The occurrence of pigmentation in conjunction with visual function, from the most primitive photoreceptors to the complex mammalian eye, suggests that pigments form an integral part of the physiology of vision.1a Schanz, making use of the known photo-electric effects of fluorescent dyes, based his theory of vision on the absorption of light by the pigment of the pigment epithelium of the retina, which was assumed to emit electrons, thus stimulating the rods and cones.2 Zikulenko considered the cells of the retinal pigment epithelium to be the primary visual elements, rather than the rods and cones.3 Eldridge-Green based his theory of vision on the visual purple (rhodopsin) present in the rods.4a The photochemical changes involved in dark adaptation have likewise offered a field for speculation as to the possible rôle of pigments of the eye as photosensitive or light-absorbing substances.
The general course of dark adaptation,