966 JUDGMENT OF SOLIDITY. [BOOK in. 



distant than the horizon ; a geometric construction will shew 

 that a body of the same size placed at different parts of the real 

 (spherical) vault will appear greater near the horizon than near 

 the zenith of the flatter, apparent vault. An amusing illustra- 

 tion 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 differ- 

 ences 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. 



60L 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 dis- 

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

 sense of the binocular adjustment, of the greater or less conver- 

 gence of the two eyes, required to make the various parts suc- 

 cessively 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 ob- 

 ject 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. 165 R, 

 while that which falls on the left eye has the form of Fig. 165 

 L ; yet the perception gained from the two images together cor- 

 responds to the form of which Fig. 165 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 mir- 



