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 introduction 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 1 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:
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. Luppo-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 emulsions. We have also the enormous advantage over the chemists of the last generation, that with the aid of suitable dyes we may render silver chloride much more responsive 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 experiments have shown to be within the limits of possibility.
It recently occurred to us to expose a sheet of glossy Gravura paper to daylight under a landscape negative having strong gradations. Printing out was necessarily a work of time, and in the long run nothing was gained by nitrating the paper. Gradually, a very beautiful and very vivid series of colours unfolded themselves to the eye. True, they were not the exact colours of nature, but at certain stages of printing the combinations of tints were far from unpleasing. The following shows the results obtained by repeated experiments:
Short Exposure: Soft tones, yellow; hard tones, pink; shadows, blue.
Prolonged Exposure: Soft tones, pink; hard tones, blue; shadows, green. Very long Exposure : High lights, pink; soft tones, blue; hard tones, green; shadows, brown.
To ensure deep printing the paper was dipped in the following solution, and dried on a ferrotype plate, before printing:
Sodium Salicylate.......10 gr.
Fixed for 30 sec. in the ordinary hypo bath, the colours completely changed; but returned on exposure to light after washing as: soft tones, pink; hard tones, blue; and shadows, green. Rapid fixing in ammonium sulphocyanide 10 per cent. solution gave slight variations of colour. The general result seemed to prove that the colour was dependent upon the thickness of the deposit of allotropic silver. Only occasional batches of the paper (Gravura No. 1) possess this property of printing out in brilliant colour. But, possibly it is shared by some other gaslight papers with which we are less well acquainted. We imagine it to depend on a slight ripening of the emulsion grain. These experiments are worth comparing with the exhaustive researches made by Mr. Chapman Jones into the effect of grain on the colour of the silver image. Toning improves the colour.
We have arrived then at one important principle, viz. that the colour of the silver image will, under certain given conditions, follow a definite rule. According to the fineness of the metal deposited in this particular allotropic state, the colours will be in order, yellow, pink, blue, green, brown; and some experiments with denitrated P.O.P. lead us to believe that with much thicker deposits of silver there is another ascending series also giving the same order - yellow, red, blue. Needless to say that, except for very brilliant autumn foliage, these tints do not correspond at all with the gradations of the ordinary negative. Whether they can be corrected by an admixture of the various sensitive silver salts is a question which only the experiments of a number of skilled chemists can determine.
Rough and steep is the path of progress; slow the march towards the perfection of an ideal. This brief summary of what has been accomplished during a hundred years warns us not to be optimistic. Much circumstantial evidence has been gathered together, but the real solution of our difficulties may yet be afar off. We can only endorse as still true words written nearly forty years ago, by a contemporary of Niepce de St. Victor: "Though but a very short portion of the road towards the great end has been opened up, still it would be wrong to despise what has been done, or to regard the final solution as an altogether Utopian or chimerical problem." Somehow or other, it is a law of nature that, wherever a strong enough demand exists, a means of supply will ultimately be found; and, with so many minds engaged on the search, we may look forward hopefully, with the faith which is the evidence of things yet to be seen.