SINGLE VISION. 



275 



consists of two plane mirrors, with their backs inclined to each other at 

 an angle of 90, near the faces of which two monocular pictures are so 

 disposed, that their reflected images are seen by the two eyes, each 

 looking into one of the mirrors on the same plane. The experiment 

 may, however, be made sufficiently well by the subjoined figures. 



Fig. 121. 



\ 



Binocular Vision. Professor Wheatstone's Experiment. 



Fix the right eye on the right-hand figure, and the left eye on the 

 left-hand figure ; hold between the eyes, in front of the nose, the board 

 of an octavo book. The two figures a a will be seen to approximate, 

 and to run into one, representing the skeleton of a truncated four-sided 

 figure in bold relief, b; a fact, which shows, that the visual apprecia- 

 tion of solidity or projection arises from the combination in the mind 

 of two different images. These could not exist in a person who has 

 never had more than one eye ; and therefore from sight alone lie could 

 form no notion of solidity. He would have to combine with sight the 

 evidence afforded by touch. 



All these facts demonstrate, that two impressions are really made in 

 all cases, one on each eye ; and yet the brain has perception of but 



Fig. 122. 



Fig. 123. 



.binocular Vision. 



one. If the law of visible direction, which Sir David Brewster has 

 pointed out (see page 258), be adopted, the cause of single vision with 

 two eyes must be admitted as a necessary consequence. If we arc 



