128 SCIENTIFIC APPARATUS. 



When paper is washed over with the dichromate of an alkali, 

 dried and submitted to the vapour of aniline, a blackening will 

 be apparent. When light, however, has reduced the chromic 

 salt, the surface will remain unchanged. In Willis's aniline 

 process advantage is taken of this reaction. A map or plan is 

 placed over the sensitized paper, and after exposure to light, it 

 is submitted to the vapour of aniline. A print is thus obtained 

 at one operation without the aid of a negative picture. 



The methods of producing relief blocks for setting up type by 

 means of photography are various. One of the most successful is 

 by exposing a zinc plate coated with asphaltum beneath a nega- 

 tive, and then dissolving away those portions unchanged by light. 

 The zinc, which is uncovered, is acted upon by acid, and after cer- 

 tain precautions are taken, a depth is obtained sufficient to 

 allow an impression to be taken in the printing press as if it were 

 a wood-cut. Gillot's is a good example of a photo-relief process. 



There are other metallic and organic compounds which are 

 acted upon by light. The ferric and uranic salts are all more or 

 less reduced to the ferrous and uranous state when in the pre- 

 sence of organic matter, such as paper, gelatine, &c. Some 

 examples of early prints produced by their aid will be found in 

 the exhibition. 



Vidal's polychromatic process is a carbon process : a negative of 

 the subject is first obtained, and others are reproduced from it, 

 the number depending on the number of colours to^be printed. 

 When it is desired that any portion of a print should have some 

 particular tint, one of the negatives is utilised, and all but the 

 required portions of it painted out with any opaque paint. A 

 gelatine picture of the necessary tint is printed, and by means of 

 a system of registration is placed accurately over a print of the 

 whole subject, which has been previously made in neutral or other 

 suitable colour as a ground. Thus, one after another, the films of 

 various transparent colours are super-imposed till the polychromatic 

 picture is built up. The process is analogous to chromo-lithography. 



W. DE W. ABNEY. 



