XV. On the connexion between the phenomena of the absorption of light, and the colours of thin plates

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Abstract
Since the phenomena of the absorption of light by coloured media began to be studied with attention, various philosophers have regarded them as inexplicable by the theory of the colours of thin plates, and have consequently regarded Sir Isaac Newton’s theory of the colours of natural bodies as either defective in generality, or altogether unfounded. Mr. Delaval was the first person who brought an extensive series of experiments to bear upon this subject. Dr. Thomas Young considered it “impossible to suppose the production of natural colours perfectly identical with those of thin plates,” unless the refractive density of the particles of colouring bodies was at least twenty or thirty times as great as that of glass or water, which he con­sidered as “difficult to believe with respect to any of their arrangements constituting the diversities of material substances.” Sir John Herschel has expressed a still more decided opinion upon this subject. He regards “the speculations of Newton on the colours of natural bodies” as only “a premature generalization,” and "limited to a comparatively narrow range; while the phenomena of absorption, to which he considers the great majority of natural colours as referable, have always appeared to him to constitute a branch of photology sui generi .” The general opinion advanced by these three philosophers I have long entertained; and with the view of supporting them I have analysed a great variety of colours which are exhibited by the juices of plants. In a paper "On the Colours of Natural Bodies,” I have shown that the green colour of plants, the most prevalent of all the colours of natural bodies, in place of being a green of the third order , as Newton and his com­mentators assert, is a colour of no order whatever, and having in its composition no relation at all to the colours of thin plates.

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