BINOCULAR VISION. 125 



The fact just stated is of course a mathematical necessity 

 in binocular vision for near objects ; but the actual demon- 

 stration of the* fusion of two dissimilar images and the conse- 

 quent formation of a single image giving the impression of 

 solidity was made by the invention of the stereoscope, by 

 Wheatstone. 1 The principle of this instrument is very sim- 

 ple. Two pictures are made, representing a solid object, one 

 viewed slightly from the right side, and the other, slightly 

 from the left, so as to imitate the differences in the images 

 formed upon the two retinae. These pictures are so placed 

 in a box that the image of one is formed upon the right ret- 

 ina, and the other, upon the left. "When these conditions are 

 accurately fulfilled, we see but a single image, and this con- 

 veys to the mind the perfect illusion of a solid object. Ex- 

 periments with the stereoscope are so familiar that they need 

 hardly be dwelt upon. With most persons, an apparatus is 

 necessary to shut off disturbing visual impressions ; but some 

 individuals are able to fuse two images in this way, placed in 

 proper position, without the aid of an instrument, by a sim- 

 ple effort of the will. 



The invention of the stereoscope has led to many curious 

 and interesting experiments, especially since the art of pho- 

 tography has enabled us to produce pictures in any position 

 with absolute accuracy ; but a simple statement of the princi- 

 ple upon which the instrument is constructed illustrates the 

 mechanism of binocular vision in the appreciation of the 

 form of objects. Experience, the aid of the sense of touch, 

 etc., enable persons with but one eye to get a notion of form, 

 but the impressions are never entirely accurate in this regard, 

 although, from habit, this defect occasions little or no incon- 

 venience. A striking illustration of these points is afforded 

 by the binocular microscope, which, especially with low mag- 

 nifying powers, produces a startling impression of relief. 



As we have just remarked, the stereoscope affords a satis- 



1 WHEATSTONE, Contributions to the Physiology of Vision. Philosophical 

 Transactions, London, 1838, vol. xxxviii., p. 371, et seq. 



