108 J. Waterhouse — The Application of Fhotography [No. 2, 



Various attempts were made to improve on these processes, but un- 

 successfully, until M. Placet showed that it was necessary to adopt in them 

 the same principle of exposing on one side of the gelatine film and develop- 

 ing on the other, which, as we have seen, had previously been shown to be 

 necessary for the preservation of the half-tones in the pigment-printing 

 process, and for a similar reason. 



M. Placet indicated several ingenious ways of obtaining his results.* 

 They may, however, be briefly summed up as follows : 



A film of chromated gelatine is exposed under a transmitted positive 

 cliche, so that the light acts on the under side of the film ; this is done either 

 by covering the collodion side of the cliche itself with the sensitive coating, 

 or by using a thin transparent sheet of transfer collodion or mica as a sup- 

 port for it. After exposure to the light, the film is soaked in water, where- 

 upon those parts which have been protected from the light swell up in 

 proportion to the amount of the action of light upon them. By treating 

 the mould in relief thus obtained with metallic solutions, an electrotype 

 copy in copper may be produced, which can be printed from in the copper- 

 plate press. 



If a negative cliche is used, the unaltered gelatine must be dissolved or 

 a second electrotype must be made. 



M. Placet also suggested the employment of a sensitive surface which 

 would become solublf under the influence of light, such as a mixture of 

 gelatine, or other colloid, with perchloride of iron and tartaric acid, as 

 recommended by Poitevin for pigment-printing. In this case the solvent 

 acting on the exposed side hollows out the image, in the same way as an 

 etching fluid does on copper, but with the advantage that each line has only 

 the exact strength given to it by the intensity of the cliche. Or the altered 

 parts of the gelatine film may simply be swollen with cold water, producing 

 an image in relief. In either case, a mould is taken from the gelatine and 

 electrotyped, or copper may be deposited on the gelatine itself. 



By his process M Placet was able to obtain very perfect gradation of 

 shade in the half-tones of his pictures, with a fine natural grain produced 

 by some means which he did not divulge. He has lately, however, de- 

 scribed a method of producing the grain, which consists in plunging the gela- 

 tine plate into a solution of bichromate of potash and then treating it with 

 a solution of protosulphate of iron containing acetic acid.f The principle 

 he lays down is first to treat the gelatine with a solvent and then with a 

 solution of some substance that will tan or contract it. 



Messrs. Fontaine, Avet and Drivet have also proposed similar processes, 

 in which they have partially overcome the difiiculty of obtaining a proper 



* See Davanne, ' Les Tr ogres de la Photographie,' p. 185. 



t See ' Bulletin de la Soc. Franc, de Fhotographie,' Vol. XXIII, p. 130. 



