THE problem of color vision is one of the most intricate which the biologist is asked to solve. The following paragraphs are intended to indicate the several methods which are being employed for its solution, together with some of the results thus far obtained.. The anatomy of color vision will be considered first; then in turn its physiology and its development; and finally, the abnormal conditions of color blindness, together with the theories of normal vision to which they have given rise. Anatomy. The mechanism of color vision is lodged in the rod and the cone cells. A ray of light, after passing through the lens of the eye and its vitreous body, penetrates several layers of the retina, thus arriving at the proximal ends of the elongated rod and cone cells. These cells are arranged in a single row. The light traverses the length of the cells to their distal ends which it stimulates. The rod and cone cells project against a single layer of heavily pigmented cells, the stratum pigment retinae (Fig. 1, S. P.). These have non-retractile processes which are found between the rods and the cones. The pigment fuscin, in the form of elongated or crystalloid granules, migrates into these* processes when the eye is illuminated; in the dark it is withdrawn into the cell body. Every rod cell consists of a rod, a rod fiber, and a nucleus, arranged as shown in Fig. 1, A. A rod, which is from 40 to 50 I. long and 1.5 to 2 , in diameter, consists of a doubly refractive, lustrous outer segment, and a singly refractive, finely granular inner segment. In serum or dilute cosmic acid the outer segment breaks into a series of regular transverse discs which are believed to indicate a stratified structure in the living rods. Visual purple is a pigment which occurs only in the outer segments of the rods...