788 



SIGHT. 



at different distances from 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 judgment 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. 



FIG. 182. 



691. 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 

 the right eye cannot be exactly like that which falls on the left, though 

 both are combined in a 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. 182, R, while 

 that which falls on the left eye has the form of Fig. 182, L ; yet the percep- 

 tion gained from the two images together corresponds to the form of which 

 Fig. 182, 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 dis- 

 similar pictures, such as would correspond to the vision of each eye sep- 

 arately, 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 perception 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 R on L (Fig. 

 182), 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 in 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 shown 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 movement of the eyes. If the flash occurred at the moment that 

 the eyes were binocularly adjusted for the bases of the pyramids, the two 



