THE PROBLEM OF COLOR VISION. 
By JoHN M. DANE. 
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 if the retina, thus 
arriving at the proximal ends of the elongated rod and cone cells. 
These sells 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 pigmenti retine. (Fig. 1,8. 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it is withdrawn into the cell body.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 » long and 1.5 
to 2 » in diameter, consists of a doubly refractive, lustrous outer seg- 
ment, and a singly refractive, finely granular inner segment. In 
serum or dilute osmic acid the outer segment breaks into a series of 
regular transverse disks 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. It bleaches rapidly in the light, 
but (unless the pigmented stratum has been removed experimentally) 
it is soon restored in the dark. Light thus appears to incite chemical 
processes in the outer segments of the rods. The inner segments are 
sometimes described as having a longitudinally fibrillar structure in 
@Reprinted, by permission, from the American Naturalist, Vol. XUI, No. 
486, June, 1907. 
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