in Single and Binocular Vision. 445 



'£> 



ing the eyes to wander from it for a moment. The entire line 

 will appear single, and in its proper relief," &c. After making 

 this experiment with the greatest care, we admit that it may 

 appear single, without being single. To us it does not appear 

 single, but exactly the same as a line having the same length 

 and the same position appears in ordinary vision. Now, 

 though this latter line appears single to most eyes, yet it is 

 certain that every point of it is double and indistinct, except- 

 ing the point on which the attention is fixed, and to which 

 the optic axes converge. The vision of objects in relief from 

 the union of dissimilar pictures, is performed by the very same 

 process as the vision of real objects in relief by the ordinary 

 agency of our two eyes ; and in establishing this principle, the 

 true cause of the phaenomenon discovered by Mr. Wheatstone 

 will be readily obtained. 



Mr. Wheatstone considers it as experimentally established, 

 that " the most vivid belief of the solidity of an object of three 

 dimensions arises from two different perspective projections of 

 it being simultaneously presented to the mind ;" and that "the 

 simultaneous vision of two dissimilar pictures suggests the re- 

 lief of objects in the most vivid manner." Having already 

 explained, in § 3, the true process by which solid bodies are 

 seen in relief, I shall now endeavour to show, that, in the vivid 

 relief produced by the union of two dissimilar plane pictures, 

 this union is merely a necessary accessory, and not the cause 

 of the phfenomenon in question. 



When two of the images of two perfectly similar objects are 

 united either by looking at a nearer or a remote object, the 

 compound image thus formed is seen at the place where the 

 two optic axes converge, and is larger and more remote than 

 the single image if we look at a more distant object, and 

 smaller and nearer if we look at a nearer object*. The best 

 mode of conducting this class of experiments is to suspend two 

 equal rings by invisible fibres, or to cement them upon a large 

 plate of glass, whose surface and figure are not visible to the 

 observer. The object of this arrangement is to prevent the 

 observer from having any knowledge of their distance from 

 the eye. When the rings, thus placed, are doubled, interpose 

 an aperture, so as to permit only the united rings to be seen ; 

 and it will be found that they appear at the place to which the 

 optic axes converge, appearing smaller and nearer, or larger 

 and more remote, according as the optic axes are converged 

 to a point nearer or more distant than the actual rings. In 



* Several curious facts establishing this result have been given by Dr. 

 Smith in his Complete System of Optics, vol. ii. 387-389; and Remarks, 

 § 526-527- 



