Excerpt from The Art of Photography: Instructions in the Art of Producing Photographic Pictures in Any Color and on Any Material
I have been engaged for several years in experiments and researches on the chemical alterations and modifications produced in various bodies by the action of light. The object which I had more immediately in view was to ascertain whether other metallic compounds, besides those of silver, might be used in photography.
In addition to this, I had also proposed to myself to try whether the purely conjectural and random way of mixing together the salts used in photographic processes, might not be reduced to something like a law of definite mixing proportions. It affords me sincere gratification to be able to state that I have attained both objects.
The results of the experiments which I made to ascertain whether other metals besides silver might not be employed in photography, fully established the fact that all metallic compounds suffer alteration under the influence of light. In many of them this alteration takes place in a very short time, as in the case of iron for instance; others require a more protracted exposure to the action of the light. By combinations of different metallic salts the most interesting variations may be produced. Next to silver, the salts of iron, and the compounds of chromium appear to me the most important. Thus, for instance, the blue or green images produced with a salt of iron and ferricyanide of potassium (commonly termed red prussiate of potash or potassa), look remarkably fine, and are well adapted for light-screens and lithophanies; to protect them effectually from injury, they need simply be enclosed between two glass-plates set in a frame.
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