The most reliable conclusions regarding the dependency of visual experience upon the retinal mechanism are comprised in the so-called duplicity theory, according to which the two histo logically differentiable types of receptors, the rods and the cones, have psychophysically distinct functions. The cones are sup posed to have a relatively low degree of sensitivity and to sub serve photopic or day vision, with chromatic discrimination. The rods, on the other hand, are characterized by a sensitivity which, at its maximum, is about io,000 times as great as that of the cones, but by an absence of chromatic response. At high intensi ties of stimulation, they are practically eliminated by adaptation, but at low intensities, they entirely replace the cones. A multi tude of spatial and temporal effects can be explained in terms of the differences in retinal distribution, spectral sensitivity and inertia between the rod and cone systems.
remaining factor in visual sensation, the depth impression, can be disposed of quite briefly, since it is customary to discuss its conditions in detail in connection with binocular visual perception. Depth elements seem introspectively to be all
of the same kind, differing only in location within subjective visual space. They must be regarded as being determined directly by processes in the visual areas of the cerebral cortex, although their number and locations are regulated by a complex assembly of peripheral factors, the so-called primary and secondary criteria of depth. The most important among these factors consist in the disparations of the images of identical object points upon the two retinas. Hering treated such disparation as a sensory variable, having positive and negative values, corresponding with crossed and uncrossed relations of the corresponding lines of view, respectively. (See VISION.) For a more detailed discussion of the problems of visual sensation, see J. H. Parson's An Introduction to the Study of Colour Vision (1024), or L. T. Troland's The Principles of Psychophysiology, vol. ii.
chap. xiv. (1929). (L. T. T.)