ii2 PHOTOGRAPHY FOR NATURALISTS. 



transparency than about making a simple carbon print ; 

 and, if we assume that the photographer buys his paper 

 supports ready coated, only one additional operation, 

 that of coating the glass with a substratum which, like 

 that on the paper support, shall be insoluble in hot 

 water. The fact that this single operation, simple as it 

 is, seems to deter many carbon workers from 

 transparency work, leads one to suppose that the 

 instructions of the textbooks on this point are vague' or 

 unintelligible. On referring to two well-known books on 

 the carbon process the writer finds that the instructions 

 are to coat the glass with a substratum of gelatine 

 rendered insoluble by the addition of .potassium 

 bichromate or chrome alum. The formula runs some- 

 what as follows : Gelatine i ounce (soaked in cold water 

 for some hours and then dissolved by aid of a hot- water 

 bath), water 20 ounces, add potassium bichromate 20 

 grains, and water 2 ounces. 



The thought that will immediately occur to the 

 amateur will be, " How are glass plates coated ? " and 

 he will promptly conjure up the vision of a pneumatic 

 holder, and a fairly large pool of liquid on the glass, 

 tilted nervously from one corner to the other, until it 

 lands itself on the floor or on the operator's garments. 

 The mere prospective stickiness of gelatine under such 

 circumstances would deter any but the most earnest 

 worker. Moreover, in the ordinary amateur's hands, 

 the dried result would probably be a conglomerate of 

 dust and yellow streakiness. Fortunately, the coating 

 of glass for carbon transparencies does not require the 



