552 VISUAL JUDGMENTS. [BOOK in. 



Wheats-tone'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 E on L (Fig. 77), 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 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 pyra- 

 mids, the two apices not falling on exactly corresponding parts 

 would give rise to two perceptions, 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 cerebral operations, resulting in 

 what we have called a judgment. 



Struggle of the two Fields of Vision. If the images of two 

 surfaces, one black and the other white, are made to fall on corre- 

 sponding parts of the eye, so as to be united into a single percep- 

 tion, 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 as plumbago, is looked at : the surface 

 appears brilliant. The reason probably is because when we look at 

 a polished surface the amount of reflected light which falls upon 

 the retina is generally different in the two eyes ; and hence we as- 

 sociate an unequal stimulation of the two retinas with the idea of 

 a polished surface. So also when the impressions of two colours 

 are united in binocular vision, the result is in most cases not a 

 mixture of the two colours, as when the same two impressions are 

 brought to bear together at the same time on a single retina, but a 

 struggle between the two colours, now one, and now the other, be- 

 coming prominent, intermediate tints however being frequently 

 passed through. This may arise from the difficulty of accommo- 

 dating at the same time for the two different colours (see p. 508) ; 

 if two eyes, one of which is looking at red, and the other at blue, 

 be both accommodated for red ravs, the red sensation will over- 



