer ,f4 ,,baird1 ,,assistant ,,secretary4 ,w4 ,w4 ,,seaton1 ,,treasurer4 ,,alexander ,d4 ,,bache1 ,,richard ,,wallach1 ,,executive ,,committee4 ,,richard ,,delafield1 ,,regents4 ---- ---- ,vice-,president of the ,united ,states4 ,,salmon ,p4 ,,chase1 ,chief ,justice of the ,united ,states4 ,,richard ,,wallach1 ,mayor of the ,city of ,washington4 ,,lyman ,,trumbull1 ,member of the ,senate of the ,united ,states4 ,,william ,p4 ,,fessenden1 ,,garrett ,,davis1 ,,samuel ,s4 ,,cox1 ,member of the ,house of ,representatives ,u4 ,s4 ,,james ,w4 ,,patterson1 ,,henry ,w4 ,,davis1 ,,william ,b4 ,,astor1 ,citizen of ,new ,york4 ,,theodore ,d4 ,,woolsey1 of ,connecticut4 ,,louis ,,agassiz1 of ,massachusetts4 7,vacancy47 ---- ---- ,,alexander ,d4 ,,bache1 of ,washington4 ,,richard ,,delafield1 of ,washington4 ,,members ,,ex-,,officio ,,of ,,the ,,institution4 ,,andrew ,,johnson1 ,president of the ,united ,states4 ---- ---- ,vice-,president of the ,united ,states4 ,,william ,h4 ,,seward1 ,secretary of ,state4 ,,hugh ,,mcculloch1 ,secretary of the ,treasury4 ,,edwin ,m4 ,,stanton1 ,secretary of ,war4 ,,gideon ,,welles1 ,secretary of the ,navy4 ,,william ,,dennison1 ,postmaster-,general4 ,,james ,,speed1 ,attorney-,general4 ,,salmon ,p4 ,,chase1 ,chief ,justice of the ,united ,states4 ,,david ,p4 ,,holloway1 ,commissioner of ,patents4 ,,richard ,,wallach1 ,mayor of the ,city of ,washington4 ,,honorary ,,member4 ,,james ,,harlan4 ,the ,secretary of the ,interior4 ,,table ,,of ,,contents4#a #a ,each memoir is separately paged and indexed4 ,,page ,,article ,i4 ,,introduction4 ,pp4 #af4 ,advertisement iii ,list of ,officers of the ,smithsonian ,institution ix ,,article ,,ii4 ,,discussion ,,of ,,the ,,magnetic ,,and ,,meteorological ,,observations ,,made ,,at ,,the ,,girard ,,college ,,observatory1 ,,philadelphia1 ,,in #ahdj1 #ahda1 #ahdb1 #ahdc1 #ahdd1 ,,and #ahde4 ,third ,section1 comprising ,parts ,,vii1 ,,viii1 ,,and ,,ix4 ,,vertical ,,force4 ,,investigation ,,of ,,the ,,eleven 7,,or ,,ten7 ,,year ,,period ,,and ,,of ,,the ,,disturbances ,,of ,,the ,,vertical ,,component ,,of ,,the ,,magnetic ,,force1 ,,and ,,appendix ,,on ,,the ,,magnetic ,,effect ,,of ,,the ,,aurora ,,borealis2 ,,with ,,an ,,investigation ,,of ,,the ,,solar ,,diurnal ,,variation1 ,,and ,,of ,,the ,,annual ,,inequality ,,of ,,the ,,vertical ,,force2 ,,and ,,of ,,the ,,lunar ,,effect ,,or ,,the ,,vertical ,,force1 ,,the ,,inclination1 ,,and ,,total ,,force4 ,by ,a4 ,d4 ,,bache1 ,,ll4 ,d41 ,f4 ,r4 ,s41 ,mem4 ,corr4 ,acad4 ,sc4 ,paris2 ,prest4 ,nat4 ,acad4 ,sciences2 ,superintendent ,u4 ,s4 ,coast ,survey4 ,pp4 #gb4 7,published ,may1 #ahfd47 ,,article ,,iii4 ,,discussion ,,of ,,the ,,magnetic ,,and ,,meteorological ,,observations ,,made ,,at ,,the ,,girard ,,college ,,observatory1 ,,philadelphia1 ,,in #ahdj1 #ahda1 #ahdb1 #ahdc1 #ahdd1 ,,and #ahde4 ,fourth ,section1 comprising ,parts ,x1 ,,xi1 ,,and ,,xii4 ,,dip ,,and ,,total ,,force4 ,,analysis ,,of ,,the ,,disturbances ,,of ,,the ,,dip ,,and ,,total ,,force2 ,,discussion ,,of ,,the ,,solar ,,diurnal ,,variation ,,and ,,annual ,,inequality ,,of ,,the ,,dip ,,and ,,total ,,force2 ,,and ,,discussion ,,of ,,the ,,absolute ,,dip1 ,,with ,,the ,,final ,,values ,,for ,,declination1 ,,dip ,,and ,,force ,,between #ahda ,,and #ahde4 ,by ,a4 ,d4 ,,bache1 ,,ll4 ,d41 ,f4 ,r4 ,s41 ,mem4 ,corr4 ,acad4 ,sc4 ,paris2 ,prest4 ,nat4 ,acad4 ,sciences2 ,superintendent ,u4 ,s4 ,coast ,survey4 ,pp4 #dd4 7,published ,january1 #ahfe47 ,,article ,,iv4 ,,on ,,the ,,construction ,,of ,a ,,silvered ,,glass ,,telescope1 ,,fifteen ,,and ,a ,,half ,,inches ,,in ,,aperture1 ,,and ,,its ,,use ,,in ,,celestial ,,photography4 ,by ,,henry ,,draper1 ,m4 ,d41 ,professor of ,natural ,science in the ,university of ,new ,york4 ,pp4 #fj4 7,published ,july1 #ahfd47 #a4 ,grinding and ,polishing the ,mirrors #b #b4 ,the ,telescope ,mounting #bg #c4 ,the ,clock ,movement #ch #d4 ,the ,observatory #da #e4 ,the ,photographic ,laboratory #df #f4 ,the ,photographic ,enlarger #ea ,,article ,v4 ,,pal,,ontology ,,of ,,the ,,upper ,,missouri3 ,a ,,report ,,upon ,,collections ,,made ,,principally ,,by ,,the ,,expeditions ,,under ,,command ,,of ,,lieut4 ,g4 ,k4 ,,warren1 ,u4 ,s4 ,top4 ,engrs41 ,,in #ahee ,,and #ahef4 ,,invertebrates4 ,by ,f4 ,b4 ,,meek ,,and ,f4 ,v4 ,,hayden1 ,m4 ,d4 ,part ,i4 ,pp4 #aeh1 and five ,plates4 7,published ,april1 #ahfe47 ,introductory ,remarks vii ,i4 ,silurian ,age4 ,potsdam ,period #a ,,ii4 ,carboniferous ,age4 ,carboniferous ,period #aa ,,iii4 ,carboniferous ,age4 ,permian ,period #dh ,,iv4 ,reptilian ,age4 ,jurassic ,period #ff ,index #aba ,explanations of ,plates4 ,,article ,,vi4 ,,cretaceous ,,reptiles ,,of ,,the ,,united ,,states4 ,by ,,joseph ,,leidy1 ,m4 ,d41 ,professor of ,anatomy in the ,university of ,pennsylvania1 ,curator of the ,academy of ,natural ,sciences of ,philadelphia4 ,pp4 #adj and twenty plates4 7,published ,may1 #ahfe47 ,introduction #a ,sauria #e ,chelonia #ajd ,a ,synopsis1 in which an attempt is made to define more closely the ,genera and ,species of ,reptiles whose remains are described in the preceding pages #aae ,index #aba ,references to the ,plates #abc ,,smithsonian ,,contributions ,,to ,,knowledge4 ----#ahj---- ,,on ,,the ,,construction ,,of ,a ,,silvered ,,glass ,,telescope1 ,,fifteen ,,and ,a ,,half ,,inches ,,in ,,aperture1 ,,and ,,its ,,use ,,in ,,celestial ,,photography4 ,,by ,,henry ,,draper1 ,m4 ,d41 ,,professor ,,of ,,natural ,,science ,,in ,,the ,,university ,,of ,,new ,,york4 ,,accepted ,,for ,,publication1 ,,january1 #ahfd4 ,,commission ,,to ,,which ,,this ,,paper ,,has ,,been ,,referred4 ,prof4 ,,wolcott ,,gibbs4 ,com4 ,j4 ,m4 ,,gilliss1 ,u4 ,s4 ,n4 ,,joseph ,,henry1 ,secretary ,s4 ,i4 ,,collins1 ,,printer1 ,,philadelphia4 ,,contents4 ,,historical ,,sketch ,,of ,,the ,,telescope4 ,,memoir ,,divided ,,into ,,six ,,sections3-- #a4 ,,grinding ,,and ,,polishing ,,the ,,mirrors #b #a4 ,experiments on a metal speculum4 ,corrosion by aqua regia2 voltaic grinding #b #b4 ,silvering glass4 ,foucault's and ,cimeg's processes2 details of silvering a mirror2 thickness and durability of silver films2 their use in daguerreotyping #b #c4 ,grinding and polishing glass4 ,division of subject #f a4 ,peculiarities of glass2 effects of pressure2 effects of heat2 oblique mirrors #f b4 ,emery and rouge2 elutriation of emery #aj c4 ,tools of iron1 lead1 pitch2 the gauges2 the leaden tool2 the iron tool2 the pitch polisher #aj d4 ,methods of examination2 two tests1 eyepiece and opaque screen2 appearance of spherical surface2 oblate spheroidal surface2 hyperbolic surface2 irregular surface2 details of tests2 atmospheric movements2 correction for parallel rays by measure2 appearances in relief on mirrors #ac e4 ,machines2 ,lord ,rosse2 ,mr4 ,lassell2 spiral stroke machine2 its construction and use2 the foot-power2 method of local corrections2 its advantages and disadvantages2 machine for local corrections2 description and use #ai #d4 ,eyepieces1 plane mirrors1 and test objects #bf #b4 ,,the ,,telescope ,,mounting #bg ,stationary eyepiece2 method of counterpoising #bg a4 ,the tube2 the mirror support2 air sac2 currents in the tube #bh b4 ,the supporting frame #ca #c4 ,,the ,,clock ,,movement #cc a4 ,the sliding plateholder2 the frictionless slide #cc b4 ,the clepsydra2 the sand-clock #cf c4 ,the sun camera #dj #d4 ,,the ,,observatory #da a4 ,the building #da b4 ,the dome2 its peculiarities #dd c4 ,the observer's chair #de #e4 ,,the ,,photographic ,,laboratory #df a4 ,description of the apartment #df b4 ,photographic processes2 washed plates2 difficulties of celestial photography #dg #f4 ,,the ,,photographic ,,enlarger #ea a4 ,low powers2 use of a concave mirror1 its novelty and advantages2 of the making of reverses #ea b4 ,high powers2 microscopic photography #ed ,,an ,,account ,,of ,,the ,,construction ,,and ,,use ,,of ,a ,,silvered ,,glass ,,telescope4 ,the construction of a reflecting telescope capable of showing every celestial object now known1 is not a very difficult task4 ,it demands principally perseverance and careful observation of minuti4 ,the cost of materials is but trifling compared with the result obtained1 and ,i can see no reason why silvered glass instruments should not come into general use among amateurs4 ,the future hopes of ,astronomy lie in the multitude of observers1 and in the concentration of the action of many minds4 ,if what is written here should aid in the advance of that noble study1 ,i shall feel amply repaid for my labor4 ,a short historical sketch of this telescope may not be uninteresting4 ,in the summer of #aheg1 ,i visited ,lord ,rosse's great reflector1 at ,parsonstown1 and1 in addition to an inspection of the machinery for grinding and polishing1 had an opportunity of seeing several celestial objects through it4 ,on returning home1 in #aheh1 ,i determined to construct a similar1 though smaller instrument2 which1 however1 should be larger than any in ,america1 and be especially adapted for photography4 ,accordingly1 in ,september of that year1 a #ae inch speculum was cast1 and a machine to work it made4 ,in #ahfj1 the observatory was built1 by the village carpenter1 from my own designs1 at my father's country seat1 and the telescope with its metal speculum mounted4 ,this latter was1 however1 soon after abandoned1 and silvered glass adopted4 ,during #ahfa1 the difficulties of grinding and polishing that are detailed in this account were met with1 and the remedies for many of them ascertained4 ,the experiments were conducted by the aid of three #ae-a_/b inch disks of glass1 together with a variety of smaller pieces4 ,three mirrors of the same focal length and aperture are almost essential1 for it not infrequently happens that two in succession will be so similar1 that a third is required for attempting an advance beyond them4 ,one of these was made to acquire a parabolic figure1 and bore a power of #a1jjj4 ,the winter was devoted to perfecting the art of silvering1 and to the study of special photographic processes4 ,a large portion of #ahfb was spent with a regiment in a campaign in ,virginia1 and but few photographs were produced till autumn1 when sand clocks and clepsydras of several kinds having been made1 the driving mechanism attained great excellence4 ,during the winter1 the art of local corrections was acquired1 and two #ae-a_/b inch mirrors1 as well as two of #i inches for the photographic enlarging apparatus1 were completed4 ,the greater part of #ahfc has been occupied by lunar and planetary photography1 and the enlargement of the small negatives obtained at the focus of the great reflector4 ,lunar negatives have been produced which have been magnified to #c feet in diameter4 ,i have also finished two mirrors #ae-a_/b inches in aperture1 suitable for a ,herschelian telescope1 that is1 which can only converge oblique pencils to a focus free from aberration4 ,this work has all been accomplished in the intervals of professional labor4 ,the details of the preceding operations are arranged as follows3 #a4 ,,grinding ,,and ,,polishing ,,the ,,mirrors2 #b4 ,,the ,,telescope ,,mounting2 #c4 ,,the ,,clock ,,movement2 #d4 ,,the ,,observatory2 #e4 ,,the ,,photographic ,,laboratory2 #f4 ,,the ,,photographic ,,enlarger4 #a4 ,,grinding ,,and ,,polishing ,,the ,,mirrors4 7#a47 ,,experiments ,,on ,a ,,metal ,,speculum4 ,my first #ae inch speculum was an alloy of copper and tin1 in the proportions given by ,lord ,rosse4 ,his general directions were closely followed1 and the casting was very fine1 free from pores1 and of silvery whiteness4 ,it was #b inches thick1 weighed #aaj pounds1 and was intended to be of #ab feet focal length4 ,the grinding and polishing were conducted with the ,rosse machine4 ,although a great amount of time was spent in various trials1 extending over more than a year1 a fine figure was never obtained--the principal obstacle to success being a tendency to polish in rings of different focal length4 ,it must1 however1 be borne in mind that ,lord ,rosse had so thoroughly mastered the peculiarities of his machine as to produce with it the largest specula ever made and of very fine figure4 ,during these experiments there was occasion to grind out some imperfections1 #h_/ajj of an inch deep1 from the face of the metal4 ,this operation was greatly assisted by stopping up the defects with a thick alcoholic solution of ,canada balsam1 and having made a rim of wax around the edge of the mirror1 pouring on nitro-hydrochloric acid1 which quickly corroded away the uncovered spaces4 ,subsequently an increase in focal length of #ae inches was accomplished1 by attacking the edge zones of the surface with the acid in graduated depths4 ,an attempt also was made to assist the tedious grinding operation by including the grinder and mirror in a ,voltaic circuit1 making the speculum the positive pole4 ,by decomposing acidulated water between it and the grinder1 and thereby oxidizing the tin and copper of the speculum1 the operation was much facilitated1 but the battery surface required was too great for common use4 ,if a sufficient intensity was given to the current1 speculum metal was transferred without oxidation to the grinder1 and deposited in thin layers upon it4 ,it was proposed at one time to make use of this fact1 and coat a mirror of brass with a layer of speculum metal by electrotyping4 ,the gain in lightness would be considerable4 ,during the winter of #ahfj the speculum was split into two pieces1 by the expansion in freezing of a few drops of water that had found their way into the supporting case4 7#b47 ,silvering ,glass4 ,at ,sir ,john ,herschel's suggestion 7given on the occasion of a visit that my father paid him in #ahfj71 experiments were next commenced with silvered glass specula4 ,these were described as possessing great capabilities for astronomical purposes4 ,they reflect more than #ij per cent4 of the light that fulls upon them1 and only weigh one-eighth as much as specula of metal of equal aperture4 ,as no details of ,steinheil's or ,foucault's processes for silvering in the cold way were accessible at the time1 trials extending at intervals over four months were made4 ,a variety of reducing agents were used1 and eventually good results obtained with milk sugar4 ,soon after a description of the process resorted to by ,m4 ,foucault in his excellent experiments was procured4 ,it consists in decomposing an alcoholic solution of ammonia and nitrate of silver by oil of cloves4 ,the preparation of the solutions and putting them in a proper state of instability are very difficult1 and the results by no means certain4 ,the silver is apt to be soft and easily rubbed off1 or of a leaden appearance4 ,it is liable to become spotted from adherent particles of the solutions used in its preparation1 and often when dissolved off a piece of glass with nitric acid leaves a reddish powder4 ,occasionally1 however1 the process gives excellent results4 ,in the winter of #ahfa1 ,m4 ,cimeg published his method of silvering looking-glasses by tartrate of potash and soda 7,rochelle salt74 ,since ,i have made modifications in it fitting the silver for being polished on the reverse side1 ,i have never on any occasion failed to secure bright1 hard1 and in every respect1 perfect films4 ,the operation1 which in many details resembles that of ,m4 ,foucault1 is divided into3 #ast1 cleaning the glass2 #b;d1 preparing the solutions2 #c;d1 warming the glass2 #dth1 immersion in the silver solution and stay there2 #eth1 polishing4 ,it should be carried on in a room warmed to #gj ,f4 at least4 ,the description is for a #ae-a_/b inch mirror4 #ast4 ,clean the glass like a plate for collodion photography4 ,rub it thoroughly with nitric acid1 and then wash it well in plenty of water1 and set it on edge on filtering paper to dry4 ,then cover it with a mixture of alcohol and prepared chalk1 and allow evaporation to take place4 ,rub it in succession with many pieces of cotton flannel4 ,this leaves the surface almost chemically clean4 ,lately1 instead of chalk ,i have used plain uniodized collodion1 and polished with a freshly-washed piece of cotton flannel1 as soon as the film had become semi-solid4 #b;d4 ,dissolve #efj grains of ,rochelle salt in two or three ounces of water and filter4 ,dissolve #hjj grains of nitrate of silver in four ounces of water4 ,take an ounce of strong ammonia of commerce1 and add nitrate solution to it until a brown precipitate remains undissolved4 ,then add more ammonia and again nitrate of silver solution4 ,this alternate addition is to be carefully continued until the silver solution is exhausted1 when some of the brown precipitate should remain in suspension4 ,the mixture then contains an undissolved excess of oxide of silver4 ,filter4 ,just before using1 mix with the ,rochelle salt solution1 and add water enough to make #bb ounces4 ,illustration3 ,fig4 #a4 ,the ,silvering ,vessel4 ,the vessel in which the silvering is to be performed may be a circular dish 7,fig4 #a7 of ordinary tinplate1 #af-a_/b inches in diameter1 with a flat bottom and perpendicular sides one inch high1 and coated inside with a mixture of beeswax and rosin 7equal parts71 ,at opposite ends of one diameter two narrow pieces of wood1 a a1 #a_/h of an inch thick1 are cemented4 ,they are to keep the face of the mirror from the bottom of the vessel1 and permit of a rocking motion being given to the glass4 ,before using such a vessel1 it is necessary to touch any cracks that may have formed in the wax with a hot poker4 ,a spirit lamp causes bubbles and holes through to the tin4 ,the vessel too must always1 especially if partly silvered1 be cleaned with nitric acid and water1 and left filled with cold water till needed4 ,instead of the above1 ,india-rubber baths have been occasionally used4 #c;d4 ,in order to secure fine and hard deposits in the shortest time and with weak solutions1 it is desirable1 though not necessary1 to warm the glass slightly4 ,this is best done by putting it in a tub or other suitably sized vessel1 and pouring in water enough to cover the glass4 ,then hot water is gradually stirred in1 till the mixture reaches #ajj ,f4 ,it is also advantageous to place the vessels containing the ingredients for the silvering solution in the same bath for a short time4 #dth4 ,on taking the glass out of the warm water1 carry it to the silvering vessel--into which an assistant has just previously poured the mixed silvering solution--and immediately immerse it face downwards1 dipping in first one edge and then quickly letting down the other till the face is horizontal4 ,the back of course is not covered with the fluid4 ,the same precautions are necessary to avoid streaks in silvering as in the case of putting a collodion plate in the bath4 ,place the whole apparatus before a window4 ,keep up a slow rocking motion of the glass1 and watch for the appearance of the bright silver film4 ,the solution quickly turns brown1 and the silver soon after appears1 usually in from three to five minutes4 ,leave the mirror in the liquid about six times as long4 ,at the expiration of the twenty minutes or half hour lift it out1 and look through it at some very bright object4 ,if the object is scarcely visible1 the silver surface must then be washed with plenty of water1 and set on edge on bibulous paper to dry4 ,if1 on the contrary1 it is too thin1 put it quickly back1 and leave it until thick enough4 ,when polished the silver ought1 if held between the eye and the sun1 to show his disk of a light blue tint4 ,on coming out of the bath the metallic surface should have a rosy golden color by reflected light4 #eth4 ,when the mirror is thoroughly dry1 and no drops of water remain about the edges1 lay it upon its back on a thoroughly dusted table4 ,take a piece of the softest thin buckskin1 and stuff it loosely with cotton to make a rubber4 ,avoid using the edge pieces of a skin1 as they are always hard and contain nodules of lime4 ,go gently over the whole silver surface with this rubber in circular strokes1 in order to commence the removal of the rosy golden film1 and to condense the silver4 ,then having put some very fine rouge on a piece of buckskin laid flat on the table1 impregnate the rubber with it4 ,the best stroke for polishing is a motion in small circles1 at times going gradually round on the mirror1 at times across on the various chords 7,fig4 #b74 ,at the end of an hour of continuous gentle rubbing1 with occasional touches on the flat rouged skin1 the surface will be polished so as to be perfectly black in oblique positions1 and1 with even moderate care1 scratchless4 ,the process is like a burnishing4 ,put the rubber carefully away for another occasion4 ,illustration3 ,fig4 #b4 ,polishing ,strokes4 ,the thickness of the silver thus deposited is about #a_/bjj1jjj of an inch4 ,gold leaf1 when equally transparent1 is estimated at the same fraction4 ,the actual value of the amount on a #ae-a_/b inch mirror is not quite a cent--the weight being less than #d grains 7#bci milligrammes on one occasion when the silver was unusually thick71 if the directions above given are followed4 ,variations in thickness of this film of silver on various parts of the face of the mirror are consequently only small fractions of #a_/bjj1jjj of an inch1 and are therefore of no optical moment whatever4 ,if a glass has been properly silvered1 and shows the sun of the same color and intensity through all parts of its surface1 the most delicate optical tests will certainly fail to indicate any difference in figure between the silver and the glass underneath4 ,the faintest peculiarities of local surface seen on the glass by the method of ,m4 ,foucault1 will be reproduced on the silver4 ,the durability of these silver films varies1 depending on the circumstances under which they are placed1 and the method of preparation4 ,sulphuretted hydrogen tarnishes them quickly4 ,drops of water may split the silver off4 ,under certain circumstances1 too1 minute fissures will spread all over the surface of the silver1 and it will apparently lose its adhesion to the glass4 ,this phenomenon seems to be connected with a continued exposure to dampness1 and is avoided by grinding the edge of the concave mirror flat1 and keeping it covered when not in use with a sheet of flat plate glass4 ,heat seems to have no prejudicial effect1 though it might have been supposed that the difference in expansibility would have overcome the mutual adhesion4 ,generally silvered mirrors are very enduring1 and will bear polishing repeatedly1 if previously dried by heat4 ,i have some which have been used as diagonal reflectors in the ,newtonian1 and have been exposed during a large part of the day to the heat of the sun concentrated by the #ae-a_/b inch mirror4 ,these small mirrors are never covered1 and yet the one now in the telescope has been there a year1 and has had the dusty film--like that which accumulates on glass--polished off it a dozen times4 ,in order to guard against tarnishing1 experiments were at first made in gilding silver films1 but were abandoned when found to be unnecessary4 ,a partial conversion of the silver film into a golden one1 when it will resist sulphuretted hydrogen1 can be accomplished as follows3 ,take three grains of hyposulphite of soda1 and dissolve it in an ounce of water4 ,add to it slowly a solution in water of one grain of chloride of gold4 ,a lemon yellow liquid results1 which eventually becomes clear4 ,immerse the silvered glass in it for twenty-four hours4 ,an exchange will take place1 and the film become yellowish4 ,i have a piece of glass prepared in this way which remains unhurt in a box1 where other pieces of plain silvered glass have changed some to yellow1 some to blue1 from exposure to coal gas4 ,i have also used silvered glass plates for daguerreotyping4 ,they iodize beautifully if freshly polished1 and owing probably to the absence of the usual copper alloy of silver plating1 take impressions with very short exposures4 ,the resulting picture has a rosy warmth1 rarely seen in ordinary daguerreotypes4 ,the only precaution necessary is in fixing to use an alcoholic solution of cyanide of potassium1 instead of hyposulphite of soda dissolved in water4 ,the latter has a tendency to split up the silver4 ,the subsequent washing must be with diluted common alcohol4 ,pictures obtained by this method will bear high magnifying powers without showing granulation4 ,unfortunately the exposure required for them in the telescope is six times as great as for a sensitive wet collodion1 though the iodizing be carried to a lemon yellow1 the bromizing to a rose red1 and the plate be returned to the iodine4 7#c47 ,,grinding ,,and ,,polishing ,,glass4 ,some of the facts stated in the following paragraphs1 the result of numerous experiments1 may not be new to practical opticians4 ,i have had1 however1 to polish with my own hands more than a hundred mirrors of various sizes1 from #ai inches to #a_/d of an inch in diameter1 and to experience very frequent failures for three years1 before succeeding in producing large surfaces with certainty and quickly4 ,it is well nigh impossible to obtain from opticians the practical minuti which are essential1 and which they conceal even from each other4 ,the long continued researches of ,lord ,rosse1 ,mr4 ,lassell1 and ,m4 ,foucault are full of the most valuable facts1 and have been of continual use4 ,the subject is divided into3 a4 ,the ,peculiarities of ,glass2 b4 ,emery and ,rouge2 c4 ,tools of ,iron1 ,lead and ,pitch2 d4 ,methods of ,examining ,surfaces2 e4 ,machines4 a4 ,peculiarities of ,glass4 ,effects of ,pressure4--,it is generally supposed that glass is possessed of the power of resistance to compression and rigidity in a very marked manner4 ,in the course of these experiments it has appeared that a sheet of it1 even when very thick1 can with difficulty be set on edge without bending so much as to be optically worthless4 ,fortunately in every disk of glass that ,i have tried1 there is one diameter on either end of which it may stand without harm4 ,in examining lately various works on astronomy and optics1 it appears that the same difficulty has been found not only in glass but also in speculum metal4 ,short used always to mark on the edge of the large mirrors of his ,gregorian telescopes the point which should be placed uppermost1 in case they were removed from their cells4 ,in achromatics the image is very sensibly changed in sharpness if the flint and crown are not in the best positions2 and ,mr4 ,airy1 in mounting the ,northumberland telescope1 had to arrange the means for turning the lenses on their common axis1 until the finest image was attained4 ,in no account1 however1 have ,i found a critical statement of the exact nature of the deformation1 the observers merely remarking that in some positions of the object glass there was a sharper image than in others4 ,before ,i appreciated the facts now to be mentioned1 many fine mirrors were condemned to be re-polished1 which1 had they been properly set in their mountings1 would have operated excellently4 ,in attempting to ascertain the nature of deformations by pressure1 many changes were made in the position of the disk of glass1 and in the kind of support4 ,some square mirrors1 too1 were ground and polished4 ,as an example of the final results1 the following case is presented3 ,a #ae-a_/b inch unsilvered mirror #a-a_/d inch thick was set with its best diameter perpendicular1 the axis of the mirror being horizontal 7,fig4 #h74 ,the image of a pin-hole illuminated by a lamp was then observed to be single1 sharply defined1 and with interference rings surrounding it as at a1 ,fig4 #c4 ,on turning the glass #ij degrees1 that is one quarter way round1 its axis still pointing in the same direction1 it could hardly be realized that the same concave surface was converging the rays4 ,the image was separated into two of about equal intensity1 as at b1 with a wing of light going out above and below from the junction4 ,inside and outside of the focal plane the cone of rays had an elliptical section1 the major axis being horizontal inside1 and perpendicular outside4 ,turning the mirror still more round the image gradually improved1 until the original diameter was perpendicular again--the end that had been the uppermost now being the lowest4 ,a similar series of changes occurred in supporting the glass on various parts of the other semicircle4 ,it might be supposed that irregularities on the edge of the glass disk1 or in the supporting arc would account for the phenomena4 ,but two facts dispose of the former of these hypotheses3 in the first place if the glass be turned exactly half way round1 the character of the image is unchanged1 and it is not to be believed that in many different mirrors this could occur by chance coincidence4 ,in the second place1 one of these mirrors has been carefully examined after being ground and polished three times in succession1 and on each occasion required the same diameter to be perpendicular4 ,as to the second hypothesis no material difference is observed whether the supporting arc below be large or small1 nor when it is replaced by a thin semicircle of tinplate lined with cotton wool4 ,illustration3 ,fig4 #c4 ,effect of ,pressure on a ,reflecting ,surface4 ,i am led to believe that this peculiarity results from the structural arrangement of the glass4 ,the specimens that have served for these experiments have probably been subjected to a rolling operation when in a plastic state1 in order to be reduced to a uniform thickness4 ,optical glass1 which may be made by softening down irregular fragments into moulds at a temperature below that of fusion1 may have the same difficulty1 but whether it has a diameter of minimum compression can only be determined by experiment4 ,why speculum metal should have the same property might be ascertained by a critical examination of the process of casting1 and the effect of the position of the openings in the mould for the entrance of the molten metal4 ,effects of ,heat4--,the preceding changes in glass when isolated appear very simple1 and their remedy1 to keep the proper diameter perpendicular1 is so obvious that it may seem surprising that they should have given origin to any embarrassment4 ,in fact it is now desirable to have a disk in which they are well marked4 ,but in practice they are complicated in the most trying manner with variations produced by heat pervading the various parts of the glass unequally4 ,the following case illustrates the effects of heat3-- ,illustration3 ,fig4 #d4 ,effects of ,heat on a ,reflecting ,surface4 ,a #ae-a_/b inch mirror1 which was giving at its centre of curvature a very fine image 7a1 ,fig4 #d7 of an illuminated pin-hole1 was heated at the edge by placing the right hand on the back of the mirror1 at one end of the horizontal diameter4 ,in a few seconds an arc of light came out from the image as at b1 and on putting the left hand on the other extremity of the same diameter the appearance c was that of two arcs of light crossing each other1 and having an image at each intersection4 ,the mirror did not recover its original condition in ten minutes4 ,another person on a subsequent occasion touching the ends of the perpendicular diameter at the same time that the horizontal were warmed1 caused the image d to become somewhat like two of c1 put at right angles to each other4 ,a little distance outside the focus the complementary appearances1 b1 c1 d1 were found4 ,by unsymmetrical warming still more remarkable forms emerged in succession1 some of which were more like certain nebul with their milky light1 than any regular geometrical figure4 ,if the glass had1 after one of these experiments1 been immediately put on the polishing machine and re-polished1 the changes in surface would to a certain extent have become permanent1 as in ,chinese specula1 and the mirror would have required either re-grinding or prolonged polishing to get rid of them4 ,this occurred unfortunately very frequently in the earlier stages of this series of experiments1 and gave origin on one occasion to a surface which could only show the image of a pin-hole as a lozenge 7b1 ,fig4 #e71 with an image at each angle inside the focus1 and as an image a with four wings outside4 ,illustration3 ,fig4 #e4 ,effects of ,heat rendered permanent4 ,but it must not be supposed that such apparent causes as these are required to disturb a surface injuriously4 ,frequently mirrors in the process for correction of spherical aberration will change the quality of their images without any perceptible reason for the alteration4 ,a current of cold or warm air1 a gleam of sunlight1 the close approach of some person1 an unguarded touch1 the application of cold water injudiciously will ruin the labor of days4 ,the avoidance of these and similar causes requires personal experience1 and the amateur can only be advised to use too much caution rather than too little4 ,such accidents1 too1 teach a useful lesson in the management of a large telescope1 never1 for instance1 to leave one-half the mirror or lens exposed to radiate into cold space1 while the other half is covered by a comparatively warm dome4 ,under the head of the ,sun-,camera1 some further facts of this kind may be found4 ,oblique ,mirrors4--,still another propensity of glass and speculum metal must be noted4 ,a truly spherical concave can only give an image free from distortion when it is so set that its optical axis points to the object and returns the image directly back towards it4 ,but ,i have polished a large number of mirrors in which an image free from distortion was produced only when oblique pencils fell on the mirror1 and the image was returned along a line forming an angle of from #b to #c degrees with the direction of the object4 ,such mirrors1 though exactly suited for the ,herschelian construction1 will not officiate in a ,newtonian unless the diagonal mirror be put enough out of centre in the tube1 to compensate for the figure of the mirror4 ,some of the best photographs of the moon that have been produced in the observatory1 were made when the diagonal mirror was #f inches out of centre in the #af inch tube4 ,of course the large mirror below was not perpendicular to the axis of the tube1 but was inclined #b #cb4 ,the figure of such a concave might be explained by the supposition that it was as if cut out of a parabolic surface of twice the diameter1 so that the vertex should be on the edge4 ,but if the mirror was turned #ahj it apparently did just as well as in the first position1 the image of a round object being neither oval nor elliptical1 and without wings4 ,the image1 however1 is never quite as fine as in the usual kind of mirrors4 ,the true explanation seems rather to be that the radius of curvature is greater along one of the diameters than along that at right angles4 ,how it is possible for such a figure to arise during grinding and polishing is not easy to understand1 unless it be granted that glass yields more to heat and compression in one direction than another4 ,after these facts had been laboriously ascertained1 and the method of using such otherwise valueless mirrors put in practice as above stated1 chance brought a letter of ,maskelyne to my notice4 ,he says1 8,i hit upon an extraordinary experiment which greatly improved the performance of the six-feet reflector04444 ,it was one made by ,short4 8,as a like management may improve many other telescopes1 ,i shall here relate it3 ,i removed the great speculum from the position it ought to hold perpendicular to the axis of the tube when the telescope is said to be rightly adjusted1 to one a little inclined to the same and found a certain inclination of about #b-a_/b 7as ,i found by the alteration of objects in the finer one of ,dollond's best night glasses with a field of #f71 which caused the telescope to show the object 7a printed paper7 incomparably better than before2 insomuch that ,i could read many of the words which before ,i could make nothing at all of4 ,it is plain1 therefore1 that this telescope shows best with a certain oblique pencil of rays4 ,probably it will be found that this circumstance is by no means peculiar to this telescope40 ,this very valuable observation has lain buried for eighty-two years1 and ignorance of it has led to the destruction of many a valuable surface4 ,as regards the method of combating this tendency1 it is as a general rule best to re-grind or rather re-fine the surface1 for though pitch polishing has occasionally corrected it in a few minutes1 it will not always do so4 ,i have polished a surface for thirteen and a half hours1 examining it frequently1 without changing the obliquity in the slightest degree4 ,glass1 then1 is a substance prone to change by heat and compression1 and requiring to be handled with the utmost caution4 b4 ,emery and ,rouge4 ,in order to excavate the concave depression in a piece of glass1 emery as coarse as the head of a pin has been commonly used4 ,this cuts rapidly1 and is succeeded by finer grained varieties1 till flour emery is reached4 ,after that only washed emeries should be permitted4 ,they are made by an elutriating process invented by ,dr4 ,green4 ,five pounds of the finest sifted flour emery are mixed with an ounce of pulverized gum arabic4 ,enough water to make the mass like treacle is then added1 and the ingredients are thoroughly incorporated by the hand4 ,they are put into a deep jar containing a gallon of water4 ,after being stirred the fluid is allowed to come to rest1 and the surface be skimmed4 ,at the end of an hour the liquid containing extremely fine emery in suspension is decanted or drawn off with a siphon1 nearly down to the level of the precipitated emery at the bottom1 and set aside to subside in a tall vessel4 ,when this has occurred1 which will be in the lapse of a few hours1 the fluid is to be carefully poured back into the first vessel1 and the fine deposit in the second put into a stoppered bottle4 ,in the same way by stirring up the precipitate again1 emery that has been suspended #cj1 #aj1 #c1 #a minutes1 and #bj1 #c1 seconds is to be secured and preserved in wide-mouthed vessels4 ,the quantity of the finer emeries consumed in smoothing a #ae-a_/b inch surface is very trifling--a mass of each as large as two peas sufficing4 ,rouge1 or peroxide of iron1 is better bought than prepared by the amateur4 ,it is made by calcining sulphate of iron and washing the product in water4 ,three kinds are usually found in commerce3 a very coarse variety containing the largest percentage of the cutting black oxide of iron1 which will scratch glass like quartz2 a very fine variety which can hardly polish glass1 but is suitable for silver films2 and one intermediate4 ,trial of several boxes is the best method of procuring that which is desired4 c4 ,tools of ,iron1 ,lead1 and ,pitch4 ,in making a mirror1 one of the first steps is to describe upon two stout sheets of brass or iron1 arcs of a circle with a radius equal to twice the desired focal length1 and to secure1 by filing and grinding them together1 a concave and convex gauge4 ,when the radius bar is very long1 it may be hung against the side of a house4 ,by the assistance of these templets1 the convex tools of lead and iron and the concave surface of the mirror are made parts of a sphere of proper diameter4 ,the excavation of a large flat disc of glass to a concave is best accomplished by means of a thick plate of lead1 cast considerably more convex than the gauge4 ,the central parts wear away very quickly1 and when they become too flat must be made convex again by striking the lead on the back with a hammer4 ,the glass is thus caused gradually to approach the right concavity4 ,ten or twelve hours usually suffice to complete this stage4 ,the progress of the excavating is tested sufficiently well by setting the convex gauge on a diameter of the mirror1 and observing how many slips of paper of a definite thickness will pass under the centre or edge1 as the case may be4 ,this avoids the necessity of a spherometer4 ,the thickness of paper is found correctly enough by measuring a half ream1 and dividing by the number of sheets4 ,in this manner differences in the versed sine of a thousandth of an inch may be appreciated1 and a close enough approximation to the desired focal length reached--the precision required in achromatics not being needed4 ,the preparation of the iron tools on which the grinding is to be finished is very laborious where personal exertion is used4 ,they require to be cast thin in order that they may be easily handled1 and hence cannot be turned with very great exactness4 ,the pair for my large mirrors are #ae-a_/b inches in diameter1 and were cast #c_/h of an inch thick1 being strengthened however on the back by eight ribs #c_/d of an inch high1 radiating from a solid centre two inches in diameter 7a1 ,fig4 #f74 ,they weighed #be pounds apiece4 ,four ears1 with a tapped hole in each1 project at equal distances round the edge1 and serve either as a means of attachment for a counterpoise lever1 or as handles4 ,illustration3 ,fig4 #f4 ,the ,iron ,grinder4 ,after these were turned and taken off the lathe chuck1 they were found to be somewhat sprung1 and had to be scraped and ground in the machine for a week before fitting properly4 ,the slowness in grinding results from the emery becoming imbedded in the iron1 and forming a surface as hard as adamant4 ,once acquired1 such grinders are very valuable1 as they keep their focal length and figure apparently without change if carefully used1 and only worked on glass of nearly similar curvature4 ,at first no grooves were cut upon the face1 for in the lead previously employed for fining they were found to be a fruitful source of scratches1 on account of grains of emery imbedding in them1 and gradually breaking loose as the lead wore away4 ,subsequently it appeared1 that unless there was some means of spreading water and the grinding powders evenly1 rings were likely to be produced on the mirror1 and the iron was consequently treated as follows3-- ,a number of pieces of wax1 such as is used in making artificial flowers1 were procured4 ,the convex iron was laid out in squares of #c_/d of an inch on the side1 and each alternate one being touched with a thick alcoholic solution of ,canada balsam1 a piece of wax of that size was put over it4 ,this was found after many trials to be the best method of protecting some squares1 and yet leaving others in the most suitable condition to be attacked4 ,a rim of wax1 melted with ,canada balsam1 was raised around the edge of the iron1 and a pint of aqua regia poured in4 ,in a short time this corroded out the uncovered parts to a sufficient depth1 leaving an appearance like a chess-board1 except that the projecting squares did not touch at the adjoining angles 7b1 ,fig4 #f74 ,i should have chipped the cavities out1 instead of dissolving them away1 but for fear of changing the radius of curvature and breaking the thin plate4 ,however as soon as the iron was cleaned1 it proved to have become flatter1 the radius of curvature having increased #g-c_/d inches4 ,this shows what a state of tension and compression there must be in such a mass1 when the removal of a film of metal #a_/ej of an inch thick1 here and there1 from one surface1 causes so great a change4 ,when the glass has been brought to the finest possible grain on such a grinder1 a polishing tool has to be prepared by covering the convex iron with either pitch or rosin4 ,these substances have very similar properties1 but the rosin by being clear affords an opportunity of seeing whether there are impurities1 and therefore has been frequently used1 straining being unnecessary4 ,it is1 however1 too hard as it occurs in commerce1 and requires to be softened with turpentine4 ,a mass sufficiently large to cover the iron #a_/h of an inch thick is melted in a porcelain or metal capsule by a spirit lamp4 ,when thoroughly liquid the lamp is blown out1 and spirits of turpentine added1 a drachm or two at a time4 ,after each addition a chisel or some similar piece of metal is dipped into the fluid rosin1 and then immersed in water at the temperature of the room4 ,after a minute or two it is taken out1 and tried with the thumb-nail4 ,when the proper degree of softness is obtained1 an indentation can be made by a moderate pressure4 ,the iron having been heated in hot water is then painted in stripes #a_/h of an inch deep with this resinous composition4 ,the glass concave to be polished being smeared with rouge1 is pressed upon it to secure a fit1 and the iron is then put in cold water4 ,with a narrow chisel straight grooves are made1 dividing the surface into squares of one inch1 separated by intervals of one-quarter of an inch 7,fig4 #g74 ,under certain circumstances it is also desirable to take off every other square1 or perhaps reduce the polishing surface irregularly here and there1 to get an excess of action on some particular portion of the mirror4 ,illustration3 ,fig4 #g4 ,the ,polishing ,tool4 ,it is well1 on commencing to polish with a tool made in this way1 to warm the glass as well as the tool in water 7page #d7 before bringing the two in contact4 ,if this is not done the polishing will not go on kindly1 a good adaptation not being secured for a length of time1 and the glass surface being injured at the outset4 ,the rosin on a polisher put away for a day or two suffers an internal change1 a species of irregular swelling1 and does not retain its original form4 ,heating1 too1 has a good effect in preventing disturbance by local variations of temperature in the glass4 ,the description of 8,local ,polishers0 will be given under ,machines4 d4 ,methods of ,examining ,surfaces4 ,i have been in the habit of testing mirrors exclusively at the centre of curvature1 not putting them in the telescope tube until nearly parabolic or finished4 ,the means of trial are so excellent1 the indications obtained so precise1 and the freedom from atmospheric disturbances so complete1 that the greatest facilities are offered for ascertaining the nature of a surface4 ,in addition the observer is entirely independent of day or night1 and of the weather4 ,i do not think that anything more is learned of the telescope1 even under favorable circumstances1 than in the workshop4 ,for the improvement of these methods of observation1 ,science is largely indebted to ,m4 ,foucault1 whose third test--the second in the next paragraph--is sufficient to afford by itself a large part of the information required in correcting a concave surface4 ,there are two distinct modes of examination3 #ast1 observing with an eye-piece the image of an illuminated pin-hole at the focus1 and the cone of rays inside and outside that plane2 #b;d1 receiving the entire pencil of light coming from the mirror through the pupil on the retina1 and noticing the distribution of light and shade1 and the appearances in relief on the face of the mirror4 ,illustration3 ,fig4 #h4 ,testing a ,concave at the ,centre of ,curvature4 ,the arrangements for these tests are as follows3 ,around the flame of a lamp 7a1 ,fig4 #h7 a sheet of tin is bent so as to form a cylindrical screen4 ,through it at the height of the brightest part of the flame1 as at b1 two holes are bored1 a quarter of an inch apart1 one #a_/cb of an inch in diameter1 the other as small as the point of the finest needle will make--perhaps #a_/bjj of an inch4 ,this apparatus is to be set at the centre of curvature of the mirror c--the optical axis of the latter being horizontal--and so adjusted that the light which diverges from the illuminated hole in use1 may1 after impinging on the concave surface of the glass1 return to form an image close by the side of the tin screen4 ,in the case of the first test1 the returning rays are received into an eye-piece or microscope1 d1 magnifying #bj times1 and moving upon a divided scale to and from the mirror4 ,in the second test the eye-piece is removed away from before the eye1 and a straight-edged opaque screen1 e1 is put in its place4 ,the mirror is supported in these trials by an arc of wood f1 lined with thick woollen stuff1 and above two wooden latches1 g1 g1 prevent it from falling forward1 but do not compress it4 ,it is1 of course1 unsilvered4 ,in the figure the table is represented very much closer to the mirror than it should be4 ,in trials on the #ae-a_/b inch it has to be #be feet distant4 ,the appearance that a truly spherical concave surface presents with the first test is3 the image of the hole is sharply defined without any areola of aberration around it1 and is surrounded by interference rings4 ,inside and outside the focus the cone of rays is exactly similar1 and circular in section4 ,it presents no trace of irregular illumination1 nor any bright or dark circles4 ,with the second test1 when the eye is brought into such a position that it receives the whole pencil of reflected rays1 and the opaque screen is gradually drawn across in front of the pupil1 the brightness of the surface slowly diminishes1 until just as the screen is cutting off the last relic of the cone of rays 7,fig4 #i71 the mirror presents an uniform grayish tint1 followed by total darkness1 and gives to the eye the sensation of a plane4 ,illustration3 ,fig4 #i4 ,action of the ,opaque ,screen4 ,illustration3 ,fig4 #aj4 ,caustic of ,oblate ,spheroidal ,mirror4 ,if1 however1 the mirror is not spherical1 but instead gradually decreases in focal length toward the edge1 the following changes result3 ,the image at the best focus is surrounded by a nebulosity1 stronger as the deviation from the sphere is greater1 and neither can a sharp focus be obtained nor interference fringes seen4 ,in order to include this nebulosity in the image1 it will be necessary to push the eye-piece toward the mirror4 ,before the cone of rays has completed its convergence1 the mass of light will be seen to have accumulated at the periphery1 and after the focus is past and divergence has commenced1 the accumulation will be around the axis4 ,that is1 a caustic 7,fig4 #aj7 is formed with its summit from the mirror4 ,by the second test1 in gradually eclipsing the light coming from the mirror1 just before all the rays are obstructed1 a part of those which have constituted the nebulosity will escape past the screen 7,fig4 #aa7 into the eye1 and cause there an extremely exaggerated appearance in relief of the solid superposed upon the true surface beneath4 ,the glass will no longer seem to be a plane1 but to have a section as in ,fig4 #ab4 ,let us examine by the aid of ,m4 ,foucault's diagrams why it is that the surface seems thus curved4 ,if the dotted line1 ,fig4 #ac1 represents the section of the mirror1 and the solid line a section of a spherical mirror of the same mean focal length1 it will be seen that the curves touch at two points1 but are separated by an interval elsewhere4 ,if this interval be projected by means of the differences of the ordinates1 the resulting curve will be found to be the same as that which the mirror apparently has4 ,illustration3 ,fig4 #ab4 ,apparent ,section of ,oblate ,spheroidal ,mirror4 ,if the opaque screen be drawn a short distance from the mirror1 the appearance of the section curve will seem to change1 the bottom of the groove 7,fig4 #ab7 between the centre and edge advancing inwards1 and the mound in the middle growing smaller4 ,if the screen be pushed toward the mirror the reverse takes place1 the central mound becoming larger1 but the edge decreasing4 ,the reason for these variations becomes apparent by considering the three diagrams1 ,fig4 #ad4 ,the dotted curve in each instance represents the real curve of the mirror described in the last paragraph1 while the solid lines are circles drawn with radii progressionally shorter in a1 b and c1 and represent sections of three spherical mirrors whose focal lengths also progressively shorten4 ,illustration3 ,fig4 #aa4 ,action of the ,opaque ,screen4 ,illustration3 ,fig4 #ac4 ,section of ,spherical and ,spheroidal ,mirrors4 ,illustration3 ,fig4 #ad4 ,relation of ,spheres to ,oblate ,spheroid4 ,when the opaque screen is at a given distance from the mirror under examination1 the only parts of the mirror which can officiate well are those which have a curvature corresponding to a radius equal to the same distance4 ,all the other parts seem as if they were covered by projecting circular masses4 ,in looking at ,fig4 #ad1 it is plain1 then1 if the opaque screen is at a maximum distance from the mirror1 that the central parts alone will seem to operate1 because the two curves 7a7 only touch there4 ,if the screen is moved toward the mirror the curves 7b7 will coincide at some point between the centre and edge1 while if carried still farther in only the edges touch and the appearance will be as if a large mound were fixed upon the centre4 ,i have been careful in explaining how a surface may thus seem to present entirely different characteristics if examined from points of view which vary slightly in distance1 because a knowledge of these facts is of the utmost importance in correcting such an erroneous figure4 ,it is now obvious that the correction will be equally effectual if the mirror be polished with a small rubber on the edge1 or on the centre1 or partly on each4 ,the only difference in the result will be1 that the mean focal length will be increased in the first instance1 and decreased in the second1 while it will remain unchanged in the third4 ,illustration3 ,fig4 #ae4 ,caustic of ,hyperbolic ,mirror4 ,illustration3 ,fig4 #af4 ,apparent ,section of ,hyperbolic ,mirror4 ,if the mirror1 instead of having a section like that of an oblate spheroid1 should have either an ellipse1 parabola1 or hyperbola1 as its section curve1 the appearances seen above are reversed4 ,whilst by the first test there is still an aberration round the image at the best focus1 the eye-piece must now be drawn from the mirror to include it4 ,the cone of rays is most dense round the axis inside1 and at the periphery outside the focus1 and the summit of the caustic 7,fig4 #ae7 is turned towards the mirror4 ,the second test shows a section as in ,fig4 #af1 a depression at the centre1 and the edges turned backwards4 ,the nature of the movement necessary to reduce the surface to a sphere is very plainly indicated1 action on a zone a between the centre and edge4 ,if1 however1 a parabolic section is required1 the zone a must not be entirely removed1 and the surface rendered apparently flat1 but as much of it must be left as experience shows to be desirable4 ,illustration3 ,fig4 #ag4 ,action of the ,opaque ,screen4 ,illustration3 ,fig4 #ah4 ,apparent ,section of ,mirror with ,rings4 ,if1 in still a fourth instance1 the mirror is not formed by the revolution of any regular curve upon its axis1 but has upon its surface zones of longer and shorter radius intermixed irregularly1 a very common case1 the two tests still indicate with precision the parts in fault1 and the correction demanded4 ,thus the mirror seen in section in ,fig4 #ag1 when the principal mass of light was obstructed by the opaque screen1 would still permit that coming from certain parts to find its way into the eye4 ,figure #ah represents an irregular mirror1 that was produced in the process of correction of a hyperbolic surface1 which had an apparent section like ,fig4 #af previously4 ,the zone a had been acted upon with a small local polisher1 and the mirror was finished by subsequently softening down b and c with a larger tool4 ,after having gained from the preceding paragraphs a general idea of the value and nature of these tests at the centre of curvature1 a more particular description of their use is desirable4 ,m4 ,foucault in his methods first brings the mirror to a spherical surface1 and then by moving the luminous pin-hole toward the mirror1 and correspondingly retracting the eye-piece or opaque screen1 carries it1 avoiding aberration continually by polishing1 through a series of ellipsoidal curvatures1 advancing step by step toward the paraboloid of revolution4 ,the length of the apartment1 however1 soon puts a termination to this gradual system of correction1 and he is forced to perform the last steps of the conversion by an empirical process1 and eventually to resort to trial in the telescope4 ,with my mirrors of #aej inches focal length1 demanding from the outset a room more than #be feet long1 this successive system had to be abandoned4 ,it was not found feasible to place the lamp in the distant focus of the ellipse--the workshop being less than #cj feet long--and putting the luminous source on stands outside1 introduced several injurious complications1 not the least of which was currents in the layers of variously refracting air in the apartment4 ,in a still room the density and hygrometric variations in its various parts only give rise to slight embarrassment4 ,the moment1 however1 that currents are produced1 satisfactory examination of a mirror becomes difficult4 ,the air is seen only too easily to move in great spiral convolutions between the mirror and the eye1 areol of aberration appear around a previously excellent image1 and were it not for the second test1 any determination of surface would be impossible4 ,by that test the real deviations from truth of figure can be distinguished from the atmospheric1 and to a practised eye sufficient indications of necessary changes given4 ,such a movement as that caused by placing the hand in or under the line of the converging rays1 will completely destroy the beauty of an image1 and by the second test give origin in the first case to the appearance ,fig4 #ai4 ,in order to be completely exempt at all times from arial difficulties1 it is desirable to have control of a long underground apartment1 the openings of which can be tightly closed4 ,as no artificial warmth is needed1 there is the minimum of movement in the inclosed air1 and conclusions respecting a surface may be arrived at in a very short time4 ,the mirror may also be supported from the ground1 so that tremulous vibrations which weary the eye1 and interfere with the accuracy of criticism1 may be avoided4 ,illustration3 ,fig4 #ai4 ,atmospheric ,motions4 ,driven then from observing an image kept continually free from aberration1 through advancing ellipsoidal changes1 it became necessary to study the gradual increase of deformation1 produced by the greater and greater departures from a spherical surface1 as the parabola was approached4 ,it was found that a sufficient guide is still provided in these tests1 by modifying them properly4 ,the longitudinal aberration of a mirror of small angular opening is easily calculated--being equal to the square of half the aperture1 divided by eight times the principal focal length4 ,that is1 if a #ae-a_/b inch mirror of #aej inches focal length were spherical1 and were used to converge parallel rays1 those from its edge would reach a focus #e_/ajj of an inch nearer the mirror than those from its central parts4 ,if now the converse experiment be tried1 and a mirror of the same size and focal length which can converge parallel rays1 falling on all its parts1 to one focus1 be examined at the centre of curvature1 it gives there an amount of longitudinal aberration #aj_/ajj of an inch1 equal to twice the preceding4 ,this latter1 then1 is the condition at the centre of curvature1 to which such mirror must be brought in order to converge parallel rays with exactness4 ,in addition1 strict watch must be kept upon the zones intermediate between the centre and edge1 both by measurement with diaphragms of their aberration1 and better yet1 by observation of the regularity of the curve of that apparent solid1 ,fig4 #af1 seen by the second test4 ,this modification of the first test is literally a method of parabolizing by measure1 and is capable of great precision when the eye learns to estimate where the exact focus of a zone is4 ,the little irregularities found round the edges of the holes through the tin screen1 ,fig4 #h1 are in this respect of material assistance4 ,they show1 too1 the increased optical or penetrating power that is gained by increase of aperture4 ,minute peculiarities1 not visible under very high powers with a #aj inch diaphragm1 become immediately perceptible even with less magnifying when the whole aperture is used1 provided the mirror is spherical4 ,illustration3 ,fig4 #bj4 ,adjusting the ,opaque ,screen4 ,in the use of the second test precautions have to be taken1 as may be inferred from page #ae1 to set the opaque screen exactly in the proper position4 ,the best method for ascertaining its location is1 having received the image into the eye1 placed purposely too near the mirror1 to cause the screen to move across the cone of rays from the right towards the left side4 ,a jet black shadow begins to advance at the same time1 and in the same direction across the mirror4 ,if the eye is then moved from the mirror sufficiently1 this black shadow can be made to originate by the same motion of the screen as before1 from the left or opposite side of the mirror4 ,midway between these extremes there is a point where the advance is from neither side4 ,this is the true position for the screen when it is desired to see the imperfections of the surface in highly exaggerated relief1 as in ,fig4 #bj1 which represents the appearance of ,fig4 #ab4b #b ,in order to examine ,fig4 #bj1 the book should be held with the left side of the page toward a window or lamp4 ,the eye should also be at least two feet distant4 ,the centre will then be seen to protrude1 and the surface present the apparent section engraved below it4 ,the interpretation of the lights and shadows upon the face of a mirror in this test is always easy1 and the observer is not likely to mistake an elevation for a depression1 if he bears in mind the fact that the surface under examination must always be regarded as illuminated by an oblique light coming from a source on the side opposite to that from which the screen advances1 coming for instance from the left hand side1 in the above description4 ,in practice1 the diaphragms commonly used for a #ae-a_/b inch mirror have been as small as the light from the unsilvered surface would allow4 ,a six inch aperture at the centre1 a ring an inch wide round the edge1 and a two inch zone midway between the two4 e4 ,machines4 ,in the beginning of this section the difficulties into which ,i fell with ,lord ,rosse's machine were stated4 ,these caused it at the time to be abandoned4 ,a machine based on the same idea as ,mr4 ,lassell's beautiful apparatus was next constructed4 ,it varied1 however1 in this1 that the hypocycloidal curve was described partly by the rotation of the mirror1 and partly by the motions of the polisher--the axes of the spindles carrying the two being capable either of coincidence or lateral separation to a moderate extent4 ,a great deal of time and labor was expended in grinding and polishing numerous mirrors with it1 but still the difficulty that had been so annoying in the former machine persisted4 ,frequently1 in fact generally1 from six to eight zones of unequal focal length were visible1 although on some occasions when the mirror was hyperbolic1 the number was reduced to two4 ,at first it was supposed that the fault lay with the polishing1 the pitch accumulating irregularly from being of improper softness1 for it was found to be particularly prone to heap up at the centre4 ,but after ,i had introduced a method of fine grinding with elutriated hone powder1 which enabled the glass to reject light before the pitch polishing1 it became evident that the zones were connected with the mode of motion of the mechanism4 ,many changes were made in the speed of its various elements1 and a contrivance to control the irregular motion of the polisher introduced1 but a really fine and uniform parabolic surface was never obtained1 the very best showing when finished zones of different focal lengths4 ,although it cannot be said that ,i have tried this machine thoroughly1 for ,mr4 ,lassell has produced specula of exquisite defining power with it1 and must have avoided these imperfections to a great extent1 yet the evident necessity of complicating the movement#c considerably1 to avoid the polishing in rings1 led me to adopt an entirely different construction1 which was used until quite recently4 ,although it has now been replaced by another machine1 which is still better in principle1 and gives fine results much more quickly1 yet as it produced one parabolic surface that bore a power of more than #ajjj1 and as it serves to introduce the process of grinding1 it is worthy of description4 ,the action of machines for grinding and polishing has been thoroughly examined in my workshop1 no less than seven different ones having been made at various times4 #c ,messrs4 ,de ,la ,rue and ,nasmyth1 who used one of ,mr4 ,lassell's machines1 as ,i have since learned1 met with the same trouble1 and were led to make two additions to the mechanism3 #a1 to control the rotation of the polisher rigorously2 and #b1 to give the whole speculum a lateral motion1 by which the intersecting points of the curves described by the polisher were regularly changed in distance from the centre of the mirror4 ,mr4 ,lassell had previously1 however1 introduced a contrivance for this latter purpose himself4 ,the machine1 which is a simplification of ,lord ,rosse's1 was intended to give spiral strokes4 ,it differed from the original1 however1 in demanding a changeable stroke1 and in the absence of the lateral motion4 ,in another most essential feature it varied from both that and ,mr4 ,lassell's1 the mirror was always uppermost while polishing1 and being uncounterpoised escaped to as great an extent as possible from the effects of irregular pressure4 ,to any one who has studied the deformations of a reflecting surface1 and knows how troublesome it is to support a mirror properly1 the advantage is apparent4 ,illustration3 ,fig4 #ba4 ,polishing ,machine4 ,illustration3 ,fig4 #bb4 ,the ,foot ,power4 ,the construction is as follows3 ,a stout vertical shaft1 a1 ,fig4 #ba1 carries at its top a circular table b1 upon which the polisher c is screwed4 ,below a band-wheel d is fixed4 ,above the table1 at a distance of four inches1 a horizontal bar e is arranged1 so as to move back and forward in the direction of its length1 and to carry with it by means of a screw l1 the mirror m1 and its iron back or chuck n4 ,the bar is moved by a connecting rod f1 attached to it at one end1 and at the other to a pin g moving a slot4 ,this slot is in a crank h1 carried by a vertical shaft i1 near the former one a4 ,the band-wheel k is connected with the foot power1 ,fig4 #bb4 ,the machine1 except those parts liable to wear by friction1 is made of wood4 ,the ends o o of the horizontal bar e1 are defended by brass tubes working in mahogany1 and have even now but little shake1 though many hundred thousands of reciprocations have been made4 ,the foot power consists of an endless band with wooden treads a a1 passing at one end of the apparatus over iron wheels b b1 which carry the band-wheel c upon their axle4 ,at the other end it goes over the rollers d d4 ,two pairs of intermediate wheels e e1 serve to sustain the weight of the man or animal working in it4 ,the treads are so arranged that they interlock1 and form a platform1 which will not yield downwards4 ,owing to its inclination when a weight is put on the platform a1 it immediately moves from b toward d and the band-wheel turns4 ,by a moderate exertion1 equivalent to walking up a slight incline at a slow rate1 a power more than sufficient to polish a #ae-a_/b inch mirror is obtained4 ,this machine1 in which very little force is lost in overcoming friction1 is frequently employed for dairy use1 and is moved commonly in the ,state of ,new ,york by a sheep4 ,i have generally myself walked in the one used by me1 and have travelled some days1 during five hours1 more than ten miles4 ,in order to give an idea of the method of using a grinding and polishing machine1 the following extract from the workshop note-book is introduced3-- 8,a disk of plate glass #ae-a_/b inches in diameter1 and #a-a_/d inch thick was procured4 ,it had been polished flat on both sides1 so that its internal constitution might be seen4#d ,it was fastened upon the table b of the machine1 by four blocks of wood as at c1 ,fig4 #ba4 ,underneath the glass were three thick folds of blanket1 #ae inches in diameter1 to prevent scratching of the lower face1 and avoid risk of fracture4 ,a convex disk of lead weighing #dj pounds having been cast1 was laid upon the upper surface of the glass1 and then the screw l was depressed so as to catch in a perforated iron plate n1 at the back of the lead m1 and press downward strongly4 #d ,the glass that ,i have used has generally been such as was intended for dead-lights and sky-lights in ships4 8,emery as coarse as the head of a pin having been introduced1 through a hole in the lead1 motion was commenced and continued for half an hour1 an occasional supply of emery being given4 ,the machine made #aej eight-inch cross strokes1 and the mirror #ej revolutions per minute4 ,the grinder m was occasionally restrained from turning by the hand4 ,at the end of the time the detritus was washed away1 and an examination with the gauge made4 ,a spot #aa inches in diameter1 and #a_/fj of an inch deep1 was found to have been ground out4 ,the same process was continued at intervals for ten hours1 measurements with the gauge being frequently made4 ,the concave was then sufficiently deep4 ,the leaden grinder was kept of the right convexity by beating it on the back when necessary4 ,a finer variety of coarse emery1 and after that flour emery were next put on1 each for an hour4 ,these left the surface moderately smooth1 and nearly of the right focal length4 ,the leaden grinder was then dismissed1 and the iron one1 ,fig4 #f1 put in its stead4 ,the mirror was removed from its place1 and ground upon a large piece of flat glass for ten minutes1 to produce a circular outline to the concavity4 ,it was cemented with soft pitch to the concave iron disk1 the counterpart of ,fig4 #f1 and again recentred on the blanketed table b4 ,emeries of #c and #bj seconds1 and #a1 #c1 #aj1 #cj1 #fj minutes' elutriation were worked on it1 an hour each4 ,the rate of cross motion was reduced to #be per minute to avoid heating1 the mirror still revolving once for every three cross strokes4 ,the screw pressure of l was stopped4 ,this produced a surface exquisitely fine1 semi-transparent1 and appearing as if covered with a thin film of dried milk4 ,it could reflect the light from objects outside the window until an incidence of #de degrees was reached1 and at night was found to be bright enough for a preliminary examination at the centre of curvature4 8,the polisher was constructed in the usual way 7page #ab71 and being smeared with rouge was fastened to the table b1 where the mirror had been4 ,the latter warmed in water to #abj ,f41 was then put face downwards upon the former1 and the screw l so lowered as to cause no pressure4 ,the machine was allowed to make #bj four-inch cross strokes per minute1 and the polisher to revolve once for every three strokes4 ,the mirror being unconstrainedly supported on the polisher1 was irregularly rotated by hand1 or rather prevented from rotating with the polisher4 ,the tendency of this method is to produce an almost spherical surface4 ,to change it to a paraboloid1 it was only necessary when the glass was polished all over to increase the length of the stroke to #h inches1 and continue working fifteen minutes at a time1 examining in the intervals by the tests at the centre of curvature4 ,the production of a polish all over occupied about two hours1 but the correction of figure took more time1 on account of the frequent examinations1 and the absolute necessity of allowing the mirror to come back to a state of equilibrium from which it had been disturbed when worked on the machine40 ,i have seen a mirror which was parabolic when just off the machine1 by cooling over night become spherical4 ,and these heat changes are often succeeded by other slower molecular movements1 which continue to modify a surface for many days after4 ,this correction1 where time and not length of stroke is the governing agent1 has once or twice been accomplished in fifteen minutes1 but sometimes has cost several hours4 ,if the figure should have become a hyperboloid of revolution1 that is1 have its edge zones too long in comparison with the centre1 it is only necessary to shorten the stroke to bring it back to the sphere1 or even to overpass that and produce a surface in which at the centre of curvature the edge zones have too short a focal length 7,fig4 #ab74 ,very much less trouble from zones of unequal focal length was experienced after this machine and system of working were adopted4 ,this was owing probably partly to the element of irregularity in the rotation of the mirror1 and partly to the fact that the surface is kept spherical until polished1 and is then rapidly changed to the paraboloid4 ,where the adjustments of an apparatus are made so as to attempt to keep a surface parabolic for some hours1 there is a strong tendency for zones to appear1 and of a width bearing a fixed relation to the stroke4 ,the method of producing reflecting surfaces next to be spoken of1 is however that which has finally been adopted as the best of all1 being capable of forming mirrors which are as perfect as can be1 and yet only requiring a short time4 ,it is the correction of a surface by local retouches4 ,in the account published by ,m4 ,foucault1 it appears that he is in ,france the inventor of this improvement4 ,the mode of practising the retouches is as follows3 ,several disks of wood1 as a1 ,fig4 #bc1 varying from #h inches to #a_/b an inch in diameter1 are to be provided1 and covered with pitch or rosin of the usual hardness1 in squares as at c1 on one side4#e ,on the other a low cylindrical handle b1 is to be fixed4 ,the mirror a1 ,fig4 #bd1 having been fined with the succession of emeries before described1 is laid face upward on several folds of blanket1 arranged upon a circular table1 screwed to an isolated post in the centre of the apartment1 which permits the operator to move completely round it4 ,an ordinary barrel has generally supplied the place of the post1 the head c1 ,fig4 #bd1 serving for the circular table1 and the rim b preventing the mirror sliding off4 ,the other end is fastened to the floor by four cleets d d4 #e ,m4 ,foucault used plano-convex lenses of glass1 of a radius of curvature slightly less than that of the mirror1 and covered with paper on the convex face4 ,illustration3 ,fig4 #bc4 ,local ,polisher4 ,illustration3 ,fig4 #bd4 ,section of ,optician's ,post4 ,the large polisher is first moved over the surface in straight strokes upon every chord1 and a moderate pressure is exerted4 ,as soon as the mirror is at all brightened1 perhaps in five minutes1 the operation is to be suspended1 and an examination at the centre of curvature made4 ,by carefully turning round1 the best diameter for support is to be found1 and marked with a rat-tail file on the edge1 and then the curve of the mirror ascertained4 ,if it is nearly spherical1 as will be the case if the grinding has been conducted with care and irregular heating avoided1 it is to be replaced on the blanketed support1 and the previous action kept up until a fine polish1 free from dots like stippling1 is attained4 ,this stage should occupy three or four hours4 ,another examination should reveal the same appearances as the preceding4 ,it is next necessary to lengthen the radius of curvature of the edge zones1 or what is much better shorten that of the centre1 so as to convert the section curve into a parabola4 ,this is accomplished by straight strokes across every diameter of the face1 at first with a #d inch1 then with a #f inch1 and finally with the #h inch polisher4 ,examinations must1 however1 be made every five or ten minutes1 to determine how much lateral departure from a direct diametrical stroke is necessary1 to render the curve uniform out to the edge4 ,care must be taken always to warm the polisher1 either in front of a fire or over a spirit lamp1 before using it4 ,perhaps the most striking feature in this operation is that the mirror presents continually a curve of revolution1 and is not diversified with undulations like a ruffle4 ,by walking steadily round the support1 on the top of which the mirror is placed1 there seems to be no tendency for such irregularities to arise4 ,if the correction for spherical aberration should have proceeded too far1 and the mirror become hyperbolic1 the sphere can be recovered by working a succession of polishers of increasing size on the zone a1 ,fig4 #af1 intermediate between the centre and edge1 causing their centres to pass along every chord that can be described tangent to the zone4 ,a most perfect and rapid control can thus be exercised over a surface1 and an uniform result very quickly attained4 ,it becomes a pleasant and interesting occupation to produce a mirror4 ,but two effects have presented themselves in this operation1 which unfortunately bar the way to the very best results4 ,in the first place the edge parts of such mirrors1 for more than half an inch all around1 bend backwards and become of too great focal length1 and the rays from these parts cannot be united with the rest forming the image4 ,in the second place1 the surface1 when critically examined by the second test1 is found to have a delicate wavy or fleecy appearance1 not seen in machine polishing4#f ,although the variations from the true curve implied by these latter greatly exaggerated imperfections are exceedingly small1 and do not prevent a thermometer bulb in the sunshine appearing like a disk surrounded by rings of interference1 yet they must divert some undulations from their proper direction1 or else they would not be visible4 ,all kinds of strokes have been tried1 straight1 sweeping circular1 hypocycloidal1 c4 without effecting their removal4 ,m4 ,foucault1 who used a paper polisher1 also encountered them4 ,eventually they were imputed to the unequal pressure of the hand1 and in consequence a machine to overcome the two above mentioned faults of manual correction was constructed4 #f ,by this it is not meant that there is a rippled polish1 like that produced by buckskin4 ,illustration3 ,fig4 #be4 ,machine for ,local ,corrections4 ,the mirror a1 is carried by an iron chuck or table b1 covered with a triple fold of blanket1 and is prevented from slipping off by four cleets c c4 ,the vertical shaft d passes through a worm-wheel e1 the endless screw of which f1 is driven by a band g1 from the primary shaft h4 ,at i is the band-wheel for connection to the foot-power4 ,at one end of the primary shaft is firmly fixed the cogwheel k1 which drives the crank-shaft l4 ,attached to the horizontal part of l1 is the crank-pin m4 ,the two bolts n n move in a slot1 so that the crank-pin may be set at any distance from #j to #b inches1 out of line with l4 ,above1 the crank-pin carries one end of the bar o1 the other end passing through an elliptical hole in the oak-block p4 ,down the middle of the bar runs a long slot1 through which the screw-pin q passes1 and which permits q to be brought over any zone from the centre to the edge of the mirror a4 ,it is retained by the bolts r r1 which are tapped into s4 ,the local polisher is seen at t4 ,the curve which the centre of the local polisher describes upon the face of the mirror1 varies with the adjustments4 ,fig4 #bf is a reduction from one traced by the machine1 the overlapping being seen on the left side4 ,the mirror is not tightly confined by the cleets c c1 for that would certainly injure the figure1 but performs a slow motion of rotation1 so that in no two successive strokes are the same parts of the edge pressed against them4 ,illustration3 ,fig4 #bf4 ,hypocycloidal ,curve4 ,the local polishers are made of lead1 alloyed with a small proportion of antimony1 and are #h1 #f1 and #d inches in diameter1 respectively4 ,the largest and smallest are most used1 the former on account of its size polishing most quickly1 but the latter giving the truest surface4 ,the rosin that covers them is just indentable by the thumb nail1 and is arranged in a novel manner4 ,the leaden basis1 as seen at t1 ,fig4 #be1 is perforated in many places with holes1 which permit evaporation1 serve for the introduction of water where needed1 and allow the rosin to spread freely4 ,grooves are made from one aperture to another1 and the rosin thus divided into irregular portions4 ,the effects of the production of heat are in this way avoided4 ,the mirror may be ground and fined on this machine1 in the same manner as on that described at page #ba1 or it may be ground with a small tool #h inches in diameter1 as recently suggested by ,m4 ,foucault1 the results in the latter case being just as good a surface of revolution as in the former4 ,it is best polished with the #h inch1 and a moderate pressure may be given by the screw q1 if the pitch is not too soft4 ,this1 however1 tends to leave an excavated place at the centre of the mirror1 the size depending on the stroke of the crank m1 which should be about #b inches4 ,the pin q ought to be half way from the centre to the edge of the mirror1 but must be occasionally moved right or left an inch along the slot4 ,when the surface is approaching a perfect polish1 the warmed #d inch polisher must be put in the place of the #h inch4 ,the pin q must be set exactly half-way between the centre and edge of the mirror1 and the crank must have a stroke of two inches radius4 ,the polisher then just goes up to the centre of the glass surface with one edge1 and to the periphery with the other1 while the outer excursion of the inner edge and inner excursion of the outer edge meet1 and neutralize one another at a midway point4 ,wherever the edge of a polisher changes direction many times in succession1 on a surface1 a zone is sure to form1 unless avoided in this manner4 ,all the foregoing description is for a #ae-a_/b inch mirror4 ,by this system of local polishing the difficulties of heat1 distribution of polishing powders1 irregular contact of the rosin1 c4 that render the attainment of a fine figure so uncertain usually1 entirely disappear4 ,a spherical surface is produced as above described1 and afterwards by moving q towards the edge1 and at the same time increasing the stroke1 it is converted into a paraboloid4 ,the fleecy appearance spoken of on a former page is not perceived1 and the surface is good almost up to the extreme edge4 7#d47 ,,eye-,,pieces1 ,,plane ,,mirrors ,,and ,,test ,,objects4 ,the telescope is furnished with several eye-pieces of various construction1 giving magnifying powers from #ge to #abjj1 or if it were desired even higher4 ,for the medium powers #cjj and #fjj ,ramsden1 or rather positive eye-pieces have been adopted4 ,they differ1 however1 from the usual form in being achromatic1 that is1 each plano-convex is composed of a flint and crown1 arranged according to formulas calculated by ,littrow4 ,in this way a large flat field and absence of color are secured1 and the fine images yielded by the mirror are not injured4 ,for the higher powers1 single achromatic lenses are used1 and for the highest of all a ,ross microscope4 ,with these means it has been found that the parabolic surfaces yielded by the processes before described1 will define test objects excellently4 ,of close double stars they will separate such as #b ,andromed1 and show the colors of the components4 ,in the case of unequal stars which seem to be more severe tests1 they can show the close companion of ,sirius--discovered by ,mr4 ,alvan ,clark's magnificent refractor--the sixth component of #a ,orionis1 and a multitude of other difficult objects4 ,as an example of light collecting power1 ,debillisima between and #e ,lyr is found to be quintuple1 as first noticed by ,mr4 ,lassell4 ,in the #ah-a_/b inch specula of ,herschel1 it was only recorded as double1 and1 according to ,admiral ,smyth1 ,lord ,rosse did not notice the fourth and fifth components4 ,jupiter's moons show with beautiful disks1 and their difference in diameter is very marked4 ,as for the body of that planet1 it is literally covered with belts up to the poles4 ,the bright and dark spots on ,venus1 and the fading illumination of her inner edge1 and its irregularities are perceived even when the air is far from tranquil4 ,stars are often seen as disks1 and without any wings or tails1 unless indeed the mirror should be wrongly placed1 so that the best diameter for support is not in the perpendicular plane1 passing through the axis of the tube4 ,it has been found that no advantage other than the decrease of atmospheric influence on the image1 results from cutting down the aperture of these mirrors by diaphragms1 while the disadvantage of reducing the separating power1 is perceived at the same time4 ,faint objects can be better seen with the whole surface than with a reduced aperture1 and this though apparently a property common to all reflectors and object glasses is not so in reality4 ,a defective edge will often cause the whole field to be filled with a pale milky light1 which will extinguish the fainter stars4 ,good definition is just as important for faint as for close objects4 ,the properties of these mirrors have been best shown by the excellence of the photographs taken with them4 ,although these are not as sharp as the image seen in the telescope1 yet it must not be supposed that an imperfect mirror will give just as good pictures4 ,a photograph which is magnified to #c feet1 represents a power of #chj4 ,as the original negative taken at the focus of the mirror is not quite #a-a_/b inch in diameter when the moon is at its mean distance1 it has to be enlarged about #be times1 and has therefore to be very sharp to bear it4 ,the light collecting power of an unsilvered mirror is quite surprising4 ,with a #ae-a_/b inch1 the companion of ,lyr can be perceived1 though it is only of the eleventh magnitude4 ,the moon and other bright objects are seen with a purity highly pleasing to the eye1 some parts being even more visible than after silvering4 ,in order to finish this description1 one part more of the optical apparatus requires to be noticed--the plane mirrors4 ,in the ,newtonian reflector the image is rejected out at the side of the tube by a flat surface placed at #de with the optical axis of the large concave4#g ,if this secondary mirror is either convex or concave1 it modifies the image injuriously1 causing a star to look like a cross1 and this though the curvature be so slight as hardly to be perceptible by ordinary means4 ,for a long time ,i used a piece #c #e inches1 which was cut from the centre of a large looking-glass accidentally broken1 but eventually found that by grinding three pieces of #f inches in diameter against one another1 and polishing them on very hard pitch1 a nearer approach to a true plane could be made4 ,they were tested by being put in the telescope1 and observing whether the focus was lengthened or shortened1 and also by trial on a star4 ,when sufficiently good to bear these tests1 a piece of the right size was cut out with a diamond1 from the central parts4 #g ,a right-angled prism cannot be used with advantage to replace the plane silvered mirrors1 because it transmits less light than they reflect1 is more liable to injure the image1 and the glass is apt to be more or less colored4 ,its great size and cost1 one three inches square on two faces being required for my purposes1 has also to be considered4 #b4 ,,the ,,telescope ,,mounting4 ,the telescope is mounted as an altitude and azimuth instrument1 but in a manner that causes it to differ from the usual instrument of that kind4 ,the essential feature is1 that the eye-piece or place of the sensitive plate is stationary at all altitudes1 the observer always looking straight forward1 and never having to stoop or assume inconvenient and constrained positions4 ,illustration3 ,fig4 #bg4 ,miss ,herschel's ,telescope4 ,the stationary eye-piece mounting was first used by ,miss ,caroline ,herschel1 who had a #bg inch ,newtonian arranged on that plan4 ,fig4 #bg4 7,smyth's ,celestial ,cycle47 ,subsequently it was applied to a large telescope by ,mr4 ,nasmyth1 the eminent engineer1 but no details of his construction have reached me4 ,he used it for making drawings of the moon1 which are said to be excellently executed4 ,when it became necessary to determine how my telescope should be mounted1 ,i was strongly urged to make it an equatorial4 ,but after reflecting on the fact that it was intended for photography1 and that absolute freedom from tremor was essential1 a condition not attained in the equatorial when driven by a clock1 and in addition that in the case of the moon rotation upon a polar axis does not suffice to counteract the motion in declination1 ,i was led to adopt the other form4 ,a great many modifications of the original idea have been made4 ,for instance1 instead of counterpoising the end of the tube containing the mirror by extending the tube to a distance beyond the altitude or horizontal axis1 ,i introduced a system of counterpoise levers which allows the telescope to work in a space little more than its own focal length across4 ,this construction permits both ends of the tube to be supported1 the lower one on a wire rope1 and gives the greatest freedom from tremor1 the parts coming quickly to rest after a movement4 ,in the use of the telescope for photography1 as we shall see1 the system of bringing the mass of the instrument to complete rest before exposing the sensitive plate1 and only driving that plate itself by a clock1 is always adopted4 ,the obvious disadvantage connected with the alt-azimuth mounting--the difficulty of finding some objects--has not been a source of embarrassment4 ,in fact the instability of the optical axis in reflecting instruments1 if the mirror is unconstrainedly supported1 as it should be1 renders them unsuitable for determinations of position4 ,a little patience will enable an observer to find all necessary tests1 or curious objects4 ,the mounting is divided into3 a4 ,the ,tube2 and b4 ,the supporting frame4 a4 ,the ,tube4 ,the telescope tube is a sixteen sided prism of walnut wood1 #ah inches in diameter1 and #ab feet long4 ,the staves are #c_/h of an inch thick1 and are hooped together with four bands of brass1 capable of being tightened by screws4 ,inside the tube are placed two rings of iron1 half an inch thick1 reducing the internal diameter to about #af inches4 ,at opposite sides of the upper end of the tube are screwed the perforated trunnions a1 ,fig4 #bh 7of which only one is shown71 upon which it swings4 ,surrounding the other end is a wire rope b b b1 the ends of which go over the pulleys c 7c not shown7 on friction rollers1 and terminate in disks of lead d d4 ,these counterpoises are fastened on the ends of levers e e1 which turn below on a fixed axle f4 ,by this arrangement as the tube assumes a horizontal position and becomes1 so to speak1 heavier1 the counterpoises do the same1 while when the tube becomes perpendicular1 and most of its weight falls upon the trunnions1 the counterpoises are carried mostly by their axle4 ,a continual condition of equilibrium is thus reached1 the tube being easily raised or depressed to any altitude desired4 ,it is necessary1 however1 to constrain the wire rope b b b1 to move in the arc of the circle described by the end of the tube and ends of the levers and hence the twelve rollers or guide pulleys g g g4 ,over some of the same pulleys a thin wire rope h h runs1 but while its ends are fastened to the lower part of the tube at b1 the central parts go twice around a roller connected with the winch i1 near the eye-piece1 thus enabling the observer to move the telescope in altitude1 without taking the eye from the eye-piece4 ,illustration3 ,fig4 #bh4 ,sectional ,view of ,observatory4 ,the iron wire rope required to be carefully made1 so as to avoid rigidity4 ,it contains #b-a_/c miles of wire1 #a_/ajj of an inch in diameter1 and has #cjj strands4 ,each single wire will support #g pounds4 ,it is1 however1 more flexible than a hempen rope of the same size1 owing to its loose twisting4 ,illustration3 ,fig4 #bi4 ,the ,mirror ,support4 ,at the lower end of the tube1 at the distance of a foot1 and crossing it at right angles1 held by three bars of iron i i i1 ,fig4 #bi1 is a circular table of oak e1 which carries an ,india-rubber air sac d1 and upon this the mirror f is placed4 ,the edge support of the mirror is furnished by a semicircular band of tin-plate a1 lined inside with cotton1 and fastened at the ends by links of chain b1 7b not seen7 to two screws c c2 g and h are the wire ropes1 marked b and h in ,fig4 #bh4 ,instead of the blanket support which ,herschel found so advantageous1 ,m4 ,foucault has suggested this use of an air sac4 ,in his instrument there is a tube going up to the observer1 by which he may adjust its degree of inflation4 ,it requires that there should be three bearings c c c1 in front of the mirror1 against which it may press when the sac behind is inflated1 otherwise the optical axis is altogether too instable1 and objects cannot be found4 ,the arrangement certainly gives beautiful definition1 bringing stars to a disk when the glass just floats1 without touching its front bearings4 ,the first sac that ,i made was composed of two circular sheets of ,india-rubber cloth1 joined around the edges4 ,but this could not be used while photographing1 because the image was kept in a state of continuous oscillation if there was a breeze1 and even under more favorable circumstances took a long time to come to rest4 ,it was not advisable to blow the mirror hard up against its three front bearings1 in order to avoid the instability1 for then every point in of an object became triple4 ,to the eye the oscillations were not offensive1 because the swaying image was sharp4 ,subsequently1 however1 an air chair cushion was procured1 and as the surface was flat instead of convex the difficulty became so much less1 that the blanket support was definitely abandoned4 ,it is necessary that the mirror should have free play in the direction of the length of the tube when this kind of support is used1 and that is the reason why the tin edge hoop must terminate in links of chain4 ,the interval1 eight or ten inches1 which separates the face of the mirror from the tube1 is occupied by a curtain of black velvet1 confined below by a drawing cord and tacked above to the tube4 ,this permits access to the mirror to put a glass cover on it1 and when shut down stops the current of air rushing up4 ,when the instrument is not being used this curtain is left open1 because the mirror and tube are in that case kept more uniform in temperature with the surrounding air4 ,in spite of such contrivances there is still sometimes a strong residual current in the tube4 ,i have tried to overcome it by covering the mouth of the tube with a sheet of flat glass1 but have been obliged to abandon that because the images were injured4 ,at one time1 too1 when it was supposed that the current was partly from the observer's body1 heated streams of air going out around the tube1 the aperture in the dome was closed by a conical bag of muslin1 which fitted the mouth of the telescope tightly4 ,the only advantages resulting were mere bodily comfort and a capability of perceiving fainter objects than before1 because the sky-light was shut off4 ,illustration3 ,fig4 #cj4 ,section of ,azimuth ,axis4 b4 ,the ,supporting ,frame4 ,the frame which carries the preceding parts is of wood1 and rests on a vertical axis a1 ,fig4 #cj1 turning below in a gun-metal cup b1 supported by a marble block resting on the solid rock4 ,the upper end of the axis is sustained by two collars1 one c c above1 and the other below an intermediate triangular box e e from the sides of which three long beams f f f #ab #c inches diverge1 gradually declining till they meet the solid rock at the limits of the excavation in which the observatory is placed4 ,these beams are fastened together by cross-pieces g g g1 ,fig4 #ca1 and go through the floor in spaces h h h1 so contrived that the floor does not touch them4 ,at the ends they are cased with a thick leaden sheathing1 to deaden vibration and prevent the access of moisture4 ,illustration3 ,fig4 #ca4 ,plan of ,observatory 7lower floor74 ,this tripod support in connection with the sustaining of the telescope by the wire rope1 gives that steadiness which is so essential in photography4 ,only a slight amount of force1 about two pounds1 is required to move the instrument in azimuth1 though it weighs almost a thousand pounds4 ,the plan of the frame centrally carried by the axis a is as follows3 ,from the corners of a parallelogram i i 7#b #ac feet7 of wooden beams1 eight inches thick and three inches broad1 perpendiculars n n1 ,fig4 #bh1 rise4 ,at the top they are connected by lighter pieces to form a parallelogram1 similar to that below1 and just large enough to contain the tube of the telescope4 ,at right angles to the parallelogram below1 and close upon it1 a braced bar o o1 ,fig4 #bh1 crosses4 ,from its extremities four slanting braces as at p p1 ,fig4 #bh1 go to the corners of the upper parallelogram1 and combine to give it lateral support4 ,at the top of one close pair of the perpendiculars n1 ,fig4 #bh1 are bronze frames carrying friction rollers upon which the trunnions move1 while similarly upon the other pair n are two pulleys1 also on friction rollers1 for the wire rope coming from the counterpoises4 ,movement in altitude is very easily accomplished1 and with the left hand upon the winch i1 under high powers1 both altitude and azimuth motions are controlled1 and the right hand left free4 ,the whole apparatus works so well1 that in ordinary observation the want of a clock movement has not been felt4 ,of course for photography that is essential4 #c4 ,,the ,,clock ,,movement4 ,the apparatus for following celestial bodies is divided into two parts2 a4 ,the ,sliding ,plate-holder2 and b4 ,the ,clepsydra4 ,in addition a short description of the ,sun-,camera1 c1 is necessary4 a4 ,the ,sliding ,plate-holder4 ,mr4 ,de ,la ,rue1 who has done so much for celestial photography1 was the first to suggest photographing the moon on a sensitive plate1 carried by a frame moving in the apparent direction of her path4 ,he never1 however1 applied an automatic driving mechanism1 but was eventually led to use a clock which caused the whole telescope to revolve upon a polar axis1 and thus compensate for the rotation of the earth1 and on certain occasions for the motion of the moon herself4 ,in this way he has produced the best results that have been obtained in ,europe4 ,lord ,rosse1 too1 employed a similar sliding plate-holder1 but provided with clock-work to move it at an appropriate rate4 ,i have not been able as yet to procure any precise account of either of these instruments4 ,the first photographic representations of the moon ever made1 were taken by my father1 ,professor ,john ,w4 ,draper1 and a notice of them published in his quarto work 8,on the ,forces that ,organize ,plants10 and also in the ,september number1 #ahdj1 of the ,london1 ,edinburgh1 and ,dublin ,philosophical ,magazine4 ,he presented the specimens to the ,new ,york ,lyceum of ,natural ,history4 ,the ,secretary of that ,association has sent me the following extract from their minutes3-- 8,march #bc;d1 #ahdj4 ,dr4 ,draper announced that he had succeeded in getting a representation of the moon's surface by the ,daguerreotype4444 ,the time occupied was #bj minutes1 and the size of the figure about #a inch in diameter4 ,daguerre had attempted the same thing1 but did not succeed4 ,this is the first time that anything like a distinct representation of the moon's surface has been obtained4 8,,robt4 ,h4 ,,brownne1 ,secretary40 ,as my father was at that time however much occupied with experiments on the ,chemical ,action of ,light1 the ,influence of ,light on the ,decomposition of ,carbonic ,acid by ,plants1 the ,fixed ,lines of the ,spectrum1 ,spectrum ,analysis1 c41 the results of which are to be found scattered through the ,philosophical ,magazine1 ,silliman's ,journal1 and the ,journal of the ,franklin ,institute1 he never pursued this very promising subject4 ,some of the pictures were taken with a three inch1 and some with a five inch lens1 driven by a heliostat4 ,in #ahej1 ,mr4 ,bond1 taking advantage of the refractor of #ae inches aperture at ,cambridge1 obtained some fine pictures of the moon1 and subsequently of double stars1 more particularly ,mizar in ,ursa ,major4 ,the driving power1 in this instance1 was also applied to move the telescope upon a polar axis4 ,besides these1 several ,english and continental observers1 ,messrs4 ,hartnup1 ,phillips1 ,crookes1 ,father ,secchi1 and others1 have worked at this branch of astronomy1 and1 since #aheg1 ,mr4 ,lewis ,m4 ,rutherfurd1 of ,new ,york1 has taken many exquisite lunar photographs1 which compare favorably with foreign ones4 ,but in none of these instances has the use of the sliding plate-holder been persisted in1 and its advantages brought into view4 ,in the first place it gets rid completely of the difficulties arising from the moon's motion in declination1 and in the second1 instead of injuring the photograph by the tremors produced in moving the whole heavy mass of a telescope weighing a ton or more1 it only necessitates the driving of an arrangement weighing scarcely an ounce4 ,my first trials were with a frame to contain the sensitive plate1 held only at three points4 ,two of these were at the ends of screws to be turned by the hands1 and the third was on a spring so as to maintain firm contact4 ,this apparatus worked well in many respects1 but it was found that however much care might be taken1 the hands always caused some tremor in the instrument4 ,it was evident then that the difficulty from friction which besets the movements of all such delicate machinery1 and causes jerking and starts1 would have to be avoided in some other way4 ,i next constructed a metal slide to run between two parallel strips1 and ground it into position with the greatest care4 ,this1 when set in the direction of the moon's apparent path1 and moved by one screw1 worked better than the preceding4 ,but it was soon perceived that although the strips fitted the frame as tightly as practicable1 an adhesion of the slide took place first to one strip and then to the other1 and a sort of undulatory or vermicular progression resulted4 ,the amount of deviation from a rectilinear motion1 though small1 was enough to injure the photographs4 ,at this stage of the investigation the regiment of volunteers to which ,i belonged was called into active service1 and ,i spent several months in ,virginia4 ,my brother1 ,mr4 ,daniel ,draper1 to whose mechanical ingenuity ,i have on several occasions been indebted for assistance in the manifold difficulties that have arisen while constructing this telescope1 continued these experiments at intervals4 ,he presented me on my return with a slide and sand-clock1 with which some excellent photographs have been taken4 ,he had found that unless the slide above mentioned was made ungovernably long1 the same trouble continued4 ,he then ceased catching the sliding frame h1 ,fig4 #cb1 by two opposite sides1 and made it run along a single steel rod a1 being attached by means of two perforated plates of brass b1 b4 ,the cord i going to the sand-clock1 was applied so as to pull as nearly as possible in the direction of the rod4 ,a piece of cork c1 gave the whole steadiness1 and yet softness of motion4 ,the lower end of the frame was prevented from swinging back and forward by a steel pin d1 which played along the glass rod e4 ,all these parts were attached to a frame k1 fitting on the eyepiece holder1 and permitting the rod a to change from the horizontal position in which it is here drawn1 to any angular one desired4 ,the thumb-screw f retained it in place2 g and g are pulleys which permit the cord to change direction4 ,illustration3 ,fig4 #cb4 ,sliding ,plate-holder4 ,subsequently1 a better method of examining the uniformity of the rate1 than by noticing the sharpness of the photograph produced1 was invented4 ,it consists in arranging a fixed microscope1 magnifying about #dj times1 at the back of the ground glass plate1 which fits in the same slide as the sensitive plate4 ,by watching the granulated appearance pass before the eye1 as the slide is moved by the clock1 the slightest variation from uniformity1 any pulsatile or jerking movement is rendered visible4 ,by the aid of this microscopic exaggeration1 it was seen that occasionally1 when there had been considerable changes in temperature1 the steadiness of the motion varied4 ,this was traced to the irregular slipping of b1 b4 ,illustration3 ,fig4 #cc4 ,frictionless ,slide 7front view74 ,sectional view4 ,a different arrangement was then adopted1 by which a lunar crater can be kept bisected as long as is necessary1 and which gives origin to no irregularities1 but pursues a steady course4 ,the principle is1 not to allow a slipping friction anywhere1 but to substitute rolling friction1 upon wheels turning on points at the ends of their axles4 ,the following wood-cut is half the real size of this arrangement4 ,a glass rod a1 a1 ,fig4 #cc1 is sustained by two wheels b1 b1 and kept in contact1 with them by a third friction roller c1 pressed downward by a spring4 ,this rod carries a circular frame d1 d1 upon which at e1 e1 e1 are three glass holders and platinum catches4 ,a spring f holds the sensitive plate in position1 by pressing against its back4 ,the circular frame d is kept in one plane by a fourth friction roller g1 which runs on a glass rod h1 and is kept against it by the inward pressure of the overhanging frame d4 ,the cord i is attached to the arm k1 and pulls in the direction of the glass rod a4 ,from m to a fixed point near b1 a strip of elastic ,india-rubber is stretched1 to keep the cord tight4 ,the ring of brass n1 n carries the whole1 serving as a basis for the stationary parts1 and in its turn being fastened to the eyepiece holder1 so as to allow the glass rod a to change direction1 and be brought into coincidence with the apparent path of the moon4 ,at o is a thumb-screw or clamp4 ,through the ring n1 n1 a groove p is cut1 into which a piece of yellow glass may be placed1 when the actinic rays are to be shut off from the plate4 ,since this contrivance has been completed1 all the previous difficulties have vanished4 ,the moving of a plate can be accomplished with such precision1 that when the atmosphere was steady1 negatives were taken which have been enlarged to three feet in diameter4 ,the length of time that such a slide can be made to run is indefinite1 depending in my case on the size of the diagonal flat mirror1 and aperture of the eyepiece holder4 ,i can follow the moon for nearly four minutes1 but have never required to do so for more than fifty seconds4 ,at the mouth of the instrument1 where no secondary mirror is necessary1 the time of running could be increased4 ,the setting of the frictionless slide in angular position is accomplished as follows3 ,a ground glass plate is put into it1 with the ground face toward the mirror4 ,upon this face a black line must have been traced1 precisely parallel to the rod a4 ,this may be accomplished by firmly fixing a pencil point against the ground side1 and then drawing the frame d and glass past it1 while the rest of the slide is held fast4 ,as the moon passes across the field1 the position of the apparatus must be changed1 until one of the craters runs along the line from end to end4 ,a cross line drawn perpendicular to the other1 serves to adjust the rate of the clepsydra as we shall see1 and when a crater is kept steadily on the intersection for twice or three times the time demanded to secure an impression1 the adjustment may be regarded as complete4 ,it is necessary of course to expose the sensitive plate soon after1 or the apparent path of the moon will have changed direction1 unless indeed the slide is set to suit a future moment4 b4 ,the ,clepsydra4 ,my prime mover was a weight supported by a column of sand1 which1 when the sand was allowed to run out through a variable orifice below1 could be made to descend with any desired velocity and yet with uniformity4 ,in addition1 by these means an unlimited power could be brought to bear1 depending on the size of the weight4 ,previously it was proposed to use water1 and compensate for the decrease in flow1 as the column shortened1 by a conical vessel2 but it was soon perceived that as each drop of water escaped from the funnel-shaped vessel1 only a corresponding weight would be brought into play4 ,this is not the case with sand1 for in this instance every grain that passes out causes the whole weight that is supported by the column to come into action4 ,in the former instance a movement consisting of a series of periods of rest and periods of motion occurs1 because power has to accumulate by floating weight lagging behind the descending water1 and then suddenly overtaking it4 ,in the latter case1 on the contrary1 there is a regular descent1 all minor resistances in the slide being overcome by the steady application of the whole mass of the weight4 ,when these advantages in the flow of sand were ascertained1 all the other prime movers were abandoned4 ,mercury-clocks1 on the principle of the hydrostatic paradox1 air-clocks1 c41 in great variety1 had been constructed4 ,illustration3 ,fig4 #cd4 ,the ,sand-,clock4 ,the sand-clock consisted of a tube a 7,fig4 #cd71 eighteen inches long and one and a half in diameter1 nearly filled with sand that had been raised to a bright red heat and sifted4 ,upon the top of the sand a leaden weight b was placed4 ,at the bottom of the tube a peculiar stopcock1 seen at 7#b7 enlarged1 regulated the flow1 the amount passing depending on the size of the aperture d4 ,this stopcock consisted of two thin plates1 fixed at one end and free at the other4 ,the one marked e is the adjusting lever1 and its aperture moves past that in the plate g4 ,the lever f serves to turn the sand off altogether1 without disturbing the size of the other aperture1 which1 once set to the moon's rate1 varies but slightly in short times4 ,a movable cover h1 perforated to allow the cord i to pass through1 closed the top1 while the vessel k retained the escaped sand1 which at suitable times was returned into the tube a1 the weight b being temporarily lifted out4 ,from the clock the cord i communicated motion to the frictionless slide1 as shown in ,fig4 #cc4 ,this cord should be as inelastic as possible1 consistent with pliability1 and well waxed4 ,one who has not investigated the matter would naturally suppose that the flow of sand in such a long tube would be much quicker when the tube was full than when nearly empty1 and that certainly that result would occur when a heavy weight was put on the shifting mass4 ,but in neither case have ,i been able to detect the slightest variation1 for1 although by shaking the tube a diminution of the space occupied by the sand may be caused1 yet no increase of weight tried could accomplish the same reduction4 ,these peculiarities seem to result from the sand arching as it were across the vessel1 like shot in a narrow tube1 and only yielding when the under supports are removed4 ,in blasting1 a heavy charge of gunpowder can be retained at the bottom of a hole1 and made to split large masses of rock1 by filling the rest of the hole with dry sand4 ,i believe that no prime mover is more suitable than a sand-clock for purposes where steady motion and a large amount of power are demanded4 ,the simplicity1 for instance1 of a heliostat on this plan1 the large size it might assume1 and its small cost1 would be great recommendations4 ,in these respects its advantages over wheelwork are very apparent4 ,the precision with which such a sand-clock goes may be appreciated when it is stated1 that under a power of #cjj a lunar crater can be kept bisected for many times the period required to photograph it4 ,to secure the greatest accuracy in the rate of a sand-clock1 some precautions must be taken4 ,the tube should be free from dents1 of uniform diameter1 and very smooth or polished inside4 ,water must not be permitted to find access to the sand1 and hygrometric varieties of that substance should be avoided1 or their salts washed out4 ,the sand should be burned to destroy organic matter1 and so sifted as to retain grains nearly equal in size4 ,the weight1 which may be of lead1 must be turned so as to go easily down the tube1 and must be covered with writing paper or some other hard and smooth material1 to avoid the proneness to adhesion of sand4 ,a long bottle filled with mercury answers well as a substitute4 ,i have used in such clocks certain metallic preparations3 ,fine shot1 on account of its equality of size1 might do for a very large clock with a considerable opening below1 but is unsuitable for a tube of the size stated above4 ,there is1 however1 a method by which lead can be reduced to a divided condition1 like fine gunpowder1 when it may replace the sand4 ,if that metal is melted with a little antimony1 and while cooling is shaken in a box containing some plumbago1 it breaks up at the instant of solidifying into a fine powder1 which is about five times as heavy as sand4 ,if after being sifted to select the grains of proper size1 it is allowed to run through a small hole1 the flow is seen to be entirely different from that of sand1 looking as if a wire or solid rod were descending1 and not an aggregation of particles4 ,it is probable1 therefore1 that it would do better than sand for this purpose4 ,i have not1 however1 given it a fair trial1 because just at the time when the experiments with the sand-clock had reached this point1 ,i determined to try a clepsydra as a prime mover4 ,the reason which led to this change was that it was observed on a certain occasion when the atmosphere was steady1 that the photographs did not correspond in sharpness1 being in fact no better than on other nights when there was a considerable flickering motion in the air4 ,a further investigation showed that in these columns of sand there is apt to be a minute vibrating movement4 ,at the plate-holder above this is converted into a series of arrests and advances4 ,on some occasions1 however1 these slight deviations from continuous motion are entirely absent1 and generally1 indeed1 they cannot be seen1 if the parts of the image seem to vibrate on account of currents in the air4 ,by the aid of the microscopic exaggeration described on a former page--which was subsequently put in practice--they may be observed easily1 if present4 ,when the negative produced at the focus of the great mirror is intended to be enlarged to two feet or more in size1 these movements injure it sensibly4 ,a variety of expedients was resorted to in order to avoid them1 but none proved on all occasions successful4 ,it is obvious that in a water-clock1 where the mobility of the fluid is so much greater than that of solid grains1 this difficulty would not arise4 ,the following contrivance in which the fault of the ordinary clepsydra1 in varying rate of flow as the column shortens1 is avoided1 was next made4 ,with it the best results are attainable1 and it seems to be practically perfect4 ,illustration3 ,fig4 #ce4 ,the ,clepsydra4 ,it consists of a cylinder a1 in which a piston b moves watertight4 ,at the top of the piston rod is a leaden five-pound weight c1 from which the cord i goes to the sliding plateholder g4 ,the lower end of the cylinder terminates in a stopcock d1 the handle of which carries a strong index rod e1 moving on a divided arc4 ,at f a tube with a stopcock is attached4 ,below1 a vessel h receives the waste fluid4 ,in using the clepsydra the stopcock of f is opened1 and the piston being pulled upwards1 the cylinder fills with water from h4 ,the stopcock is then closed1 and if d also is shut1 the weight will remain motionless4 ,the string i is next connected with the slide1 and the telescope turned on the moon4 ,as soon as the slide is adjusted in angular position 7page #cf7 the stopcock d is opened1 until the weight c moves downwards1 at a rate that matches the moon's apparent motion4 ,in order to facilitate the rating of the clepsydra1 the index rod e is pressed by a spring k 7#b71 against an excentric l4 ,as the excentric is turned round1 the stopcock d is of course opened1 with great precision and delicacy4 ,the plug of this stopcock 7#c7 is not perforated by a round hole1 but has a slit4 ,this causes equal movements in the rod e1 to produce equal changes in the flow4 ,the rating requires consequently only a few moments4 ,the object of the side tube f is to avoid disturbing d when it becomes necessary to refill the cylinder1 for when it is once opened to the right degree1 it hardly requires to be touched again during a night's work4 ,in order to arrest the downward motion of the piston at any point1 a clamp screws on the piston rod1 and can be brought into contact with the cylinder head1 as in the figure4 ,that this instrument should operate in the best manner1 it is essential to have the interior of the brass cylinder polished from end to end1 and of uniform diameter4 ,if any irregularity should be perceived in the rate of going1 it can be cured completely by taking out the piston1 impregnating its leather stuffing with fine rotten stone and oil1 and then rubbing it up and down for five minutes in the cylinder1 so as to restore the polish4 ,the piston and cylinder must of course be wiped1 and regreased with a mixture of beeswax and olive oil 7equal parts7 after such an operation4 ,in replacing the piston1 the cylinder must be first filled with water1 to avoid the presence of air1 which would act as a spring4 ,although it may be objected that this contrivance seems to be very troublesome to use1 yet that is not the case in practice4 ,even if it were1 it so far surpasses any prime mover that ,i have seen1 where the utmost accuracy is needed1 that it would be well worth employing4 c4 ,the ,sun ,camera4 ,in taking photographs of the sun with the full aperture of this telescope1 no driving mechanism is necessary4 ,on the contrary1 the difficulty is rather to arrange the apparatus so that an exposure short enough may be given to the sensitive plate1 and solarization of the picture avoided4 ,it is not desirable to reduce the aperture1 for then the separating power is lessened4 ,the time required to obtain a negative is a very small fraction of a second1 for the wavy appearance produced by atmospheric disturbance is not unfrequently observed sharply defined in the photograph1 though these aerial motions are so rapid that they can scarcely be counted4 ,some kind of shutter that can admit and cut off the solar image with great quickness is therefore necessary4 ,illustration3 ,fig4 #cf4 ,the ,spring ,shutter4 ,in front of an ordinary camera a1 ,fig4 #cf1 attached to the eyepiece holder of the telescope1 and from which the lenses have been removed1 a spring shutter is fixed4 ,it consists of a quadrant of thin wood b1 fastened by its right angle to one corner of the camera4 ,over the hole in this quadrant a plate of tin d can be adjusted1 and held in position by a screw moving in a slot so as to reduce the hole if desired to a mere slit4 ,it may vary from #a-a_/b inch to less than #a_/ej of an inch4 ,the quadrant is drawn downwards by an ,india-rubber spring g1 #a inch wide1 #a_/h of an inch thick1 and #h inches long4 ,this spring is stretched when in action to about #ab inches1 and when released draws the slit past the aperture c in the camera4 ,two nicks in the edge of the quadrant serve with the assistance of a pin e1 which can easily be drawn out by a lever 7not shown in the cut71 to confine the slit either opposite to or above c4 ,a catch at f prevents the shutter recoiling4 ,the sensitive plate is put inside the box as usual in a plate-holder4 ,when a photograph is taken1 the spring shutter is drawn up so that the lower nick in the edge of the quadrant is entered by the pin e1 and the inside of the camera obscured4 ,the front slide of the plateholder is then removed in the usual manner1 and the solar image being brought into proper position by the aid of the telescope finder1 the trigger retaining e is touched1 the shutter flies past c1 and the sensitive plate may then be removed to be developed4 ,to avoid the very short exposure needed when a silvered mirror of #ahh square inches of surface is used1 ,i have taken many solar photographs with an unsilvered mirror1 which only reflects according to ,bouguer #b-a_/b per cent4 of the light falling upon it1 and should permit an exposure #cg times as long as the silvered mirror4 ,this is the first time that a plain glass mirror has been used for such a purpose1 although ,sir ,john ,herschel suggested it for observation many years ago4 ,but eventually this application of the unsilvered mirror had to be abandoned4 ,it has1 it is true1 the advantage of reducing the light and heat1 but ,i found that the moment the glass was exposed to the ,sun1 it commenced to change in figure1 and alter in focal length4 ,this latter difficulty1 which sometimes amounts to half an inch1 renders it well nigh impossible to find the focal plane1 and retain it while taking out the ground glass1 and putting in the sensitive plate4 ,if the glass were supported by a ring around the edge1 and the back left more freely exposed to the air1 the difficulty would be lessened but not avoided1 for a glass mirror can be raised to #abj ,f4 on a hot day by putting it in the sunshine1 though only resting on a few points4 ,other means of reducing the light and heat1 depending on the same principle1 can however be used4 ,by replacing the silvered diagonal mirror with a black glass or plain unsilvered surface1 as suggested by ,nasmyth1 the trouble sensibly disappears4 ,i have in this way secured not only macul and their penumbr1 but also have obtained facul almost invisible to observation4 ,on some occasions1 too1 the precipitate-like or minute flocculent appearance on the ,sun's disk was perceptible4 ,it seems1 however1 that the best means of acquiring fine results with solar photography1 would be to use the telescope as a ,cassegrainian1 and produce an image so much enlarged1 that the exposure would not have to be conducted with such rapidity4 ,magnifying the image by an eyepiece would in a general way have the same result1 but in that case the photographic advantages of the reflector would be lost1 and it would be no better than an achromatic4 #d4 ,,the ,,observatory4 ,this section is divided into a1 ,the ,building2 b1 ,the ,dome2 and c1 ,the ,observer's ,chair4 a4 ,the ,building4 ,the ,observatory is on the top of a hill1 #bbe feet above low water mark1 and is in ,latitude #dj #ei #be north1 and ,longitude #gc #eb #be west from ,greenwich1 according to the determinations of the ,coast ,survey4 ,it is near the village of ,hastings-upon-,hudson1 and is about #bj miles north of the city of ,new ,york4 ,the surrounding country on the banks of the ,north ,river is occupied by country seats1 on the slopes and summits of ridges of low hills1 and no offensive manufactories vitiate the atmosphere with smoke4 ,our grounds are sufficiently extensive to exclude the near passages of vehicles1 and to avoid tremor and other annoyances4 ,illustration3 ,fig4 #cg4 ,dr4 ,draper's ,observatory4 ,an uninterrupted horizon is commanded in every direction1 except where trees near the dwelling house cut off a few degrees toward the southwest4 ,the advantages of the location are very great1 and often when the valleys round are filled with foggy exhalations1 there is a clear sky over the ,observatory1 the mist flowing down like a great stream1 and losing itself in the chasm through which the ,hudson here passes4 ,the foundation and lower story of the building are excavated out of the solid granite1 which appears at the edge of the hill4 ,this arrangement was intended to keep the lower story cool1 and avoid1 in the case of the metal reflector1 sudden changes of temperature4 ,the eastern side of the lower story1 however1 projects over the brow of the hill1 and is therefore freely exposed to the air1 furnishing1 when desired1 both access and thorough ventilation through the door4 ,the second story or superstructure is of wood1 lined inside with boards like the story below4 ,they serve to inclose in both cases a non-conducting sheet of air4 ,the inside dimensions of both stories taken together are #ag-a_/b feet square1 and #bb feet high1 to the apex of the dome4 ,this space is unnecessarily large for the telescope1 which only requires a cylinder #ac feet in diameter and #ac feet high4 ,a general idea of the internal arrangement is gained from ,fig4 #bh4 ,in ,fig4 #ch1 a a is the floor of the gallery1 b b b the circular aperture in which the telescope c c turns4 ,the staircase is indicated by d4 ,the ,enlarger1 #f1 rests on the shelf e1 the heliostat being outside at f4 ,the door going into the photographic room is at g1 h h are tables1 i the water tank1 k the tap and sink1 l the stove1 m a heliostat shelf1 n the door1 o the window4 ,illustration3 ,fig4 #ch4 ,plan of ,observatory 7upper floor74 ,the building is kept ventilated by opening the door in the lower part1 and the dome shutter1 seen in ,fig4 #cg1 for some time before using the instrument4 ,on a summer day the upper parts1 and especially those close under the dome1 become without this precaution very hot1 and this occurred even before the tin roof was painted4 ,bright tinplate seems not to be able to reflect by any means all the heat that falls upon it1 but will become so warm in ,july that rosin will melt on it1 and insects which have lighted in a few moments dry up1 and soon become pulverizable4 ,a knowledge of these facts led to the abandonment of wooden sheathing under the tin1 for without it when night comes on the accumulated heat radiates away rapidly1 and ceases to cause aerial currents near the telescope4 ,the interior of the building is painted and wainscoted1 and the roof is ornamented partly in blue and oak1 and partly with panels of tulip-tree wood4 ,there are only two windows1 and they are near the southern angles of the roof4 ,while they admit sunshine on some occasions1 they can on others be closed1 and the interior be reduced to darkness4 ,in the southeast corner a small opening e may allow a solar beam three inches in diameter to come in from a heliostat outside4 ,the greatest facilities are thus presented for optical and photographical experiments1 for in the latter case the whole room can be used as a camera obscura4 b4 ,the ,dome4 ,the roof of the observatory is #bj feet square4 ,the angles are filled in solid1 and a circular space #ae feet in diameter is left to be covered by the revolving dome4 ,although such a construction is architecturally weak and liable to lose its level1 yet the great advantages of having the building below square1 and the usefulness of the corners1 determined its adoption1 the disadvantages being overcome by a very light dome4 ,the dome is #af feet in outside diameter1 and rises to a height of #e feet above its base4 ,it is1 therefore1 much flatter than usual1 in fact1 might have been absolutely flat1 with this method of mounting4 ,it would then have been liable1 however1 to be crushed in by the deep winter snows4 ,it consists of #cb ribs1 arcs of a circle1 uniting at a common centre above4 ,each one is formed of two pieces of thin whitewood1 b1 ,fig4 #ci1 fastened side by side1 with the best arrangements of the grain for strength4 ,they are three inches wide and one inch thick at the lower end1 and taper gradually to #b-a_/b by #a4 ,illustration3 ,fig4 #ci4 ,joints in ,tin of ,dome4 ,over these ribs tinplate is laid in triangular strips or gores1 about #ah inches wide at the base1 and #aj feet long4 ,where the adjacent triangles of tin a a meet1 they are not soldered1 but are bent together4 ,this allows a certain amount of contraction and expansion1 and is water-proof4 ,it strengthens the roof so much1 that if the ribs below were taken away1 this corrugated though thin dome would probably sustain itself4 ,the tin is fastened to the dome ribs b by extra pieces c inserted in the joint and doubled with the other parts1 while below they are nailed to the ribs4 ,in the figure the tin is represented very much thicker than it is in reality4 ,this dome1 although it has #bej square feet of surface1 only weighs #bej pounds4 ,that at the ,cambridge 7,massachusetts7 ,observatory1 #bi-a_/b feet in diameter1 weighs #bh1jjj pounds4 ,the slit or opening is much shorter than usual1 only extending half way from the base towards the summit4 ,it is in reality an inclined window1 #b-a_/b feet wide at the bottom1 #a-a_/b wide at the top1 and #d feet long4 ,it is closed by a single shutter1 as seen in ,fig4 #cg1 and this when opened is sustained in position by an iron rod furnished with a hinge at one end and a hook at the other4 ,the principal peculiarity of the dome1 the means by which it is rotated1 remains to be described4 ,usually in such structures rollers or cannon balls are placed at intervals under the edge1 and by means of rack work1 a motion of revolution is slowly accomplished4 ,here1 on the contrary1 the whole dome b b b 7,fig4 #dj7 is supported on an arch h h h1 carrying an axis a at its centre1 around which a slight direct force1 a pull with a single finger1 will cause movement1 and by a sudden push even a quarter of an entire revolution may be accomplished4 ,it is desirable1 however1 to let it rest on the edge b b1 when not in use4 ,at c there is an iron catch on the arch1 by which the lever e1 that raises the dome1 is held down4 ,the fulcrum is at d4 ,the lever is hinged near c1 so that when by being depressed it should have come in the way of the telescope below1 the lower half g can be pushed up1 the part from c toward d still holding the dome supported4 ,illustration3 ,fig4 #dj4 ,the ,dome ,arch4 ,the arch can be set across the observatory in any direction1 north and south1 east and west1 or at any intermediate position1 because the abutments where the ends rest1 are formed by a ring l l l1 fastened round the circular aperture1 through the stationary part of the roof4 ,when the telescope is not in use1 and the dome is let down1 so that there is no longer an interval of a quarter of an inch between it and the rest of the roof1 it is confined inside by four clamps and wedges4 ,otherwise1 owing to its lightness1 it would be liable to be blown away4 ,these clamps a1 ,fig4 #da1 are three sides of a square1 made of iron one inch square4 ,they catch above by a point in the wooden basis-circle of the dome b1 and below are tightened by the wedge c4 ,illustration3 ,fig4 #da4 ,a ,dome ,clamp4 ,when the dome is raised it is prevented from moving laterally and sliding off by three rollers1 one of which is seen at f1 ,fig4 #dj4 ,these catch against its inner edge1 and only allow slight play4 ,at first it was thought necessary to have a subsidiary half arch at right angles to the other to hold it up1 but that is now removed4 ,all the parts work very satisfactorily1 and owing to the care taken to get the roof-circle and basis-circle flat and level1 no leakage takes place at the joint1 and even snow driven by high winds is unable to enter4 c4 ,the ,observer's ,chair4 ,this is not a chair in the common acceptation of the word1 but is rather a movable platform three feet square1 capable of carrying two or more persons round the observatory1 and maintaining them in an invariable position with regard to the telescope eyepiece4 ,illustration3 ,fig4 #db4 ,the ,observer's ,chair4 ,its general arrangement is better comprehended from the sketch1 ,fig4 #db1 than from a labored description4 ,below1 it runs on a pair of wheels a 7one only is visible7 #i inches in diameter1 whose axles point to the centre of the circle upon which they run4 ,they are prevented from shifting outwards by a wooden railroad b1 b1 and inwards by the paling l1 l4 ,above1 the chair moves on a pair of small rollers c1 which press against a circular strip or track d1 d1 nailed around the lower edge of the dome opening4 ,access to the platform is gained by the steps e1 e4 ,attached to the railing of this platform1 and near it on the telescope1 are two tables 7not shown in the figure7 for eyepieces1 the sliding plateholder1 c4 #e4 ,,the ,,photographic ,,laboratory4 ,this section is divided into a1 ,description of the ,apartment2 and b1 ,photographic ,processes4 a4 ,description of the ,apartment4 ,the room in which the photographical operations are carried on1 adjoins and connects with the observatory on the southeast1 as is shown in ,figs4 #bh and #ch4 ,it is #i by #aj feet inside1 and is supplied with shelves and tables running nearly all the way round1 which have upon them the principal chemical reagents4 ,it is furnished1 too1 with an opening to admit1 from a heliostat outside1 a solar beam of any size1 up to three inches in diameter4 ,the supply of water is derived from rain falling on the roof of the building1 and running into a tank i1 ,fig4 #ch1 which will contain a ton weight4 ,the roof exposes a surface of #ecb square feet1 and consequently a fall of rain equal to one inch in depth1 completely fills the tank4 ,during the course of the year the fall at this place is about #cb inches1 so that there is always an abundance4 ,in order to keep the water free from contamination1 the roof is painted with a ground mineral compound1 which hardens to a stony consistence1 and resists atmospheric influences well4 ,the tank is lined with lead1 but having been in use for many years for other purposes1 is thoroughly coated inside with various salts of lead1 sulphates1 c4 ,in addition the precaution is taken of emptying the tank by a large stopcock when a rainstorm is approaching1 so that any accumulation of organic matter1 which can reduce nitrate of silver1 may be avoided4 ,it has not been found feasible to use the well or spring water of the vicinity4 ,the tank is placed close under the eaves of the building1 so as to gain as much head of water as is desirable4 ,from near its bottom a pipe terminating in a stopcock k1 ,fig4 #ch1 passes into the ,laboratory4 ,in the northeast corner of the room1 and under the tap is a sink for refuse water and solutions1 and over which the negatives are developed4 ,it is on an average about twelve feet distant from the telescope4 ,in another corner of the room is a stove1 resembling in construction an open fireplace1 but sufficient nevertheless to raise the temperature to #hj ,f4 or higher1 if necessary4 ,as a provision against heat in summer1 the walls and roof are double1 and a free space with numerous openings above is left for circulation of air1 drawn from the foundations4 ,the roof is of tinplate1 fastened directly to the rafters1 without sheathing1 in order that heat may not accumulate to such an extent during the day as to constitute a source of disturbance when looking across it at night4 ,for containing negatives1 which from being unvarnished require particular care1 there is at one side of the room a case with twenty shallow drawers each to hold eighteen4 ,they accumulate very rapidly1 and were it not for frequent reselections the case would soon be filled4 ,on some nights as many as seventeen negatives have been taken1 most of which were worthy of preservation4 ,not less than #aejj were made in #ahfb and '#fc4 b4 ,photographic ,processes4 ,in photographic manipulations ,i have had the advantage of my father's long continued experience4 ,he worked for many years with bromide and chloride of silver in his photo-chemical researches 7,journal of the ,franklin ,institute1 #ahcg71 and when ,daguerre's beautiful process was published1 was the first to apply it to the taking of portraits 7,phil4 ,mag41 ,june1 #ahdj7 in #ahci2 the most important of all the applications of the art4 ,subsequently he made photographs of the interference spectrum1 and ascertained the existence of great groups of lines ,m1 ,n1 ,o1 ,p1 above ,h1 and totally invisible to the naked eye 7,phil4 ,mag41 ,may1 #ahdc74 ,the importance of these results1 and of the study of the structure of flames containing various elementary bodies1 that he made at the same time1 are only now exciting the interest they deserve4 ,in #ahej1 when his work on ,physiology was in preparation1 and the numerous illustrations had to be produced1 ,i learnt microscopic photography1 and soon after prepared the materials for the collodion process1 then recently invented by ,scott ,archer4 ,we produced in #ahef many photographs under a power of #gjj diameters1 by the means described in the next section4 ,at first the usual processes for portrait photography were applied to taking the ,moon4 ,but it was soon found necessary to abandon these and adopt others4 ,when a collodion negative has to be enlarged--and this is always the case in lunar photography1 where the original picture is taken at the focus of an object glass or mirror--imperfections invisible to the naked eye assume an importance which causes the rejection of many otherwise excellent pictures4 ,some of these imperfections are pinholes1 coarseness of granulation in the reduced silver1 liability to stains and markings1 spots produced by dust4 ,these were all avoided by washing off the free nitrate of silver from the sensitive plate1 before exposing it to the light1 and again submitting it to the action of water1 and dipping it back into the nitrate of silver bath before developing4 ,the quantity of nitrate of silver necessary to development when pyrogallic acid is used1 is however better procured by mixing a small quantity of a standard solution of that salt with the acid4 ,the operation of taking a lunar negative is as follows4 ,the glass plates #b-c_/d #c-a_/d inches are kept in nitric acid and water until wanted4 ,they are then washed under a tap1 being well rubbed with the fingers1 which have of course been properly cleaned4 ,they are wiped with a towel kept for the purpose4 ,next a few drops of iodized collodion are poured on each side1 and spread with a piece of cotton flannel4 ,they are then polished with a large piece of this flannel1 and deposited in a close dry plate box4 ,this system of cleaning with collodion was suggested by ,major ,russel1 to whose skilful experiments photography is indebted for the tannin process4 ,it certainly is most effective1 the drying pyroxyline removing every injurious impurity4 ,there is never any trouble from dirty plates4 ,the stock of plates for the night's work1 a dozen or so1 being thus prepared1 one of them is taken1 and by movement through the air is freed from fibres of cotton4 ,it is then coated with filtered collodion being held near the damp sink4 ,the coated plate1 when sufficiently dry1 is immersed in a #dj grain nitrate of silver bath1 acidified with nitric acid until it reddens litmus paper4 ,the exact amount of acid in the bath makes in this 8,washed ,plate ,process0 but little difference4 ,when the iodide and bromide of silver are thoroughly formed the plate is removed1 drained for a moment1 and then held under the tap till all greasiness1 as it is called1 disappears4 ,both front and back receive the current in turn4 ,it is then exposed1 being carried on a little wooden stand1 ,fig4 #dc1 covered with filtering paper to the telescope1 and deposited on the sliding plateholder which has been set to the direction and rate of the moon1 while the plate was in the bath4 ,the time of exposure is ascertained by counting the beats of a half-second pendulum4 ,the method by which exposure without causing tremor is accomplished1 is as follows3 ,a yellow glass slides through the eyepiece-holder1 ,fig4 #cc1 just in front of the sensitive plate1 and is put in before the plate4 ,the yellow-colored moon is centred on the collodion film1 and the clepsydra and slide are set in motion1 the mass of the telescope being at rest4 ,a pasteboard screen is put in front of the telescope1 and the yellow glass taken out4 ,after #bj seconds the instrument remaining still untouched and motionless1 the screen is withdrawn1 and as many seconds allowed to elapse as desirable4 ,the screen is then replaced and the plate taken back to the photographic room4 ,illustration3 ,fig4 #dc4 ,plate ,carrier4 ,after being again put under the tap to remove any dust or impurity1 it is dipped into the nitrate bath for a few seconds4 ,two drachms of a solution of protosulphate of iron #bj grains1 acetic acid #a drachm1 and water #a ounce1 is poured on it4 ,as soon as the image is fairly visible this is washed off1 and the development continued if necessary with a weak solution of pyrogallic acid and citro-nitrate of silver--pyrogallic and citric acids each #a_/e grain1 nitrate of silver #a_/aj grain1 water #a drachm4 ,in order to measure these small quantities standard solutions of the substances are made1 so that two drops of each contain the desired amount4 ,they are kept in bottles1 through the corks of which pipettes descend to just below the level of the liquid4 ,this avoids all necessity of filtering1 and yet no blemishes are produced by particles of floating matter4 ,illustration3 ,fig4 #dd4 ,pipette ,bottle4 ,during the earlier part of the development1 when the protosulphate of iron is on the film1 an accurate judgment can be formed as to the proper length of time for the exposure in the telescope4 ,if the image appears in #aj seconds1 it will acquire an appropriate density for enlargement in #de seconds1 and will have the minimum of what is called fogging and the smallest granulations4 ,if it takes longer to make its first appearance the exposure must be lengthened1 and vice versa4 ,illustration3 ,fig4 #de4 ,developing ,stand4 ,the latter part of the development1 when re-development is practised1 is purposely made slow1 so that the gradation of tones may be varied by changing the proportion of the ingredients4 ,as it would be tiresome and uncleanly to hold the plates in the hand1 a simple stand is used to keep them level4 ,it consists of a piece of thin wood a1 ,fig4 #de1 with an ordinary wood screw1 as at b1 going through each corner4 ,four wooden pegs1 as at c1 furnish a support for the plate d4 ,by the aid of this contrivance and the washing system1 ,i seldom get my fingers marked1 and what is much more important1 rarely stain a picture4 ,when the degree of intensity most suitable for subsequent enlargement is reached1 that is1 when the picture is like an overdone positive1 the plate is again flooded with water1 treated with cyanide of potassium or hyposulphite of soda1 once more washed and set upon an angle on filtering paper to dry4 ,it is next morning labelled1 and put away unvarnished in the case4 ,to the remark that this process implies a great deal of extra trouble1 it can only be replied that more negatives can be taken on each night than can be kept1 and that1 even were it not so1 one good picture is worth more than any number of bad ones4 ,although the above is the method at present adopted1 and by which excellent results have been obtained1 it may at any moment give place to some other1 and is indeed being continually modified4 ,the defects it presents are two--first1 the time of exposure is too long1 and second1 there is a certain amount of lateral diffusion in the thickness of the film1 and in consequence a degree of sharpness inferior to that of the image produced by the parabolic mirror4 ,the shortest time in which the moon has been taken in this observatory has been one-third of a second1 on the twenty-first day1 but on that occasion the sky was singularly clear1 and the intrinsic splendor of the light great4 ,the full moon under the same circumstances would have required a much shorter exposure4 ,a person1 however1 who has put his eye at the focus of such a silvered mirror will not be surprised at the shortness of the time needed for impressing the bromo-iodide film2 the brilliancy is so great that it impairs vision1 and for a long time the exposed eye fails to distinguish any moderately illuminated object4 ,the light from #ahh square inches of an almost total reflecting surface is condensed upon #b square inches of sensitive plate4 ,occasionally a condition of the sky1 the reverse of that mentioned above1 occurs4 ,the moon assumes a pale yellow color1 and will continue to be of that non-actinic tint for a month or six weeks4 ,this phenomenon is not confined to special localities1 but may extend over great tracts of country4 ,in ,august1 #ahfb1 when our regiment was encamped in ,virginia1 at ,harper's ,ferry1 the atmosphere was in this condition there1 and was also similarly affected at the observatory1 more than #bjj miles distant4 ,as to the cause1 it was not forest or prairie fires1 for none of them of sufficient magnitude and duration occurred1 but was probably dust in a state of minute division4 ,no continued rain fell for several weeks1 and the clay of the ,virginia roads was turned into a fine powder for a depth of many inches4 ,the ,upper ,potomac river was so low that it could be crossed dry-shod4 ,on a subsequent occasion when the same state of things occurred again1 ,i exposed a series of plates 7whose sensitiveness was not less than usual1 as was proved by a standard artificial flame7 to the image of the full moon in the #ae-a_/b inch reflector for #bj seconds1 and yet obtained only a moderately intense picture4 ,this was #dj times as long as common4 ,upon all photographic pictures of celestial objects the influence of the atmosphere is seen1 being sometimes greater and sometimes less4 ,to obtain the best impressions1 just as steady a night is necessary as for critical observations4 ,if the image of ,jupiter is allowed to pass across a sensitive plate1 a streak almost as wide as the planet is left4 ,it is easily seen not to be continuous1 as it would have been were there no atmospheric disturbances1 but composed of a set of partially isolated images4 ,besides this planet1 ,i have also taken impressions of ,venus1 ,mars1 double stars1 c4 ,an attempt has been made to overcome lateral diffusion in the thickness of the film by the use of dry collodion plates1 more particularly those of ,major ,russel and ,dr4 ,hill ,norris4 ,these present1 it is true1 a fine and very thin film during exposure1 but while developing are so changed by wetting in their mechanical condition that no advantage has resulted4 ,it was while trying them1 that ,i ascertained the great control that hot water exercises over the rapidity of development1 and time of exposure1 owing partly no doubt to increase of permeability in the collodion film1 but also partly to the fact that chemical decompositions go on more rapidly at higher temperatures4 ,i have attempted in vain to develop a tannin plate when it and the solutions used were at #cb ,f41 and this though it had had a hundred times the exposure to light that was demanded when the plate was kept at #adj ,f4 by warm water4 ,protochloride of palladium1 which ,i introduced in #ahei1 is frequently employed when it is desired to increase the intensity of a negative without altering its thickness4 ,this substance will augment the opacity #af times1 without any tendency to injure the image or produce markings4 ,it is only at present kept out of general use by the scarcity of the metal4 #f4 ,,the ,,photographic ,,enlarger4 ,two distinct arrangements are used for enlarging1 a1 for ,low ,powers varying from #a to #be2 and b1 for ,high ,powers from #ej to #gjj diameters4 a4 ,low ,powers4 ,the essential feature in this contrivance is an entire novelty in photographic enlargement1 and it is so superior to solar cameras1 as they are called1 that they are never used in the observatory now4 ,it consists in employing instead of an achromatic combination of lenses1 a mirror of appropriate curvature to magnify the original negatives or objects4 ,the advantages are easily enumerated1 perfect coincidence of visual and chemical foci1 flat field1 absolute sharpness of definition4 ,if the negative is a fine one1 the enlarged proofs will be as good as possible4 ,illustration3 ,fig4 #df4 ,the ,photographic ,enlarger4 ,the mirror is of #i inches aperture1 and #aa-a_/b inches focal length4 ,it was polished on my machine to an elliptical figure of #h feet distance between the conjugate foci1 and was intended to magnify #g times4 ,at first the whole mirror was allowed to officiate1 the object being illuminated by diffused daylight4 ,but it was soon apparent1 that although a minute object placed in one focus was perfectly reproduced at the other1 seven times as large1 yet a large one was not equally well defined in all its parts4 ,i determined then to produce the enlarged image by passing a solar-beam #a-a_/b inch in diameter through the original lunar negative--placed in the focus nearest to the mirror--and allowing it to fall on a portion of the concave mirror1 #a-a_/b inch in diameter1 at one side of the vertex4 ,being reflected1 it returns past the negative1 and goes to form the magnified image at the other focus of the ellipse4 ,in ,fig4 #df1 a is the heliostat on a stone shelf outside2 b a silvered glass mirror1 to direct the parallel rays through c1 the negative2 d is the elliptical mirror2 e an aperture to be partly closed by diaphragms2 f a rackwork movement carried by the tripod g2 the curtain h h shuts out stray light from the interior of the observatory4 ,the aperture i is also diaphragmed1 but is shown open to indicate the position of the heliostat1 the shelf of which joins the outside of the building at l4 ,the dotted line points out the course of the light1 which coming from the sun falls on the heliostat mirror a1 then on b1 through c to d1 and thence returning through e to the sensitive plate in the plate holder k4 ,the distance of this last can be made to vary1 being either two feet or twenty-eight feet from d4 ,in the latter case a magnifying power of about #be results1 the moon being made three feet in diameter4 ,the sensitive plate is carried by a frame1 which screws to the side wall of the building1 and can be easily changed in position4 ,the focussing is accomplished by the rack f4 ,where so small a part 7#a-a_/b inch7 of the surface of the mirror is used1 a rigid adherence then to the true foci of this ellipse is not demanded1 the mirror seeming to perform equally well whether magnifying #g or #be times4 ,theoretically it would seem to be limited to the former power4 ,if instead of placing a lunar photograph1 which in the nature of the case is never absolutely sharp1 at c1 some natural object1 as for instance a section of bone1 is attached to the frame moved by f1 then under a power of #be times it is as well defined as in any microscope1 while at the same time the amount of its surface seen at once is much larger than in such instruments1 and the field is flat4 ,if the intention were1 however1 to make microscopic photographs1 a mirror of much shorter focal length would be desirable1 one approaching more to those of ,amici's microscopes4 ,by the aid of a concave mirror used thus obliquely1 or excentrically1 all the difficulties in the way of enlarging disappear1 and pictures of the greatest size can be produced in perfection4 ,i should long ago have made lunar photographs of more than #c feet in diameter1 except for the difficulties of manipulating such large surfaces4 ,in order to secure a constant beam of sunlight a heliostat is placed outside the observatory1 at its southeast corner f1 ,fig4 #ch4 ,this beam1 which can be sent for an entire day in the direction of the earth's axis1 is intercepted as shown at b1 ,fig4 #df1 and thus if needed an exposure of many hours could be given4 ,the interior of the observatory and photographic room being only illuminated by faint yellow rays1 no camera box is required to cut off stray light4 ,the eye is by these means kept in a most sensitive condition1 and the focussing can be effected with the critical accuracy that the optical arrangement allows1 no correction for chromatic aberration being demanded4 ,i have made all the parts of this apparatus so that they can be easily separated or changed4 ,the flat mirrors are of silvered glass1 and are used with the silvered side toward the light1 to avoid the double image produced when reflection from both sides of a parallel plate of glass is permitted4 ,the large concave mirror happens to be of speculum metal1 but it can be repolished if necessary by means of a four inch polisher1 passed in succession over every chord of the face4 ,a yellow film of tarnish easily accumulates on metal specula if they are not carefully kept1 and decreases their photographic power seriously4 ,of the making of ,reverses4--,in addition to the use of the ,enlarger for magnifying1 it is found to have important advantages in copying by contact4 ,the picture of the image of the moon produced in the telescope is negative1 that is1 the lights and shades are reversed4 ,in enlarging such a negative reversal again takes place1 and a positive results4 ,this positive cannot1 however1 be used to make prints on paper1 because in that operation reversing of light and shade once more occurs4 ,it is necessary then at some stage to introduce still another reversal4 ,this may be accomplished either by printing from the original negative a positive1 which may be enlarged1 or else printing from the enlarged positive a negative to make the paper proofs from4 ,in either case a collodion film1 properly sensitized1 is placed behind the positive or negative1 and the two exposed to light4 ,if diffused light or lamplight is used1 the two plates must be as closely in contact as possible1 or the sharpness of the resulting proof is greatly less than the original4 ,this is because the light finds its way through in many various directions4 ,if the two plates1 however1 are placed in the cone of sunlight coming from the ,enlarger1 and at a distance of fifteen or twenty feet from it1 the light passes in straight lines and only in one direction through the front picture to the sensitive plate behind4 ,i have not been able to see under these circumstances any perceptible diminution in sharpness1 though the plates had been #a_/af of an inch apart4 ,it is perfectly feasible to use wet collodion instead of dry plates1 no risk of scratching by contact is incurred1 and the whole operation is easily and quickly performed4 ,the time of exposure1 #e seconds1 is of convenient length1 but may be increased by putting a less reflecting surface or an unsilvered glass mirror in the heliostat4 ,a diaphragm with an aperture of half an inch if placed at e1 ,fig4 #df1 to shut out needless light1 and avoid injuring the sharpness of the reverse by diffusion through the room4 ,in enlarging other diaphragms are also for the same reason put in the place of this one4 ,for a half moon for instance1 a yellow paper with a half circular aperture1 whose size may be found by trial in a few minutes1 is pinned against e4 ,the enlarged pictures obtained by this apparatus are much better than can be obtained by any other method known at present4 ,the effect1 for instance1 of a portrait1 made life-size1 is very striking4 ,some astronomers have supposed that advantages would arise from taking original lunar negatives of larger size in the telescope1 that is1 from enlarging the image two or three times by a suitable eyepiece or concave achromatic1 before it reached the sensitive plate4 ,but apart from the fact that a reflector would then have all the disadvantages of an achromatic1 the atmospheric difficulties1 which in reality constitute the great obstacle to success1 would not be diminished by such means4 ,the apparent advantage1 that of not magnifying defects in the collodion1 is not of much moment1 for when development of the photographs is properly conducted1 and thorough cleanliness practised1 imperfections are not produced1 and the size of the silver granules is not objectionable4 b4 ,high ,powers4 ,although negatives of astronomical objects have not as yet been made which could stand the high powers of the arrangement about to be described1 yet they bear the lower powers well1 and give promise of improvement in the future4 ,photography of microscopic objects as usually described1 consists in passing a beam of light through the transparent object into the compound body of the microscope1 and receiving it on its exit from the eyepiece upon a ground glass or sensitive plate4 ,the difficulty which besets the instrument generally1 and interferes with the production of fine results1 arises from the uncertainty of ascertaining the focus or place for the sensitive plate4 ,for if the collodion film be put where the image on ground glass seems best defined1 the resulting photograph will not be sharp1 because the actinic rays do not form their image there1 but either farther from or nearer to the lenses1 depending on the amount of the chromatic correction given by the optician4 ,practically by repeated trials and variation of the place of the sensitive compound1 an approximation to the focus of the rays of maximum photographic intensity is reached4 ,illustration3 ,fig4 #dg4 ,microscope for ,photography4 ,during my father's experiments on light1 and more particularly when engaged in the invention of portrait photography1 he found that the ammonio-sulphate of copper1 a deep blue liquid1 will separate the more refrangible rays of light1 the rays concerned in photography1 from the rest4 ,if a beam of sunlight be passed through such a solution1 inclosed between parallel plates of glass1 and then condensed upon an object on the stage of a microscope1 a blue colored image will be formed on the ground glass1 above the eyepiece4 ,if the place of best definition be carefully ascertained1 and a sensitive plate put in the stead of the ground glass1 a sharp photograph will always result4 ,besides1 there is no danger of burning up the object1 as there would be if the unabsorbed sunlight were condensed on it1 and hence a much larger beam of light and much higher powers can be used4 ,the best results are attained when an image of the sun produced by a short focussed lens is made to fall upon and coincide with the transparent object4 ,in #ahef we obtained photographs of frog's blood disks1 navicula angulata1 and several other similar objects under a power of #gjj diameters1 excellently defined4 ,since then several hundreds of microscopic pictures have been taken4 ,in the figure1 a is the heliostat1 b a lens of three inches aperture1 c the glass cell for the ammonio-sulphate of copper1 d the object on the stage of the microscope e1 f the camera for the ground glass or sensitive plate4 ,above the figure the course of the rays is shown by dotted lines4 99 99 99 99 99 ,in concluding this account of a ,silvered ,glass ,telescope ,i may answer an inquiry which doubtless will be made by many of my readers1 whether this kind of reflector can ever rival in size and efficiency such great metallic specula as those of ,sir ,william ,herschel1 the ,earl of ,rosse1 and ,mr4 ,lassell8 ,my experience in the matter1 strengthened by the recent successful attempt of ,m4 ,foucault to figure such a surface more than thirty inches in diameter1 assures me that not only can the four and six feet telescopes of those astronomers be equalled1 but even excelled4 ,it is merely an affair of expense and patience4 ,i hope that the minute details ,i have given in this paper may lead some one to make the effort4 ,,hastings1 ,,westchester ,,county1 ,,new ,,york1 #ahfc4 ,postscript4--,since writing the above ,i have completed a photograph of the moon #ej inches in diameter4 ,the original negative from which it has been made1 bears this magnifying well1 and the picture has a very imposing effect4 ,,published ,,by ,,the ,,smithsonian ,,institution1 ,,washington ,,city1 ,,july1 #ahfd4