the originally green substance fades primarily to a red intermediate-product, like the dying leaves of the wild vine. This is itself further decomposed by rays of mean wave-length; and then, from the final decomposition-products, the original green substance is reformed by the forces of the organism. From orange to green-yellow the absorption-spectra of the two substances are coincident. Energy is set free in these two decompositions, and its operation upon the nervous apparatus again comes to consciousness as brightness. In both cases, the excitation is carried over to the nerve with a specific secondary-character, in a special rhythm, by virtue of which the brightness-sensation obtains a chromatic toning. We thus sense the decomposition of the original substance as red, that of its red intermediate product as green. Finally, these two specific rhythms are again somewhat antagonistic, mutually disturbing; so that the chromatic characters of the two sensations neutralize one another when red and green are suitably mixed, and we sense only the sum of their brightness, as white.—So much in respect to normal vision. With regard to its principal pathological modifications, ordinary color-blind persons are individuals who lack the green-red substance, and who in consequence sense, of colors in the narrower signification, only yellow and blue. The way in which they see these distributed in the spectrum is altogether conditioned by the absorption-spectra of the visual purple and the visual yellow. The further difference observed in such cases between so-called red-blindness and green-blindness is due to the occurrence of the visual purple in two modifications. In the condition of so-called color-weakness, or in those of disproportionate injury to antagonistic colors, there exist disturbances, somewhere centrad from the rod and cone layer, by which the excitation-rhythms are more or less capriciously altered in their transference to the central organ. Injuries of this kind are also present in cerebral affections of color-vision, resulting from hysteria, apoplexy, etc. Lastly, in the case of total color-blindness, either the two chromatic substances are entirely wanting, or the chromatic rhythms depending on them are entirely cancelled by disturbances existing centrad, while a conduction of the mere excitation-quantum remains still possible.
Recent investigations have led to three theories in explanation of those phenomena, hypnotic and otherwise, that are usually referred
