CONE VISION; COLOR 81 



(B) Photopic Vision 



Cone Vision — Turning now to the cones, we are confronted with the 

 complex matter of color vision — assuming for the nonce that all cone- 

 bearing vertebrates do discriminate hues. We can imagine subtracting 

 color vision from the whole performance of the cone — but what we 

 would have left, we could describe in terms of a rod mechanism that 

 had little summation and very little rhodopsin. So, we cannot well avoid 

 considering the elementary and purely qualitative aspects of color vision 

 if we are to attempt to picture the mechanism involved and thus round 

 out our survey of visual physiology. 



Color — Color, or better, 'hue', exists only in the mind. No light or 

 object in nature has hue — rather, the quality of hue aroused as a sen-y 

 sation is projected back to the object as one of its attributes, just as the 

 patterns of brightness and darkness in consciousness are projected back 

 into the visual field to endow objects with their size, shape, tone values, 

 and movement. For, we perceive objects rather than lights. We can 

 see objects falsely as to size, shape, and motion, and just as falsely as 

 to color since color is purely subjective. The color of a surface depends 

 not only upon its chemico-physical nature, but also upon the kind of 

 light by which we see it, and upon our memory of the impression it 

 may have given us under some more familiar illumination. Thus, a par- 

 ticular dress may look red only in daylight, yet we still call it red under 

 an artificial light when it may actually be reflecting more yellow light 

 and should then be seen as orange. 



The hue sensation aroused by a light depends primarily upon the 

 frequency of its vibration, usually expressed as the distance between 

 successive waves in the vibration, the wavelength. The longest visible 

 wavelengths, in the neighborhood of 760m|,i, arouse the sensation we call 

 red; the shortest ones, around 390m[l, give us the sensation of violet, 

 which must be seen in a spectroscope to be appreciated (since the violets 

 of textiles and pigments in general are not true violets, but diluted pur- 

 ples). In-between wavelengths give us the other hues of the spectrum. 



When all of the visible wavelengths are being received on the same 

 area of the retina, either simultaneously or in such rapid succession that 

 their physiological images persist long enough to overlap or fuse, we see 

 what we call white light. The removal of some wavelengths from the full 

 assortment makes the remainder of the light appear, collectively, as a 

 color. Such a removal may be effected by selective reflection or by selec- 



