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CHAPTER II COLOR FILTERS OR COLOR SCREENS
It was but natural that in the early experiments with the spectrum, advantage should be taken of the selective action of colored glasses and fluids. Most of the experiments were to find out the various effects of the rays, that is to say, whether they were, to use the old terms, calorific or chemical in action. To record all such would take up too much space, entail too much work without serving any useful purpose as it is practically outside the scope of our subject.
A prominent experimenter in this field was E. Becquerel, and he seems to have been the first to have suggested the use of filters or screens for the purpose which they are now used. He said:1 (p. 94) "One may also notice that bright blue objects will appear like white objects, for the one sends almost as many active rays to the sensitive plate as the other ; green objects, such as the leaves on trees, on the contrary will appear almost like black. I have suggested that if one were to place in front of the lens of a camera a colored screen of bright green in such a way that all objects would be seen under the same aspect, that is to say, to remove the too active and more refrangible rays, one would increase the exposure in the camera considerably, but one would obtain more of the half-tones of the subject, which are of a yellow or green color."
Becquerel had discovered the absorptive effect of quinin, uranium and yellow glasses on the ultra-violet, and their action, with that of other colored glasses and solutions, on the most refrangible rays in the visible spectrum. He had also pointed out the increased exposure required to obtain chemical action under the same on various salts of silver.
In an editorial article,2 which was in all probability written by Sir Wm. Crookes, it says, after referring to the sensitiveness of silver iodide to indigo and the ultra-violet, and the greater sensitiveness of silver bromide to blue and green : "another important step is, to destroy the action of the powerful actinic rays (including in this term, the violet and lavender, as well as the invisible rays). This is not difficult; a thin layer of solution of sulfate of quinin, between parallel glass plates, interposed between the lens and the object, will effectually cut off all above the indigo; a thin piece of yellow glass employed in the same way will act even more vigorously."
It will thus be seen that the use of niters was recognized in the very earliest days of photography.
Before proceeding to the consideration of the actual manufacture of
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