ABSTRACT
IN respect to most of the characters found in the human animal, one has only the lower animals in which to study the course of their gradual development, but in considering the colour-sense one has the unique advantage of being able to perceive its successive stages spread out upon one’s own retina. It has been found that anatomically the structure of the retina is of a high form of development in the centre, and that it gradually becomes less highly developed towards the periphery; of the retinal visual elements, the cones alone (which have been shown by Ramon y Cajal to be more highly developed rods) are found in the fovea, and they occur more and more sparingly farther out until there are practically none in the extreme periphery. Corresponding with this fact of structure is the fact of sensation that we get full tetrachromatic colour vision only in the central portion of the retina, and nothing but achromatic vision (that in which all objective light, of whatever light-ray constitution, looks white in quality) in the extreme periphery. This fact, together with the circumstance that the retina of night birds-of-prey (who have no occasion for colour vision) is very deficient in cones, led Max Schultze to form the hypothesis that chromatic vision is mediated by the cones only, and that the rods furnish nothing but achromatic vision. This view was further strengthened when it was advocated independently by Parinaud, upon reasons based on the facts of hemeralopia, a disease which consists in the non-functioning of the mechanism for darkness adaptation. The disjunction of function of the rods and cones—the specific rod-cone function (Ladd-Franklin)—was rendered indubitable by certain discoveries made in König’s laboratory in Berlin in 1892. They are: (1) The exact coincidence of the distribution through the spectrum of the subjective intensity of night-vision in the normal individual (which coincides with that of night and day vision in the achromatic defectives) with the objective spectral absorption of light by the visual purple. The latter substance appears only—or at least in vastly greater quantity—after adaptation has taken place (König). (2) The following two closely connected facts (Ladd-Franklin, Sitzber. Akad. der Wiss. Berlin, 21 Juni, 1892, p. 362): first, the normal night-blindness of the fovea, viz., the fact that that form of substitute vision which the normal individual acquires after twenty minutes in a dark room—night-vision, as it may be called, or scotopia—he does not acquire in the fovea; and, second, the complete blindness in the fovea of those individuals who have the typical (non-cortical) form of total chroma-blindness (achromatopia).
