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PHOTOGRAPHY IN NATURAL COLOURS 229
the older process, as employed by Seebeck and Herschel, i.e. they prepared pictures on paper, only the preparation of this paper was peculiar. Paper saturated with salt was made sensitive in a solution of silver, like the photographic positive paper previously described, then washed to remove the solution of silver, and afterwards exposed to the light in a solution of subchloride of tin. By this means violet subchloride of silver is formed from the white chloride of silver. The subchloride of tin only operates as a reducing medium. This paper is in itself little sensitive to coloured light ; but if it be treated with a solution of chromate of potash and sulphate of copper, its sensitiveness increases considerably, so that it is easy to copy with it transparent coloured pictures. Nevertheless, the colours are never so vivid as in the original, the red tones showing themselves the strongest. After printing, the pictures are washed with water, to make them less sensitive to light. In this condition they showed tolerably well in a subdued light, but no means were found to make them perfectly durable. Hyposulphite of sodium could not be used, as it destroys the colours immediately.
With reference to the action of light upon the salts of silver, mention must be made of the observations and experiments of Carey Lea (1887). He found that the halogen compounds of silver are not only capable of changing colour when acted upon by light, but may also be caused to do so by chemical action alone. These particular salts he named Photo Salts, and he found that with a red variety of the chloride he could obtain the best results. With the aid of this salt Lea was able to obtain a photograph of the spectrum colours with the exception of the yellow and green. This active chloride can be produced from the ordinary variety by the action of a reducing agent.
Certain chemical compounds when mixed with this salt