A Manual Of Photography

GO HISTORY OF PHOTOGRAPHY. it (after lying a minute or two) into fresli water, in which, being again rinsed and dried, it is now perfectly fixed.” Section III.—Photographic Properties of Mercury. "As an agent in the daguerreotype process, it is not, strictly speaking, photographically affected. It operates there only in virtue of its readiness to amalgamate witli silver properly pre- pared to receive it. That it possesses direct photographic sus- ceptibility, however, in a very eminent degree, is proved by the following experiment. Let a paper be washed over with a weak solution of periodide of iron, and, when dry, witli a solution of proto-nitrate of mercury. A bright yellow paper is produced, which (if the right strength of the liquids be lût) is exceedingly sensitive while wet, darkening to a brown colour in a very few seconds in the sunshine. Withdrawn, the impression fades rapidly, and the paper in a few hours recovers its original colour. In operating tins change of colour, the whole spectrum is effec- tive, with the exception of the thermic rays beyond the red. “ Proto-nitrate of mercury simply washed over paper, is slowly and feebly blackened by exposure to sunshine. And if paper be impregnated with the ammonio-citrate of iron, already so often mentioned, partially sunned, and then washed witli the proto-nitrate, a reduction of the latter salt, and consequently blackening of the paper, takes place very slowly in the dark over the sunned portion, to nearly the same amount as in the direct action of the light on the simply nitrated paper. " But if the mercurial salt be subjected to the action of light in contact with the ammonio-citrate or tartrate, the effect is far more powerful. Considering, at present, only the citric double salt, a paper prepared by washing first with that salt and then with the mercurial proto-nitrate (drying between) is endowed witli considerable sensibility, and darkens to a very deep brown, nay, to complete blackness, on a moderate exposure to good sun. Very sharp and intense photographs of a negative cha- racter may be thus taken. They are, however, difficult to fix. The only method which I have found at al1 to succeed has been by washing them witli bichromate of potash and soaking them for twenty-four hours in water, which dissolves out the chromate of mercury for the most part; leaving, however, a yellow tint on the ground, which resists obstinately. But though pretty effectually fixed in this way against light, they are not so against time, as they fade considerably on keeping. “When the proto-nitrate of mercury is mixed, in solution, witli