Chapter 10 



ADAPTATIONS TO SPACE AND MOTION 



(A) Accommodation and its Substitutes 



Any object in visual space may have a number of perceptible attri- 

 butes: size, shape, pattern, brightness, color, position, motion, and dis- 

 tance. Our awareness of most of these derives fairly directly from the 

 character of the retinal image itself, whose size and shape, tempered by 

 experience and memory, tell us the 'true' size and shape of the object. 

 We translate subjective luminosity into objective intensity, making un- 

 conscious allowances for our adaptation-condition and for the illumin- 

 ation of the moment. A white object thus seems white even when it is 

 reflecting less light than some black object seen under other conditions. 

 So also with color, which remains remarkably constant in our minds even 

 though the illumination be qualitatively altered. Pattern resolves into 

 variations of brightness and color, motion into a varying stimulation of 

 successive retinal regions. 



Dependence of Apparent Distance upon Size — When we consider 

 distance however, we are dealing with an object-attribute concerning 

 which the retinal image, alone, can give us no information whatever. A 

 light ray is a straight line, and when one end of that line lies in a visual 

 cell the physiological result is the same no matter how near or how far 

 the other end of the line may be. For our knowledge of the distance of 

 a visual object, we are much more dependent upon past experience than 

 upon what our eyes can tell us at the time. 



To know either the size or the distance of an object, we must know the 

 other. If we do not know one of these facts, we do not know either. The 

 farther an object is, the smaller its retinal image and the smaller its 

 apparent size. If it looks small and we know it to be large, we judge it 

 to be far away. If it looks large and we know it to be small, we judge it to 

 be close at hand. Brightness, haziness, overlapping of other objects, per- 

 spective, vertical position with respect to the horizon, and parallax are 

 other factors in monocular judgement of distance; but like size itself, 

 most of these are in a sense interchangeable with distance and can aid us 

 to accurate estimations of distance only in so far as they themselves are 



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