CHAP. iii.J SIGHT. 163 



thus combine in one perception two dissimilar images, one of the 

 one, and the other of the other eye, we judge that the object 

 giving rise to the images is solid. 



This is the simple principle of the stereoscope, in which two 

 slightly dissimilar pictures, such as would correspond to the vision 

 of each eye separately, are, by means of reflecting mirrors, as in 

 Wheatstone's original instrument, or by prisms, as in the form 

 introduced by Brewster, made to cast images on corresponding 

 parts of the two retinas so as to produce a single perception. 

 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 per- 

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

 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 Eon L(Fig. 161), 

 it is evident that when the bases coincide the truncated apices 

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

 sponding 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 duration 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 summits 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 operations, 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. 



800. If the images of two surfaces, one black and the other 

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

 to be united into a single perception, the result is not always a 

 mixture of the two impressions, that is a grey, but, in many cases, 

 a sensation similar to that produced when a polished surface, such 



