Trans. N. V. Ac. Scz. 1Y8 . 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 rub it 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 ran 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- 



