536 



THE STEREOSCOPE. 



That the impression of solidity is produced by our 

 seeing two different images of the same object when we 

 use both eyes, is strikingly demonstrated by the 

 stereoscope. Two pictures, each representing the same 

 object as seen by either eye alone, are by means of the 

 stereoscope presented to the eyes, but in such a manner 

 that each eye sees only one picture, while the apparent 

 positions of the pictures coincide and both pictures are 

 therefore seen in the same place. The solidity of the 

 object is brought out the more strikingly, the greater 

 the correctness of the two pictures which form the com- 

 bination. 



Let the left eye be at A in fig. 293, and the right 



FIG. 293 (-J real size). 



at B\ let a and b be the corresponding pictures for each 

 eye, and p^ p^ two prisms of glass through which the 

 pictures are seen. A prism, as we have seen, refracts 



