SENSE OF VISION. 885 



even supposing the fact to be as stated, would be far from affording the solution 

 of the problem and it would be entirely inapplicable to that very important 

 series of phenomena to be next described, which show how large an amount of 

 information we derive, not merely from the repetition, but from the difference, 

 of the sensory impressions made by the same object upon our two retinae; and 

 which indicate that here, as in the case of erect vision, the mental interpretation 

 of the sensory impressions is a process altogether removed from the simple 

 affection of the consciousness by those impressions, and is not to be accounted 

 for by any structural arrangements of the Sensorial apparatus. 



887. We shall next consider the mode in which our notion of the solid forms 

 and relative projection of objects is acquired ; on which great light has been 

 thrown by the interesting experiments of Prof. Wheatstone. 1 It seems per- 

 fectly evident, both from reason and experience, that the flat picture upon 

 the retina, which is the immediate source of our sensation, could not itself con- 

 vey to our minds any notion but that of a corresponding plane surface. In 

 fact, any notion of solidity, which might be formed by a person who had never 

 had the use of more than one eye, would entirely depend upon the combination 

 of his visual and tactile sensations. This view is fully confirmed by the case 

 already referred to, as recorded by Cheselden. The first visual idea formed by 

 the youth was that the objects around him formed a flat surface, which touched 

 his eyes, as they had previously been in contact with his hands ; and after this 

 notion had been corrected, through the education of his sight by his touch, he 

 fell into the converse error of supposing that a picture, which was shown to 

 him, was the object itself represented in relief on a small scale. But where 

 both eyes are employed, it has been ascertained by Prof. Wheatstone that they 

 concur in exciting the perception of solidity or projection, which arises from the 

 mental combination of the two dissimilar pictures formed upon the two retinae. 

 It is easily shown, that any near object is seen in two different modes by the 

 two eyes. Thus let the reader hold up a thin book, in such a manner that its 

 back shall be exactly in front of his nose, and at a moderate distance from it ; 

 lie will observe, by closing first one eye and then the other, that his perspective 

 view of it (or the manner in which he would represent it on a plane surface) is 

 very different, according to the eye with which he sees it. With the right eye, 

 he will see its right side, very much foreshortened j with the left, he will gain 

 a corresponding view 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 association 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 two different pictures upon the 

 retinae does of itself give rise to the idea of solidity. This proof is afforded 

 by Prof. Wheatstone' s ingenious instrument, the Stereoscope, first described by 

 him in 1838. 3 



1 "Philosophical Transactions," 1838 and 1852. 



2 Various modifications of this instrument have been subsequently introduced; and 

 there is one which has recently (1852) come into very -extensive use, in which the two 

 monocular pictures placed side by side, as in Figs. 214, 215, are viewed by the two eyes 

 respectively through two prisms, or two halves of a convex lens. The great advantage of 

 this instrument is its portability ; but it is limited to pictures of small size, since the dis- 

 tance between corresponding 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. As Sir D. Brewster has recently put forth his 

 claim as an original discoverer in regard to the truths of binocular vision which have been 

 established by the Stereoscope, on the strength of some trivial improvements in the con- 

 struction of the instrument, the Author feels it due to Prof. Wheatstone to state his own 

 conviction, founded upon a careful examination of the whole history of the invention, that 



