1 882. 1T9 Tra7is. N. V. Ac. Scz. 



eral reduction ot the silver salt. Those who are familiar with photo- 

 graphic process will keep in mind the fact that in developing dry plates, 

 all the silver salt is in the film, while, in the wet plate process, it is cus- 

 tomary to mix some silver nitrate with the developer and flood the 

 plate with it. In the case of dry plates the fingers are not soiled dur- 

 ing this process, while with wet plates they are sure to be blackened 

 by the silver. [Plate developed.] 



The time is so limited that I cannot discuss and explain the chemical 

 process of developing a picture as fully as I would wish. I am quite 

 sure that if we were to spend the entire evening in considering this 

 single operation, there would be no lack of interest on the part of this 

 audience. In studying the process of development, explaining how the 

 invisible niiage is intensified and brought out in all its details, we are 

 dealing with chemical changes which are so slight as to almost baffle 

 our efforts to detect them. The light acts upon the molecules of the 

 the silver salt in the plate, perhaps only the re oo'tju part of a second of 

 time ; but that is enough to overcome, or in some way to weaken, the 

 force which binds together the constituent atoms. When the develop- 

 ing solution is applied, each particle of silver salt that has been thus 

 changed, acts, we may say, as a nucleus to start the action of the devel- 

 oper. The tendency of the latter, as already stated, is to reduce any 

 silver salt that may be present ; but if a soluble bromide, such as potas- 

 sium bromide, be present in sufficient quantity, this tendency is re- 

 strained, and no reduction will take place, unless the action is started 

 by the partially decomposed silver salt. The balance of the chemical 

 forces is so perfect in a well-made developer, that wherever there is a 

 molecule of silver bromide on the plate which the light has affected, 

 there decomposition takes place and black metallic silver is deposited, 

 while all the rest of the plate remains white. Thus every line and 

 every shadow and half-tint in an object is faithfully reproduced in the 

 photograph. 



After development there remains upon the plate a quantity of un- 

 changed bromide of silver, which must be removed or the light would 

 act upon it, and destroy the picture. The picture must, therefore, be 

 fixed by dissolving the unchanged silver salt in sodic hyposulphite. 

 [Plate fixed.] 



In the short account that has been given of the preparation and de- 

 velopment of dry plates, no allusion has b^en made to many questions 

 of great theoretical interest, which I hope to make the subject of a fu- 

 ture atticle. It has seemed best to confine th's article to the strictly 

 practical part of the subject ; and I now wish to speak more particu- 

 larly of the advantages of the dry-plate process, not for the photog- 

 rapher in business, but for the traveller and explorer, the naturalist 

 and the student in various branches ot science. 



