Chap, hi.] 



SIGHT. 



967 



rors, as in Wheatstone's original instrument, or by prisms, as in 

 the form introduced by Brewster, made to cast images on corre- 

 sponding parts of the two retinas so as to produce a single per- 

 ception. Though each picture is a surface of two dimensions 

 only, the resulting perception is the same as if a single object, 

 or group of objects, of three dimensions had been looked at. 



It might be supposed that the judgment of solidity which 

 arises when two dissimilar images are thus combined in one 

 perception, was due to the fact that all parts of the two images 



Fig. 165. 



cannot fall on corresponding parts of the two retinas at the same 

 time, and that therefore the combination of the two needs some 

 movement of the eyes. Thus, if we superimpose R on L (Fig. 

 165), it is evident that when the bases coincide the truncated 

 apices will not, and vice versa; hence, when the bases fall on 

 corresponding parts, the apices will not be combined into one 

 image, and vice versa; in order that both may be combined, 

 there must be a slight rapid movement of the eyes from the one 

 to the other. That, however, no such movement is necessary 

 for each particular case is shewn by the fact that solid objects 

 appear as such when illuminated by an electric spark, the dura- 

 tion of which is too short to permit of any movements of the 

 eyes. If the flash occurred at the moment that the eyes were 

 binocularly adjusted for the bases of the pyramids, the two sum- 

 mits not falling on exactly corresponding parts would give rise 

 to the perceptions of two summits, and the whole object ought 

 to appear confused. That it does not, but, on the contrary, 

 appears a single solid, must be the result of psychical opera- 

 tions, resulting in what we have called a judgment. 



As we have seen, in any one position of the two eyes, only a 

 small portion of the field of sight lies in the horopter and falls 

 on corresponding points of the two retinas. Most of the objects 

 in a scene on which we look give rise to dissimilar images in 

 the two eyes ; and we attribute solidity to them by reason on 

 the one hand of the movements of the eyes, and on the other 

 hand of the psychical processes just mentioned. Conversely the 

 same processes which thus give rise to apparent solidity assist 

 us in forming judgments of distance. 



§ 602. If the images of two surfaces, one black and the 

 other white, are made to fall on corresponding parts of the eye, 



