CHAP, viii.] U-JSLIOGRAPUY PHOTOLITHOGRAPHY. 315 



M. Poitevin describes his process in the following terms : 



" To reproduce by printing ink the counter proof of a photo- 

 graphic drawing, on paper, lithographic stone, metallic substance or 

 wood, we apply to the surface intended for the reception of the 

 drawing, one or more coatings of a mixture in . equal parts of a 

 concentrated solution of albumen, ti brine, and gum arable, and a 

 concentrated solution of a chromate or bichromate with an earth}' or 

 metallic alkaline base not precipitating the organic matter of its 

 solution. Generally the bichromate of potassium is used ; after 

 desiccation or before, if the impression has to be made in the camera, 

 it is exposed to the light, and after the insulation, a uniform coating 

 of printing or coloured ink is. applied with a stopple or by a press ; 

 the ink is washed off : the ink only remains on the parts impressed 

 by the light." 



To obtain reliefs or depressions by the action of light alone, 

 without employing the corrosion of acids,, or the tool of the graver, 

 in a word to produce blocks engraved by light alone, the inventor 

 proceeds as follows : He spreads on whatever surface he is using a 

 uniform coating of a solution of gelatine impregnated with bichromate 

 of potash. After desiccation, a positive or negative proof obtained 

 by photography is placed on the coating and they are submitted to 

 direct or diffused sun-light. The same plate can be exposed in 

 the camera, 1 if a view from nature is to be taken : after exposure 

 the gelatine coating is immersed in water ; when all those parts 

 which have not received the luminous impresaion absorb this fluid, 

 the gelatine swells and gives the reliefs, while the unimpressed parts 

 which become very slightly moist, form the hollows. The reliefs 

 correspond to the shadows of the drawing; and the hollows to the 

 lights. 



By these means a block engraved on gelatine is obtained, which is 

 afterwards transformed into a block on copper by the ordinary processes 

 of electro-plating. 



The carbon process with printing inks only justifies its name by the 

 printing off the proofs by the means of impression with printing ink. 

 The permanence of the proofs is due to the use of this ink, into which 



1 The exposure required renders this method of obtaining a photograph practi- 

 cally useless. 



