FIG. 43.—MAN IN A BOTTLE.
Blue prints may be given a black tone by plunging them into a solution of 4 parts of caustic potash in 100 parts of water; then, when the blue color has entirely disappeared under the action of the potash, and a yellowish color has taken its place, they are immersed in a solution of 4 parts of tannin in 100 parts of water; then washing them again, we obtain prints whose tone may be assimilated to that of pale writing ink.
In the process giving blue lines upon a white ground, it is necessary that the action of the light shall be to convert the iron compound into one that can be discharged from instead of being fixed on the paper, so that we obtain a positive from a positive. Abney describes the process as follows: Thirty volumes of gum solution (water 5 parts, gum 1 part) are mixed with 8 volumes of a citrate of iron and ammonia solution (water 2 parts, double citrate 1 part), and to this is added 5 volumes of a solution of ferric chloride (water 2 parts, ferric chloride 1 part). This solution thus formed is limpid at first, but will gradually become thicker, and should be used soon after mixing. It is then applied with a brush to the paper (which should be well sized) and dried in the dark. Exposure is accomplished in a few minutes, the paper being placed under the drawing in the printing frame. It is then developed with potassium ferrocyanide, 50 grains, water 1 ounce, applied with a brush until all the details appear of a dark-blue color. The print is then rapidly rinsed, and placed in a dish containing the clearing solution, made of 1 ounce of hydrochloric acid and 10 ounces of water.
The third process, which gives violet-black lines on a white ground, is the following: Make up the sensitive solution with water, 16 ounces; gelatine, 4 drachms; perchloride of iron (in a syrup condition), 1 ounce; tartaric acid, 1 ounce; sulphate of iron, 4 drachms. The paper is floated on or brushed over with this and dried. The exposure is about the same as with the last process. When sufficient, the greenish-yellow color will turn white, except the lines, which should be somewhat dark. The developing solution is composed of 1 part of gallic acid in 10 parts of alcohol and 50 of water. When immersed in this solution the lines will turn blacker. The finish is then made by thoroughly washing in water.
[SYMPATHETIC PHOTOGRAPHS.]
These are obtained as follows: A sheet of paper is coated with a ten per cent solution of gelatine, and when dry this is floated on a ten per cent solution of bichromate of potash. Again dry and expose beneath a positive transparency. The print thus obtained is then immersed in a ten per cent solution of chloride of cobalt. The parts unacted upon by light will absorb the solution. Wash and dry. We then have a faint image which will alter its color according to the state of the atmosphere. In damp weather it will be almost if not entirely invisible, but when the weather is fine and dry, or if the image be heated before a fire it will turn to a bright blue color.