CHAP, ii.] 



SIGHT. 



551 



diminished seems to recede. In these cases the influence on our 

 judgment of the muscular sense of binocular adjustment, or 

 monocular accommodation, is thwarted by the more direct influence 

 of the association between size and distance. 



Judgment of Solidity. When we look at a small circle all 

 parts of the circle are at the same distance from us, all parts are 

 equally distinct at the same time, whether we look at it with one 

 eye or with two eyes. When, on the other hand, we look at a 

 sphere, the various parts of which are at different distances from 



FIG. 77. 



us, a sense of the accommodation, but much more a sense of the 

 binocular adjustment, of the convergence or the opposite of the two 

 eyes, required to make the various parts successively distinct, 

 makes us aware that the various parts of the sphere are unequally 

 distant ; and from that we form a judgment of its solidity. As 

 with distance of objects, so with solidity, which is at bottom a 

 matter of distance of the parts of an object, we can form a judg- 

 ment with one eye alone ; but our ideas become much more exact 

 and trustworthy when two eyes are used. And we are much 

 assisted by the effects produced by the reflection of light from the 

 various surfaces of a solid object ; so much so, that raised surfaces 

 may be made to appear depressed, or vice versa, and flat surfaces 

 either raised or depressed, by appropriate arrangements of shadings 

 and shadow. 



Binocular vision, moreover, affords us a means of judging of the 

 solidity of objects, inasmuch as the image of any solid object which 

 falls on to the right eye cannot be exactly like that which falls on 

 the left, though both are combined in the single perception of the 

 two eyes. Thus, when we look at a truncated pyramid placed in 

 the middle line before us, the image which falls on the right eye is 

 of the kind represented in Fig. 77 E, while that which falls on the 

 left eye has the form of Fig. 77 L; yet the perception gained 

 from the two images together corresponds to the form of which 

 . Fig. 77 B is the projection. Whenever we 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 



