7G J. Watorhouse — The Application of Photogrnphj [No. 2, 



salt to be reduced in the parts exposed to the light. The prints are fixed 

 first with hyposulphite of soda, and then with oxalate of potash and finally 

 washed with water. 



Gollo-chr ornate printing. — We now come to the more important pro- 

 cesses depending on the reaction of the salts of chromium, particularly the 

 alkaline bichromates, on gelatine, gum, albumen and other colloid substances 

 under the influence of light, whereby these substances become more or less 

 insoluble in and unabsorbent of water in proportion to the amount of the 

 action of light, and further acquire the property of taking up greasy ink 

 and not attracting j)lumbago or other fine dry powder, also in proportion to 

 the amount of the action of light upon them. 



This simple reaction, only partially discovered in 1839 by Mungo 

 Ponton, was first worked out and turned to practical account, some tw^elve 

 years afterwards, by Fox Talbot in his process of photoglyphic engraving ; 

 and after him Pretsch and, notably, Poitevin employed it in processes which 

 have been the foundation of nearly all the modern methods of permanent 

 photographic printing. 



The simplest of all these processes, and one which may render useful 

 service in the cases already noticed where only a few copies are required, 

 was one of the first published by Poitevin. It consists in coating paper 

 with a mixture of albumen, gum, or gelatine and bichromate of potash, 

 coloured with Indian ink or any other suitable pigment ; or, if preferred, 

 the paper may be coated with coloured gelatine and then made sensitive 

 in a separate bath of bichromate of potash, and this is sometimes the best 

 method, because the paper will not keep good for long in its sensitive state. 

 The sensitive coloured paper is exposed under a very clear line negative 

 in a copying frame for a few minutes, and then taken out and plunged 

 into water, either hot or cold, according as gelatine, gum or albumen have 

 been used. The unaltered colloid in the lights of the print, which have 

 been protected from the light under the dark parts of the negative, dissolves 

 in the water, leaving a clear image in pigment on a white ground. 



This simple method is capable of extensive use in copying maps or 

 topographical sketches, but is only applicable to subjects in line, well drawn 

 in black and white in accordance with the rules in Sect. III. These prints 

 have the advantage of being quite permanent and, as the collo-chromate 

 mixture is more sensitive to light than the chloride of silver, they can be 

 produced at a quicker rate than the silver prints, and are, of course, cheaper 

 on account of the inexpensiveness of the materials used. 



For reproducing subjects in half tones a different procedure must be 

 followed. In the process just described the exposure to light and the develop- 

 ment of the print by washing are effected on the coloured side of the 

 paper, and as the light can act with full power through the clear spaces on 



