526 



THE SENSE OF SIGHT. 



connected together by the commissural fibres of the corpus 

 callosum, and so enabled to exercise unity of function. 



But, on the whole, it is more probable, that the power of 

 forming a single idea of an object from a double impression 

 conveyed by it to the eye is the result of a mental act. This 

 view is supported by the same facts as those employed by 

 Professor Wheatstone to show that this power is subservient to 

 the purpose of obtaining a right perception of bodies raised in 



FIG. 195. 



relief. When an object is placed so near the eyes that to view 

 it the optic axes must converge, a different perspective pro- 

 jection of it is seen by each eye, these perspectives being more 

 dissimilar as the convergence of the optic axes becomes greater. 

 Thus, if any figure of three dimensions, an outline cube, for 

 example, be held at a moderate distance before the eyes, and 

 viewed with each eye successively, while the head is kept per- 

 fectly steady, A (Fig. 195) will be the picture presented to the 

 right eye, and B that seen by the left eye. Mr. Wheatstone 

 has shown that on this circumstance depends in a great meas- 

 ure our conviction of the solidity of an object, or of its pro- 

 jection hi relief. If different perspective drawings of a solid 

 body, one representing the image seen by the right eye, the 

 other that seen by the left (for example, the drawing of a 

 cube A, B, Fig. 195), be presented to corresponding parts of 

 the two retinae, as may be readily done by means of the stereo- 

 scope, an instrument invented by Professor Wheatstone for 

 the purpose, the mind will perceive not merely a single rep- 

 resentation of the object, but a body projecting in relief, 

 the exact counterpart of that from which the drawings were 

 made. 



