STEREOPSIS IN MAN 315 



increases or decreases. The amount of convergence, evaluated quite un- 

 consciously via kinesthetic reception from the internal rectus muscles, is 

 a potent cue to distance. It is effective up to the greatest distances for 

 which we converge at all appreciably — up to a hundred feet or more, 

 which is far beyond the distances for which we accommodate. 



The stimulus to converge seems to be the psychic impression of near- 

 ness. Convergence is then guided to the point of precision by the urge to 

 unify the two one-eyed images of the object being attended to, with 

 accommodation tagging along as a dependent reflex. When the object is 

 seen singly, convergence and accommodation freeze; and the parallactic 

 angles of convergence of the two eyes, being simultaneously recorded in 

 the nervous system, afford a precision of distance-judgment which succes- 

 sive monocular parallaxes can never yield. The perception of singleness 

 is inseparable from the perception of the distance of the object; and in 

 fact both are attributed to the object — the latter's distance from us seem- 

 ing as much a part of the object as its size and shape. In man, at least, 

 singleness of a solid object is also inseparable from the perception of its 

 solidity — the psychic process which we call stereopsis. 



Stereopsis in Man — Stereopsis means, literally, 'seeing solid'. As a 

 word, it has been loosely used as a synonym for distance- or depth-per- 

 ception (which is better known as bathopsis) ; but we can perceive depth 

 without solidity, or solidity without depth. For the estimation of dis- 

 tances in the visual field, convergence must be allowed; and it must be 

 allowed to 'play' or vary back and forth until it finds its dead center on 

 the object. But the perception of solidity is literally lightning fast, for it 

 is obtained in a stereoscope even when the pictures are illuminated by a 

 single electric spark lasting a ten-thousandth of a second. This, 'Dove's 

 experiment', is conclusive evidence that solidity does not depend upon a 

 play of convergence, for no time is allowed for that process. Nor is 

 convergence as such even necessary, for prisms can take its place as they 

 do in the ordinary stereoscope. As Javal pointed out years ago, the idea 

 of relief is one thing, and its measurement is another. Estimation of dis- 

 tance, depth, and thickness is closely associated with the recognition of 

 solidness, for both involve the idea of tridimensionality; but the one pro- 

 cess is dynamic and the other, static. 



For stereopsis, the prime essential is a particular blend of likeness and 

 difference between the images on the two retinae, and a particular position 

 of each image, this position being governed in ordinary experience by the 

 degree of convergence. But it does not really matter what the positions 



