In i866 Wharton Simpson treated collodio-chloride of silver on an opal plate with excess of nitric acid, and then dried it before a fire till it became lavender grey. He succeeded in getting black, orange, ruby, and magenta correctly.
The Red Photo-Chloride.—With the investigations of Carey Lea a much more hopeful stage seemed to have been reached. His instructions for the preparation of photo chloride run somewhat as follows : In a 4 oz. bottle, dissolve 4o gr. silver nitrate in an ounce of distilled water, and add sufficient hydrochloric acid to precipitate silver. Shake up well, to agglomerate the particles of the precipitate, and then pour off water, not worrying about losing a grain or two of the precipitate in the process. The precipitate is then washed three times in succession with distilled water. The next step is to fill the bottle one-third full of water and add strong ammonia till precipitate redissolves, when 6o gr. of proto-sulphate of iron, in just sufficient water to dissolve it, must be added. A black precipitate will be produced, which must be allowed to settle, and is then washed in succession with dilute sulphuric acid, dilute nitric acid, and lastly with water. If these operations have been correctly performed the result will be pink or red chloride of silver, producing with fair accuracy many of the colours of the spectrum. That is to say, that when exposed to red light it remains red, under blue light it turns blue, and violet light changes it to violet. But alas ! it is useless for practical photographic purposes because, when exposed to high lights, instead of bleaching it becomes black ! Carey Lea was convinced that there was a great future for his red photo-chloride ; by the addition of lead and zinc chlorides a certain amount of bleaching took place in the high lights, while sodium salicylate increased its sensitivity threefold. But ill-health prematurely put an end to his experiments.
Gelatzno-chloride Einulsions.—We have not advanced very far on the road since the death of Mr. Carey Lea, but a renewed interest in the matter has arisen with the intro duction of chloride of silver developing emulsions for plates and papers. The range of colours obtainable simply by varying the exposure, seems to constitute a new link in the chain of evidence pointing to the photo salt. By employing
a dilute developer, such as rodinal i in 80, or glycin, especially with Gravura, Rotox, and Gevaert gaslight papers, we can obtain a succession of colours embracing a great part of the spectrum. Thus, if a short exposure give us a dark blue, the following are all possible : Relative Exposure Colour 2 . . . . . Olive green 3 • • • . . . Sepia 5 Red 6 . . . . . . Reddish-brown 10 . . . . . . Orange 15 . . . . . Yellow By varying the constituents and strength of the developer as well as length of exposure, a very much richer range of intense colours may be secured. But this would not help us very much. The object should be rather to decide what developer, and at what strength, has most considerable range.
Acetone sulphite is said to have useful properties when added to such developers as hydrokinone and edinol. It will be observed that the last colour reached by prolonged exposure is yellow, and that yellow rays predominate in the source of light employed for gaslight exposures. The result of exposures under slips of different-coloured glasses is encouraging, in view of the slowness of the emulsion, and the fact that it is not chemically affected to any great extent, except by the violet and blue-violet rays. Liippo Cramer has proved that, by the acid-boiling process and ripening, a fine-grain chloride emulsion may be manufactured, which will compare for sensitivity with usual bromide emul sions. We have also the enormous advantage over the chemists of the last generation, that with the aid of suit able dyes we may render silver chloride much more respon sive to the longer vibrations of light at the red end of the spectrum.
But, is it necessary to employ the colour salt in making the negative ? In an ortho-chromatic negative, and still more in a negative made by the Thames Plate separate method, the wave-lengths of the spectrum are correctly represented by gradations which the colour salt should once more translate into their proper colours on the positive. This our experi ments have shown to be within the limits of possibility.