Trans. N. Y. Ac. See. 178 May, 29, 
cess. Since the only essential part of this process is the suspension of 
minute particles of bromide of silver in a suitable fluid, which will dry 
and form a film upon a plate, we might precipitate the bromide as we 
did a few moments ago, only in a dark room, and rubit up in a mortar 
with a solution of gelatin or gum arabic, until a fine, milk-like emulsion 
is formed. A better method, however, and the one usually followed, I 
propose to carry out before you now. But first I must explain, that, 
unless the silver salt can be protected from the action of light, the 
process must be carried on in a room lighted only by a red light. With- 
out spending any time to explain the reason, I will simply state that 
potassic bichromate prevents the action of light upon the silver salt, 
and it is therefore possible to prepare an emulsion in daylight, if this 
compound is used. After the emulsion is made, the bichromate can 
be removed by washing in a dark room, when the mixture becomes 
sensitive to light. I now wish to prepare a gelatin emulsion of silver 
bromide containing some iodide. [Emulsion prepared. ] 
After the plate has been exposed in the camera, the image is devel- 
oped by any one of several methods; but all of them depend upon the 
fact that, in the presence of what chemists designate as reducing agents 
—substances which greedily absorb oxygen—a salt of silver is decom- 
posed with the deposition of metallic silver. When such a reducing 
agent is caused to flow over the plate, the reducing action is most 
powerful at those places where the light has acted upon the sensitive 
silver salt and decomposed it. Therefore at such points metallic silver is 
deposited in very minute, black particles, and, as these increase in quan- 
tity, the image becomes visible, acquires density, and details are faith- 
fully brought out. 
We can readily illustrate the action of the developer by an experi- 
ment. Here is a quantity of silver bromide, which has been precipi- 
tated and exposed to sunlight. I have here another precipitate of the 
same compound, which has been protected from the light. To each 
of these I will add a developing solution, and you will see the rapidity 
with which the silver is reduced in one case, and how much more 
slowly it changes in the other. [Experiment made.] Precisely the same 
effect is produced in developing the invisible image on the exposed 
plate, I will now develop a plate which was exposed in my camera, 
one week ago to-day. It is a stereoscopic view of a modern windmill, 
which is to be found just above High Bridge. The developer to be 
used is a mixture of ferrous sulphate, or green vitriol, as it is called, 
with potassic oxalate, containing some potassium bromide. By mixing 
the two solutions first mentioned, a solution of ferrous oxalate is ob- 
tained, which is the active agent in effecting the reduction, the bromide 
of potassium being a restrainer—preventing the too rapid and too gen- 
