534 



BINOCULAE VISION. 



left eye are alternately shut or covered by the -hand, 

 a difference will be discovered in the two aspects of the 

 pencil which corresponds to that between A and B in 

 fig. 292. When we are looking at an object with both 

 eyes at the same time, we are unconscious of this 

 difference between the two images, but it is precisely 

 the coalescence of these two images into a common one 

 which gives the impression of solidity. As long as we 

 use only one eye in looking at bodies they present only 

 length and breadth, like the flat surface of a picture; 



FIG. 292. 



but simultaneous vision with both eyes produces the 

 idea of relief, that is, the perception of depth in space. 

 It is true that we scarcely fail to arrive at the same 

 perception when we look at objects with one eye shut, 

 but this is the result of a rapid mental process of which 

 we have become unconscious in consequence of our con- 

 stant experience of the succession of things in depth. 

 Still our estimation of depth in space is not trustworthy 

 if we rely on the information conveyed by one eye only. 



The difficulty of estimating distances with one eye may be shown 

 by the following experiment. Bend one end of a wire, 20 cm long and 

 2 or 3 mm thick, into a ring of about 4 cm diameter ; file the other end 

 to a point, and fasten it into the top of the bar of the retort-stand, 



