162 



JUDGMENT OF SOLIDITY. 



[BOOK in. 



the flatter, apparent vault. An amusing illustration of visual 

 judgments may be obtained by asking a number of persons in, 

 succession what they regard as the size of the moon in mid 

 heavens. Even making allowance for dioptric differences in 

 individual eyes the size of the retinal image of the moon must be 

 about the same in all eyes. And yet while some persons will be 

 found ready to compare the moon in mid heavens with a three- 

 penny piece, others will liken it to a cart-wheel ; and others will 

 make intermediate comparisons. 



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

 a sense of the binocular adjustment, of the greater or less con- 

 vergence 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. We are 

 further much assisted by the effects produced by the reflection 

 of light from the various surfaces of a solid object, and the shadows 

 cast by its raised parts ; 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 percep- 

 tion 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. 161 K, while that 



FIG. 161. 



which falls on the left eye has the form of Fig. 161 L; yet the 

 perception gained from the two images together corresponds to 

 the form of which Fig. 161 B is the projection. Whenever we 



