CLUES TO DEPTH AND DISTANCE 313 



(D) Depth- and Solidity-Perception 



In the first section of this chapter we considered the methods by which 

 vertebrate eyes adjust themselves for the distance of the object being 

 viewed. It was pointed out that this adjustment, accommodation, has 

 nothing to do with giving the animal an awareness and estimate of the 

 distance. This awareness of the 'third dimension', or toward-and-away 

 distances and movements, is a perceptual matter and not, like accommo- 

 dation, an optical one. Moreover, it is unrelated to the perception of 

 movements in the other two dimensions of space — horizontal and vertical 

 displacements of visual objects. This latter kind of perception, which is 

 movement-perception in the usual sense of the term, is considered in the 

 next section. Here, we are concerned with the means by which man and 

 animals judge visually the distances, depth, and thickness of objects; and 

 with the question of whether, and how, vertebrates perceive solidity — 

 whether, for any of them, stereopsis is possible as it is for man. 



Clues to Depth and Distance — The estimation of distance is an 

 exclusive monopoly of the sense of sight in all vertebrates except the bats. 

 It is quite impossible to be sure of distances when walking in the dark, 

 and our judgment of the distance from which a sound has come is faulty 

 in the extreme. In human vision, a number of clues exist which we inte- 

 grate perceptually to arrive at an evaluation of distance and the relative 

 distances of several objects. Most of these clues are as readily employed 

 in monocular vision as in binocular — in fact, they are incorporated by 

 any good artist into his two-dimensional painting in order to promote the 

 illusion of depth (see also p. 194). But when the two eyes are in use in a 

 three-dimensional visual field, a special and important factor is intro- 

 duced which is of particular value when the object is close at hand; and, 

 of course, it is the closest objects which are most important visually, as 

 any blind man knows. There is no more vexed question in all of compar- 

 ative ophthalmology than the one whether this binocular factor in depth- 

 perception exists for vertebrates below the mammals. But certainly the 

 same monocular clues that we humans employ are available to all verte- 

 brates. Whether a given animal can use a particular one of them, how- 

 ever, depends upon his powers of observation, his learning capacity, and 

 his equipment of instincts. These monocular clues are : 



A. Retinal image size. Where the object is a familiar one, its apparent 

 size, as determined by the size of its image on the retina, is a cue to its 



