TRICHROMATIC VISION 91 



The manner in which a mixed color, for instance purple, may be seen 

 by one eye (or both) presented with purple, or with one eye offered red 

 and the other violet, is diagrammed in Figure 31. The purple stimulus 

 in 'a' may of course be steady, or may consist of rapid alternations of 

 red and violet lights; for, as mentioned earlier, fusion of colors may 

 occur temporally as well as spatially. When purple strikes a single retina, 

 impulses somehow tagged 'redness' and 'violetness' pass along the optic 

 nerve to be combined into 'purpleness' by the same central machinery 

 that makes purpleness out of redness from one eye and violetness from 

 the other. In the retina, then, there is some analytical mechanism, two 

 separate parts of which respond independently to the short and long 

 wavelengths in the purple light. We suppose the whole of this analytical 

 mechanism to be a group of (three) photochemical substances. 



Left Eye 



Right Eye Left Eye 



I (none) I ■• — stimulus — - 1 red 



i \ peripheral 

 \ \ analysis ' 



Right Eye 



central synthesis -—> 

 binocular- mix lure locus 

 consciousness 



Fig. 3 1 — Perception of a compound color : purple. 



a, monocularly (or, a purple stimulus might be supplied to each eye), b, by binocular 

 mixture of red and violet. The inactive components of the visual system are labelled in 

 faint lettering — all components would of course be active in the perception of the all- 

 inclusive compound white. 



Central Events in Trichromatic Vision — When the dark-adapted 

 eye is presented with an equal-energy spectrum, that spectrum appears 

 colorless (some say, faintly violet) but not homogeneous. At the locus 

 of wavelength 510m[X the spectrum is maximally bright, the luminosity 

 falling off toward the ends and becoming zero, at the long-wave end, 

 at a point corresponding to the orange-red of the photopic spectrum. 

 Konig and Trendelenburg, around the turn of the century, established 



