CHAP. V.] 



THE STEREOSCOPE. 



281 



If to this we add the necessity of accommodating the eye for 

 accurate vision according to distance we shall understand the differ- 

 ence, already explained, between the impression produced by the 

 binocular sight of real objects and the impression made by the 

 most accurate picture representing them. In the case of the 

 picture similar images are painted on each retina, and vision in 

 relief, stereoscopic vision (<7Tf/jeu9, solid, and GKOTrelv, to see) is 

 impossible. 



It is to the analysis of these phenomena that we owe the inven- 

 tion of the optical instruments known as stereoscopes. A celebrated 

 English natural philosopher, the late Sir Charles Wheatstone, was 

 the first who had this idea, and he realized it in the little apparatus 

 which bears the name of reflecting stereoscope. 



This very simple arrangement is as follows : M and M' are two 

 vertical mirrors placed at right angles to each other on a rectangular 



6 



FKI. 201). Wheatstone's reflecting stereoscope. 



board, so as to form with the edges of this board angles of 45. Two 

 lateral uprights A and A' are furnished with cross-head guides, and 

 can thus receive two images of the same object, of the same view. 

 It is evident that these images will be reproduced reflected by each 

 mirror, and form two vertical images, apparently placed behind each 

 mirror symmetrically with regard to the actual object. Thus a b will 

 produce the image a l b l the two similar points a' 1)' of the object on 

 the right will form an image a\ b\ which will be placed exactly over 

 the first. 



If then the two eyes 00' are placed in front of the mirrors, and 

 if by means of a diaphragm each is prevented from seeing the image 

 produced on the other mirror, the two images a^ 7^ and a\ b\ will 



