Page:Philosophical Review Volume 3.djvu/110

94 summary, which I transcribe.—Normal color vision is due to the presence of three light-sensitive substances in the most external retinal layers. The distribution of these, their capacity of light-absorption, and their decomposability are different. One of them, the white-substance, is distributed over the whole retina, and is, at the same time, the most sensitive to light. It absorbs the rays of almost the whole visible spectrum, predominantly those of mean wave-length. This absorbed light serves to decompose it. Energy is thus set free in a form adapted to neural excitation; and the result of this stimulation manifests itself to our consciousness as sensation of brightness (white or grey). The substance is continuously decomposed (by external or internal stimuli), and at the same time continuously reformed by the organism. In this reformation are concerned not only the stimulated area, but also neighboring areas, and indeed, more or less, the total retina. For consciousness, however, in the case of this as of the other visual-stuffs, it is only the decomposition processes, and not those of regeneration also which make themselves known.—A second substance is contained in the external members of the so-called visual cells (rods and cones). It does not quite extend, therefore, to the most extreme periphery of the retina. It is also not so sensitive to light as is the white substance, to compensate for which it is present in very large quantity. This substance is identical with the visual purple; its behavior as regards light can, therefore, be investigated entirely independently of all hypothetical constructions, and has already been so investigated. In its original condition it is purple in color, and it is to be noted that there exist a more reddish and a more violet modification of it. It absorbs predominantly the (sensationally) yellow-red to green rays. The absorption-maxima of its two modifications lie between D and E. This substance is also decomposed by the operation of suitable light-rays; not, however, at once to its final decomposition-products. There is an intermediate stage. First of all, it fades to yellow, and this visual yellow is then further decomposed by the green to violet rays (so that green light-rays affect both substances simultaneously). The products of the final decomposition are turned to account by the organism for the reformation of the original substance, the visual purple. It is possible that this process is assisted by the operation of short-waved light; for the rest, the whole retina is again concerned in it.—Now, the effects of these processes upon the organ of vision manifest themselves for consciousness, on my assumption, in two ways. First of