PHOTOGRAPHY 



147 



making Profiles by the agency of Light upon 

 Nitrate of Silver; with Observations by H. Davy.' 

 The misfortune was that no attempts made either 

 by Wedgwood or Davy to prevent the uncoloured 

 portions from being acted on by light (or, as we 

 now say, tojix the picture) were successful. 



Nicephore Niepce of Chalou-sur-Saone was the 

 first to enjoy the satisfaction of producing per- 

 manent pictures by the influence of solar radiations. 

 This was accomplished in 1814, .and the name chosen 

 to designate his process was Heliography. It con- 

 sisted in coating a piece of plated silver or glass 

 with asphaltum (bitumen). The plate so prepared 

 was then exposed in the camera olwseura for a length 

 of time, varying from four to six hours. Wherever 

 the light acted it rendered the asphaltum insoluble 

 in its usual solvents. Hence, on subsequent treat- 

 ment with one of these solvents, the shadows of the 

 image dissolved away, ami tin- lights were repre- 

 sented by the insoluble asphaltum remaining on 

 the plate. 



Daguerre appears to have begun in 1824 the 

 experiments which eventually led to the discovery 

 of the daguerreotype process. On Daguerre learn- 

 ing that Niepce was working in the same direction 

 as himself, the two formed a partnership in 1829. 

 The discovery of the Daguerreotype (ri.v.) was an- 

 nounced in January 1889, but (lie detail- of the pro- 

 cess were not made public till August of the same 

 It consists in exposing a metal plate covered 

 with iodide of silver for a suitable time in a photo- 

 graphic camera, the plate being afterwards trans- 

 ferred to a dark room, and exposed to the vapour 

 of mercury, which develops the latent image, it 

 being afterwards fixed. Although tlii-< process has 

 Woioi- almost obsolete, it was really the first which 

 was of any practical value, and experts all agree 

 that no other known process renders some subjects 

 e.g. the human face with such fidelity and 

 beauty. 



W. H. Fox Talbot read a paper to the Royal 

 Society on ' Photogenic Drawings ' on 31st January 

 1839, just six months previous to the publication 

 of Daguerre's process. He produced these in this 

 way : Writing-paper was steeped in a solution of 

 common salt (chloride of sodium), and dried. It 

 was then dipped in a solution of nitrate of silver, 

 which is changed by the action of the common salt 

 into the chloride of silver, some of the nitrate, 

 however, remaining unaltered. Paper so treated 

 is sensitive to light, so that when a fern-leaf, for 

 example, is placed close down upon it between two 

 plates of glass, and daylight is allowed to act on the 

 prepared surface, the paper blackens except where 

 it is covered, and thus a reversed picture of the 

 leaf on a black ground is obtained. This was then 

 placed over another sheet of paper, prepared in the 

 same way, and the light allowed to act through it. 

 Another picture of the leaf was thus produced, but 

 tlii- time with its lights and shades tne same s in 

 nature. The white image on a dark ground was 

 called by Fox Talbot a negative, and the print 

 froin it he called a positive. These terms are still 

 current in photography ; the negative image l>eing 

 produced in the camera, as the first and leading 

 oiiciation any nnrntar of positives being obtain- 

 able from it on paper, glass, or any other material 

 capable of forming a support for a photographic 

 image. The Calotypc pun-ess was patented by Fox 

 Talbot in 1841. In this process Talbot produced 

 his negative by preparing paper on the surface with 

 iodide of silver, subsequently washing it over with 

 a mixture of nitrate of silver, wit b gallic and acetic 

 acids, and then exposing it in the camera to the 

 object be wished to copy. The invisible image 

 or picture thus obtained was developed by aceto- 

 nitrate of silver and gallic acid. The paper nega- 

 tive was then rendered translucent with wax, and 



used for the production of many positives in the 

 way described above. The introduction of collo- 

 dion by Archer marked the next great step in 

 photographic progress, and this, known as the wet- 

 plate process, has been almost eclipsed by the gela- 

 tine dry plate now almost universally used. 



Photographic Apparatus. The most important 

 piece of apparatus used by the photographer is a form 

 of the Camera Obscurafq.v.), generally called simply 

 a camera, with its attached lens that throws the 

 image on a ground glass screen placed at the back, 

 to enable it to be sharply focused. A thin flat 

 box with a shutter, together called a dark slide or 

 ' back,' contains the sensitised plate. When the 

 picture is focused the screen is withdrawn ami the 

 1 back ' inserted in its place ; the shutter is then 

 drawn out, and the sensitised plate, which exactly 

 occupies the place of the glass screen, being now 

 exposed, receives the picture. In a brief time, 

 which nowadays varies from a fraction of one to 

 several seconds in a good light, the shutter is 

 closed, and the slide returned to a room illumi- 

 nated by a light not chemically active, generally 

 red, orange or yellow green, where the plate is 

 taken out and developed. 



The introduction of dry plates for photography, 

 which may be used in the camera .a long time after 

 their preparation, has had a great influence in 

 modifying apparatus; and more especially is ibis 

 true olf the photographic camera. Under the older 

 system ( wet-plate photography ) the plate had to be 

 used immediately after it hail been coated and 

 furnished with its sensitive film, or it became 

 useless. One dark slide or back, to hold the plate 

 during exposure was therefore all that was neces- 

 sary, for only one plate could be prepared and used 

 at a time, a dark room or tent being necessary 

 for the operations. But now that the plates will 

 keep almost indefinitely l>etween preparation and 

 use, any convenient number can be made ready 

 for insertion in the camera, to be exposed to the 

 action of the light one after another. For this 

 purpose what are called double dark backs are 

 employed, each holding two plates one on each 

 side, and each side being furnished with a light- 

 tight shutter which is drawn out so as to allow the 

 lens to cast the image on the plate inside as soon 

 as the back is fixed on the camera. 



Much ingenuity has l>een applied to camera con- 

 struction of late years, but, although many new 

 modes of carrying the plates and bringing them 

 under the influence of the lens have been devised, 

 the double-back system, as just described, is the 

 one most generally adopted when glass plates are 

 employed. Various cluing ing -boxes have been de- 

 vised, which contain a dozen or more plates, and 

 dispense with all but one dark slide, that is con- 

 structed to receive and discharge any plate of the 

 series at will. Hare's changing-box is the one 

 most generally known. This has a special form 

 of dark back, which can be charged witti one plate 

 from the box at a time, and is then inserted in the 

 camera for exposure. Recently, for small-sized 

 photographs, a device has l>een largely adopted 

 whereby the camera itself becomes a storage for 

 the plates, a simple mechanical arrangement per- 

 mitting the exposed plate to fall to the bottom 

 while another takes its place. 



Hut the most recent change in photographic 

 apparatus is due to the introduction, or rather the 

 revival, of sensitive films supported, not on glass, 

 hut on a flexible material. We have already seen 

 that Fox Talbot employed paper for his negatives ; 

 and, although paper was superseded by glass when 

 the collodion process came into existence, photo- 

 graphers were quick to recognise that such a brittle 

 material had serious disadvantages. Many experi- 

 menters endeavoured to produce or find some 



