332 



ADAPTATIONS TO SPACE AND MOTION 



may be 'summated' in some vertebrates even though it is not in our- 

 selves (see p. 308). 



It seems odd that binocular resolving power should not be always 

 higher than monocular. When we consider that the receptor mosaic of 

 one retina, like a halftone reproduction, can register only certain of the 

 points on an object's surface, then obviously the chances are prepon- 

 derant that at any one moment the other retina will be recording a set 

 of points which fall mostly in between those of the first set — just as one 

 retina 'fills in' the blind spot produced by the head of the optic nerve 

 in the other retina. If now the two sets of object points are interdigitated 

 in a fusion-image, why is not that image as well resolved as would be the 

 monocular image supplied by a retina containing nearly twice as many 

 receptors per unit area as either member of the pair of retinae we are 



Fig. 122 — Retinal rivalry for patterns, 



a, stereoscope card bearing unlike patterns, b, the sort of mosaic which one may see, at 

 any one instant, while observing a in a stereoscope. 



considering? One would expect this, just as one expects to gain a better 

 idea of the form and texture of an object by holding it in two hands 

 instead of in only one. The bothersome fact remains that when the two 

 eyes are in use our resolution of details is just as good as, and no better 

 than, the resolution afforded by the better of the two eyes when that 

 eye alone is used. 



This calls for explanation. We can offer a weaselly sort of teleological 

 reason why binocular brightness should not be raised over monocular — 

 at least in diurnal animals, such as man: if binocular brightness were 

 allowed to be higher than monocular, contours would be created between 

 the binocular and uniocular visual fields, and these might be as per- 

 petually disturbing as those seen by a person who cannot get used to 



