1882. 179 tivans: Na Vara Slee 
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 
invisibie image is intensified and brought out in allits 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 tsvvoo 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, asa 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 Lromide, 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 been made to many questions 
of great theoretical interest, which I hope to make the subject of a fu- 
ture article. It has seemed best to confine th’s article to the str'ctly 
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 of science. 
