SENSE OF VISION PERCEPTION OF SOLIDITY. 



765 



cording to the eye with which he sees it. With the right eye he will see its 

 right side, very much foreshortened ; with the left he will gain a corre- 

 sponding view r of the left side; and the apparent angles, and the lengths of 

 the different lines, will be found to be very different in the two views. On 

 looking at either of these views singly, no other notion of solidity can be 

 acquired from it, than that to which the mind is conducted, by the associa- 

 tion of such a view with the touch of the object which it represents. But it 

 is capable of proof, that the mental association of the different pictures upon 

 the two retinre, does of itself give rise to the idea of solidity. This proof is 

 afforded by Professor Wheatstone's ingenious instrument, the Stereoscope, 

 first described by him in 1838. * 



623. The Stereoscope in its original form essentially consists of two plane 

 mirrors, inclined with their backs to one another, at an angle of 90. If 

 two perspective drawings of any solid object, as seen at a given distance with 

 the two eyes respectively, such as those at A and B, Fig. 272, be so placed 

 before these mirrors, one before each, that their two images shall be made to 

 fall upon the corresponding parts of the two retiuse, in the same manner as 

 the two images formed by the solid object itself would have done, the mind 

 will perceive, not a single representation of the object, nor a confused blend- 

 ing of the two, but a projecting or receding surface, the exact counterpart 

 of that from which the drawings were made. 2 The solid form is forcibly 



FIG. 272. 



impressed on the mind, even when outlines only are given, especially if these 

 be delineations of simple geometrical figures, easily suggested to the mind ; 

 and it may be readily shown that the very same outline will suggest differ- 

 ent conceptions, according to the mode in which they are placed. Thus in 



1 Various modifications of this instrument have been subsequently introduced ; 

 and there is one which has come into very extensive use, in which the two monocular 

 pictures placed side by side, as in Figs. 272, 273, are viewed by the two eyes respec- 

 tively through two halves of a convex lens. The great advantage of this instrument 

 is its portability, and its enlargement of the pictures by the magnifying power of the 

 lenses; but it is limited to pictures of small size, since the distance between corre- 

 sponding points of the two pictures must not exceed the distance between the centres 

 of the two eyes ; and it is incapable of many adaptations which can be made with 

 the mirror-stereoscope. In the Stereomonoscope of Claudet the idea of relief is ob- 

 tained by looking with both eyes at once on a ground glass plate at the image pro- 

 duced by the coalescence of the two images of a stereoscopic slide, each refracted by 

 a separate lens. (See Proceed, of Royal Society, vol. ix, 1857-59, p. 194.) 



8 The most striking effect is produced by two Photographic pictures, taken at the 

 same time by two cameras, so placed that their axes shall form the same angle with 

 each other as that which the axes of the two eyes would form when looking at the 

 same object. This adaptation, though the credit has been assumed by others, was 

 originally devised by Prof. Wheatstone. 



