TRICHROMATIC VISION 89 



Young formulated a theory of color-vision based upon LeBlond's find- 

 ings, in which he proposed three sets of receptors in the retina, each 

 most sensitive to one of three primary colors. Sensations of non-primary 

 colors were regarded as due to the simultaneous enaction, to varying 

 extents, of two or all three sets of receptors. Whiteness was due to the 

 equal stimulation of all three. 



Much support for this three-component theory of color vision was 

 given by Helmholtz in the last century, and nowadays the theory goes 

 under the hyphenated names of the two men. The Young-Helmholtz 

 theory calls for three 'somethings' in color vision; but ideas have 

 changed, from time to time, as to what these somethings are. Young 

 thought of them as three kinds of nerve endings. Helmholtz thought of 

 them as three photochemical substances or processes, which he at first 

 believed to be in three separate sets of cones. Later, he considered that 

 they probably all occurred in each cone. 



Other theorists have complicated matters considerably and, in the 

 light of the most recent developments, unnecessarily. The perception of 

 yellow, white, and black formerly gave much trouble and seemed to call 

 for a minimum of four components in cone vision, as in the theory of 

 Hering, the principal rival of that of Young and Helmholtz. The binoc- 

 ular fusibility of red and green into yellow, and the modern concept of 

 the difference between blackness and darkness as being due wholly to con- 

 trast, makes the assumption of more than three components unnecessary. 



It is entirely likely that the three processes are mediated through each 

 and every cone. White stimuli do not take on hue when made very small 

 in area, as we should expect them to do if they then struck only one or 

 two out of a total of three or more kinds of cones. Again, if there were 

 three kinds of cones with respect to color sensitivity, visual acuity would 

 necessarily be very low in a monochromatic illumination which effec- 

 tively stimulated only one-third of the cones. But visual acuity is not 

 lower in any monochromatic light (except, perhaps, red) than it is in 

 white light of equal objective or subjective intensity; and in some such 

 lights it is even higher. This could mean, as Hecht claims, that all the 

 cone-types respond nearly equally to any given monochromatic light. 

 It can also mean that the cones are all alike — at least in any given small 

 retinal area. They may vary progressively along meridians of the retina, 

 for the number of hues we can discriminate diminishes from the center 

 toward the ora terminalis, unless the intensity is very high. Even this 

 'deficiency' may really have its basis far from the cones themselves. 



