Page:Popular Science Monthly Volume 13.djvu/458

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mersion in the developing liquid, instead of pouring the latter on the collodion layer, as in the case of plates of glass, an operation that requires some dexterity and long practice.

This paper will retain all its sensibility for about two years, provided it be sheltered from light and moisture; it is not affected either by cold or heat. Hence it is destined to be of great use to travelers who explore remote regions.

After the light-impression has been made on the paper, it remains to develop the image. This operation presents no difficulty, success depending, so to say, only on the time of exposure. First, the paper is dipped in common water, care being taken to make the immersion complete. There it must remain for at least five minutes, or until the paper, which was beginning to curl, becomes perfectly flat. In the mean time the following solution is prepared in quantity only sufficient for the pictures to be developed at once, for oftentimes it decomposes in the course of a day or two:

Distilled water 1 litre.
Glacial acetic acid 20 grammes.
Citric acid 20 "
Pyrogallic acid 3 "

Into a basin with flat bottom, and of a size corresponding to that of the proof treated, is poured enough of the above solution to completely submerge the proof; a depth of three or four millimetres is amply sufficient. Into this is dipped the proof after taking it from the water and draining it, the collodionized side uppermost. After inclining the basin in every direction, so as to cause the liquid to pass several times over the proof, a portion of it is poured into a glass, and then we add to it a few drops of the following solution:

Distilled water 100 grammes.
Crystallized nitrate of silver 5 "

Stir well, so as to mix thoroughly. The whole is poured into the basin, which again is inclined as before. The image now appears; seven or eight minutes suffice to completely develop it, with the sky or the lighted parts of an intense black.

When the proof is sufficiently developed it is put in water, and then dipped in a solution of hvposulphate of soda, 40 per cent., to fix it; it is then freely washed in water in the usual way. It is now dried between leaves of silk-paper or blotting-paper.

Treated in this way the proof is perfectly secure: it is not affected by changes of temperature, and may be exposed either to damp or to drought without the least injury. To preserve it, we have only to place it in a book or in a portfolio, so that it may not be creased or rubbed on the collodionized side.

When we would take positive proofs, we detach from the paper the layer of collodion, thus getting the image on a thin transparent pellicle.