154 



PHOTOl.KAl'HY 



PHOTOGRAVURE 



that they are only observable when a refracting 

 teleoco|te is employed. Hence the use of reflecting 

 telesco|M's for this work has been suggested ; but, 

 although by this means some of the fault- men- 

 tioned are banished, other inconvenience- arise 

 which form oh-tacle- to good work. Pr Muggins 

 has done much valuable work in photographing 

 the spectra of the heavenly bodies. 



Ihiniliililij nf I'liiiliHirii/iliii- I'rintt. Exjierience 

 has proved that silver prints, however carefully 

 luepared. cannot be distended upon for permanency. 

 Much vexation hits frequently arisen from tiie 

 fading of these, and on this account they are no 

 longer used for liook illustration. They will keep 

 U-ttcr unmounted than mounted, and they should 

 lie kept in a dry situation, as damp increases their 

 tendency to fade. Platinotype prints are believed 

 to lie permanent by those best able to judge. 

 Autot\|M', \Voodburyt\|ie, and other prints in pig- 

 inentcd gelatine arc |>ermanent if stable colours are 

 employed, and of course those obtained by any of 

 the piioto mechanical processes are certainly so 

 when printed, as they usually are, in lithographic 

 or printers' ink. 



mistrltaneotis Applications and Improvements. 

 The report that the art of photographing in the 

 colours of nature has lieen discovered crops up year 

 after year with curious jicrsistency, and may lie 

 generally traced to the work of unscrupulous persons 

 who seek to deceive the public for their own advan- 

 tage. It is difficult to see how the much-talked of 

 photographs in colours a- popularly understood run 

 ever lie achieved, liy the introduction of specially 

 prepared gelatine dr\ plate- known as iirlhnrhni- 

 viiitirC right colour' i or imirtiriinitii-(' equal colour'), 

 Imth very vague terms it is possible to reproduce 

 colours in their true shade relation to one another. 

 For instance, suppose that one seeks to photograph 

 by ordinary plates a blue vase containing yellow 

 flowers. In the resulting picture the vase will lie 

 w bite and the flowers will he black. But if we use 

 Lsochromatic plates the vase will be rendered as a 

 gray and the flowers will appear almost white, 

 which is obviously more true to the way in which 

 the eye olisi-rves the original. This change in the 

 behaviour of the sensitive surface is brought alwmt 

 by adding to the gelatine emulsion of which it is 

 composed a minute quantity of certain dyes. 

 Vogel in IH73 discovered that certain coal-tar 

 lye, produce a change of sensitiveness in silver 

 com|Mtunds -. and in the same year Tailfer and 

 C'layton secured a [latent for the preparation of 

 colour-sensitive plate- prepared by the same 

 agency. They n-c an ammoniacal solution of 

 cosine : and plates made under the patent are 

 now supplied commercially. They are much 

 used in copying all coloured objects, such as 

 oil paintings ; and there is little doubt that they 

 will play an important part in sidereal photo 

 graphy, in the registration of coloured stars. 

 Meteoi.,|ogi-t- ate now dc|iending u|in photo- 

 graphy to furnish them with cloud studies, and 

 with picture- of Lightning i ii. v. I. A study of tin- 

 latter is likely to extend our knowledge concerning 

 the phenomena connected with thunderstorms, and 

 hasalready elucidated a few problems. Muybridge, 

 in the I' nitcd States, introduced the system of 



analysing by ans of photography tin; motion- of 



a trotting horse, running dog, BO. liy means of 

 special appanitu- be found it |iossible to take a 

 do/en < oil-cent ivc pictures of a .-ingle movement. 

 M.nev of I'aii- further develoiied this phase. In 

 the cinematograph a -erics of photographs taken in 

 rapid succession from a moving scene i- thrown on 

 a screen by the aid of a magic-lantern in the same 

 rapid manner in which they 'were taken, the result 

 being a life like reproduction of the original scene. 



Photography through an opaque object by means 



of the Rontgen rays (see RONTGEX) has already 

 been of immense use in surgery in ti \ing tin- exact 

 position of bullets, &c., in the human body as a 

 preliminary to their extraction. 



A 1 1 hough photographing in colours has long occu- 

 pied the attention ot scientific photographers, no 

 direct method of doing so has yet been described. 

 One, and perhaps the most important of the in- 

 direct methods, is as follows : Three separate 

 (colourless) negatives of a coloured object are 

 taken through coloured screens. From these posi- 

 t iv .- are taken, and colour is supplied by mean- of 

 inks or dyes. When these three-coloured mono 

 chromatic positives are su[>eriiii|Mised, and seen b\ 



projection on a screen or through a photo-ohromo- 



sn>| ><. a coloured image of the original is the result. 

 BIBLIOGRAPHY. Chapman Jones, Science and Practice 



>f I'hol&jraphff ; Abney, fntttructioti in Photograithti, 

 and I'hotntjrn i>hn irith Einnlxinnx ; Kdcr. J/'Wrrn />;// 

 J'lnli.i : Iliiliin-nii and Abney, Art and I'ructife of Si/rrr 

 J'rittiuuj ; Kohinson, PiftHn-matimi >>! rhn 

 J. K. Sawyer, The A. B.C. Guidr t Aulolii/f ; 

 Manual of Carbon Prorru ; Meldola, J'h 

 Chemiitry ; Chadwick, The Matjic jMttlern Manual; 

 Hepworth, FhobHiraiihy fur Amateuri and Boot of the 

 Lantern; Wilkinson, I'liolii-ewimriug and Ltihouraphti ; 

 Honckhoven, Photoitrnp/iii- Ojitii-t ; Burton, O/itirx I'm- 

 Photoijraphert ; Dallmcvcr. <'l.-iict and Ute ni I'liolo- 

 graphic Ltntrt ; Harrison, /{i.ifort/ of Photography ; 

 Sclinauss, Collotype and Pholo-lit/iofiniyihy. See alto 

 periodical literature, and especially the annual publica- 

 tions. Year-book of Photoiiraphii, Almanac, Ac. 



l'hotti;r;n liro. liy this process the finest 

 possible results are obtained, but the expense of 

 producing pictures by its aid, which is akin to the 

 operation of copperplate printing, limits it* use to 

 high-class book-work. It is also used by lioussod, 

 Valadon & Co., by Durand of Paris, by the 

 Autotype Company, by Messrs Annan & Swan 

 of Glasgow, and others for the production of 

 large pictures which rival the finest steel engrav- 

 ings in their delicacy and finish. Photographs 

 can be reproduced in this form, but the process 

 seems to be more largely employed for obtaining 

 engraving like copies of celebrated pictures. The 

 process is so perfect that every touch of the painter's 

 brush is clearly seen in the copy, ami even the 

 upstanding ridges of paint in the bolder touches 

 are rigidly reproduced. There are naturally dilb-r 

 ent ways by which printing plates for use in this 

 process are made, and a brief description of two 

 met hods only must here suffice to give an indication 

 of the line of operations. (1)A gelatine relief is 

 obtained by exposing hichromatcd gelatine to the 

 action of light beneath a negative. Hut the gela- 

 tine employed is mingled with a certain quantity 

 of graphite (black lead) in a more or less granular 

 form. This addition causes the resulting relief to 

 have a surface which is granular in character, and 

 which is also a conducting one to electricity. If 

 therefore the relief lie placed in an electrotype 

 bath it will speedily liecome covered with a deimsit 

 of cop|icr. From the coii|ier plate so formed copies 

 on pa|-r can lie obtained liy the usual copperplate 

 printing proce-s. (>} A bichroniated gelatine print 

 alive in character is developed upon the 

 s|M-cially prepared surface of a copper plate, which 

 is then subjected to the action of a solution of 

 pel-chloride of iron. This penetrates the gelatine 

 iiion- or less i|iiick!Y according to its varying thick- 

 lie--, and then attacks the copper, which is eaten 

 awiiv by the chemical action that ensues. Thus 

 in the end the copper plate bears on its surface 

 an etched image, iienetrating mme or le-s in depth 

 accoiding to tlie shadows and lights (> f the gelatine 

 image previously aflixcd to it. The plate is next 

 steel'-faced ami printed in the copperplate press. 

 ( It is often necessary to resort to hand-finishing in 

 order to get the finest results.) 



