418 ON SENSATION, AND THE ORGANS OF THE SENSES. 



that they concur in exciting the perception of solidity or projection, 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 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 ; 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 retina?, 

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

 Wheatstone's ingenious instrument, the Stereoscope. 



547. 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 retina?, 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 consequence 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 com- 

 bination of the images furnished by the two eyes is a mental act, resulting 

 from an inherent law of our psychical constitution ; 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, how- 

 ever, the difference in the images formed by the two eyes is so slight, that it 

 cannot aid in the determination ; and hence it is, that, whilst we have no dif- 

 ficulty in distinguishing a picture, however well painted, from a solid object, 

 when placed near our eyes, (since the idea, which might be suggested by the 

 image formed on one eye, will then be corrected by the other,) we are very 

 liable to be misled by a delineation, in which the perspective, light and shade, 

 &c., are faithfully depicted, if we are placed at a distance from it, and are pre- 



