1S7<^.] to the 'Reproduction of 2Iaps and Flans. 91 



film was removed from the waxed glass j^late, and the side which had been 

 next to the glass was exposed under a reversed negative in the usual waj, 

 and, then, as in Albert's process, the back surface of the film was hardened 

 by exposure to light. After this, the film was attached under water to a 

 metal plate, preferably pewter, coated with india-rubber, and ^ squeegeed' into 

 perfect contact Tvith it. The bichromate salt was then removed by washing 

 and the plate was ready to be printed in an ordinary Albion printing 

 press. 



In this process the peculiarities were the use of chrome alum for 

 hardening the gelatine ; the separation of the colloid film from its original 

 support, by which perfect contact with the negative was secured, as well 

 as less risk of breakage of the latter ; the subsequent transference of the 

 film to a metal plate, by which the liability to breakage of glass plates in 

 the progress of printing was obviated, and, lastly, the substitution of vertical 

 instead of a scraping pressure in printing, by which the gelatine films were 

 not exposed to injury by wear and scraping of the surface. 



This process is still, I believe, largely practised and full details of it, 

 with various improvements suggested by Capt Abney, R. E., will be 

 found in the latter's excellent little work — " Instruction in FliotograpliyT 



About the same time, Herr Obernetter, of Munich, proposed another 

 process of the same kind offering some peculiarities, and said to produce 

 very satisfactory results. 



A sheet of glass is coated mth a mixture of gelatine, albumen, sugar 

 and bichromate of potash, dried and exposed to light under a negative. 

 The plate is then dusted over with finely powdered zinc, which attaches 

 itself only to the parts protected from the light and in proportion to the 

 amount of protection they have received. The plate is then heated to about 

 309° F., or exposed to light till the whole surface of the film has been 

 rendered insoluble. Before printing, the plates are treated with dilute 

 muriatic or sulphuric acid. By 'this operation the parts of the gelatine 

 film covered with zinc, are rendered, by the formation of hydrogen, sus- 

 ceptible of attracting water to a greater or less degree, while the other 

 portions, upon which no zinc has settled, are capable of receiving a fatty 

 ink. The printing is then proceeded with in the usual manner.* 



Since 1869, when these processes first began to come into practical use, 

 many methods of working have been introduced, chiefly in Germany and 

 France, but so far as known they are nearly all of them more or less 

 modifications of one or other of the above, merely differing in the manner 

 of preparing and hardening the gelatine film. A good deal of information 

 on the subject will be found in Husnik's '' Gesammtgehiet des Lichtdrucks,'" 

 Geymet's " Phototypie,^' Moock's " Traite pratique dHmpressions photo- 

 * '■ Thotographic News^' Vol. XTII, p. 483. 



