ILLUSTRATIONS. 



DESCRIPTION OF THE GELATINE PROCESS OF 

 PICTURE MAKING. 



As the art of reproducing photographs by the gelatine process 

 is quite recent and not yet very generally understood, a 

 few words in explanation of it may not be out of place 

 as an introduction to the description of the plates in 

 this book, which have been made wholly by this process. 



All photo-mechanical w^ork depends on two peculiar proper- 

 ties of gelatine. 1st : Warm water readily dissolves gela- 

 tine, while cold water only soaks or is absorbed by it 

 without dissolving any portion of it. 2nd : A mixture 

 of a solution of gelatine with bichromate of potassa 

 remains a simple mechanical mixture as long as it is 

 kept in the dark ; but when exposed to white light it 

 becomes a chemical compound, hard and impervious to 

 water. 



In the first place then, a thin film of this mixture of gela- 

 tine and bichromate is spread over a square of plate 

 glass in the photographer's dark room. A photographic 

 negative is then placed in close contact with this film, 

 and it is exposed to sun light. The film becomes hard 

 and insoluble under the light parts of the negative, 

 partially so under the half tints, and remains soft and 

 unchanged under the dark shades. After sufficient ex- 

 posure the plate is taken to the dark room and all the 

 uncombined bichromate washed out by cold water. It 

 is then in condition to print from, and may be taken 

 directly to the printing press. 



For each, copy that is taken from it the plate is sponged 

 with cold water and lightly wiped, leaving the hard 

 parts dry, the semi-hardened parts more or less damp, 

 and the soft parts quite wet. In this condition the 

 ordinary " greasy " ink of the lithographers will adhere 

 only to the dry parts of it and be repelled from the wet. 

 Rollers coated with this thick ink, first of the quality 

 called " coarse," then of " fine," are successively passed 



