460 SPECIAL PHYSIOLOGY. 



These correspondences are very liable to delusion, or misinterpreta- 

 tion ; thus, if we look with one eye, at a raised caineo, or medal, 

 illuminated from the right-hand side, we perceive that its surface is in 

 relief; but, if by an effort of imagination, we suppose the light to 

 come from the left-hand side, the design appears to be in intaglio, or 

 hollow. The same effect may be produced by looking at the medal, 

 through a convex lens held at a distance from the single eye, so that 

 the image is reversed, the light still remaining on the right-hand side. 

 These experiments show how completely the ultimate notions derived 

 from our sense of sight are mental. 



But, in actual vision, we employ both eyes, each of which receives its 

 own image of any given solid object. When such objects are within 

 the range of the binocular parallax, the optic axes converge, and, 

 moreover, the images or figures, formed in the two eyes, do not exactly 

 agree ; for each eye sees a different aspect of the same solid body, as, 

 for example, of a sphere, a cube, or a book ; the right eye seeing 

 more of the right side, and the left, more of the left side, of the object. 

 The difference between the two images, is regulated by the distance of 

 the object, and by the interocular distance. We frequently, indeed, 

 view an object, first with one eye, and then with the other, in order to 

 gain a better knowledge of its form, or its position in space. By a 

 mental combination of these two different perspective impressions, the 

 idea of the solidity of the object is produced, not, however, as a 

 simple sensation, or even as an intuition, but, in the very earliest 

 period of our lives, as the result of a joint action of sight and touch, 

 leading to the formation of a notion of solidity, as producing certain 

 visual appearances of form, light, and shade. Such notions may 

 seem to be intuitive in after life, and have been named secondary in- 

 tuitions. 



A good illustration of the effect of the two eyes in giving the notion 

 of solidity, is furnished by the philosophical apparatus known as the 

 stereoscope. By the combination, through optical means, of two draw- 

 ings of a solid object, taken at different points of view, and showing, 

 therefore, two different aspects of the same, this instrument commu- 

 nicates to the mind, the appearance of a solid body, or of a body of 

 three dimensions. The reflecting stereoscope invented by Wheatstone, 

 consists of two mirrors, placed with their backs towards each other, at 

 an angle of 90 ; by means of two sliding frames, one at each side, 

 two different perspective drawings of the same solid object, can be 

 fixed and adjusted, so that their images in the mirrors, are separately 

 seen by the two eyes, placed in front of the converging mirrors. The 

 images thus formed in the two eyes, which resemble the natural im- 

 ages of the object, when this is regarded directly by them, are then 

 mentally combined, as in the case of such natural images, into a single 

 perception. But this only happens, when the eyes receive the images 

 on the corresponding or identical parts of the two retinae; for if either 

 image is out of place, the two do not coalesce, but are seen separately, 

 zudflat, not solid. The refracting stereoscope, invented by Sir David 

 Brewster, consists of two eccentric double convex lenses, each connected 

 with a sliding tube, by which they can be adjusted to suit the sight 



