THE STEREOSCOPE. 



671 



retinae of the two eyes. It is supposed, indeed, that these 

 fibres may connect the corresponding parts of the two 

 retinae, and may thus explain their unity of action ; in the 

 same way that corresponding parts of the cerebral hemi- 

 spheres are believed to be connected together by the com- 

 missural 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 im- 

 pression 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 percep- 

 tion of bodies raised in relief. When an object is placed 

 Fig. 1 88. 



\ 



so near the eyes that to view it the optic axes must con- 

 verge, a different perspective projection 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 ex- 

 ample, be held at a moderate distance before the eyes, 

 and viewed with each eye successively, while the head is 

 kept perfectly steady, A (fig. 188) will be the picture pre- 

 sented to the right eye, and B that seen by the left eye. 

 Mr. Wheatstone has shown that on this circumstance 

 depends in a great measure our conviction of the solidity 

 of an object, or of its projection in relief. If different 

 perspective drawings of a solid body, one representing the 



