SENSE OF VISION. 251 



idea formed by the youth was, that the objects around him formed a flat sur- 

 face, 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 Mr. 

 Wheatstone that they concur in exciting the perception of solidity or pro- 

 jection, which arises from the combination of two different images in the 

 mind. 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 ; he 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 surfabe) 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 ; 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 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 Mr. Wheatstone's ingenious instrument, the Stereoscope. 

 340. The Stereoscope essentially consists of two plane mirrors, inclined 

 with their backs to one another at an angle of 90. If two perspective draw- 

 ings of any solid object, as seen at a given distance with the two eyes respect- 

 ively, be placed before these mirrors, in such a manner that their images 

 shall be made to fall upon the corresponding parts of the two retinae, 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 union of the two, but a body projecting in relief, the exact 

 counterpart of that from which the drawings were made. Mr. Wheatstone 

 further shows, by means of the Stereoscope, that similar images, differing to a 

 certain extent in magnitude, when presented to the corresponding parts of the 

 two retinae, give rise to the perception of a single object, intermediate in size 

 between the two monocular pictures. Were it not for this, objects would 

 appear single only when at an equal distance from both eyes, so that their 

 pictures upon the retina are of the same size ; which will only happen when 

 they are directly in front of the median line of the face. Again, if pictures 

 of dissimilar objects be simultaneously presented to the two eyes, the conse- 

 quence will be similar to that which is experienced when the rays come to 

 the eye through two differently-coloured media; the two images do not 

 coalesce, nor do they appear permanently superposed upon one another ; but 

 at one time one image predominates to the exclusion of the other, and then 

 the other is seen alone ; and it is only at the moment of change that the two 

 seem to be intermingled. It does not appear to be in the power of the will, 

 Mr. Wheatstone remarks, to determine the appearance of either ; but, if one 

 picture be more illuminated than the other, it will be seen during a larger 

 proportion of the time. :Many other curious experiments with this simple 

 instrument are related by Mr. Wheatstone ; and they all go to confirm the 

 general conclusion, that the combination of the images furnished by the two 

 eyes is a mental act, resulting from an inherent law of our psychical constitu- 

 tion ; and that our perceptions of the solidity and projection of objects, near 

 enough to be seen in different views with the two eyes, result from this cause. 

 In regard to distant objects, however, the difference in the images formed by 



