4 6 



.THE POPULAR SCIENCE MONTHLY. 



middle line f. E E' are panels, to which the pictures are fixed in such 

 a manner that their corresponding horizontal lines shall be on the 

 same level ; these panels are capable of sliding backward and forward 

 in grooves on the upright boards, DD'. The observer must place his 

 eyes as near as possible to the mirrors, the right eye before the right- 

 hand mirror, and the left eye before the left-hand mirror ; and he must 

 move the sliding-panels E E' to or from him, until the two reflected 

 images coincide at the intersection of the optic axes, and form an 

 image of the same apparent magnitude as each of the component pict- 



ures. 



55 



In using this stereoscope, of which a perspective view is given in 

 Fig. 4, the two conjugate pictures must be on separate cards, but may 

 be much larger than those which are now so extensively used with 

 more modern instruments. The arrangement is obviously such that 

 no side-images can be perceived, since it is impossible for either eye 

 to receive more than one image, and this is reflected from the oblique 

 mirror directly in front. As an instrument it is unwieldy and incon- 

 venient in comparison with those to which we are accustomed ; but 



Fig. 6. The Binocular Camera. 



with it the great secret of binocular vision was brought into open day- 

 light. Wheatstone had the genius to find out how the door was to be 

 unlocked, and it was left for others to devise the special forms that 

 would be employed in making most acceptable to the world the treas- 

 ure which he had found. His predecessors had more or less distinct 

 conceptions of an hypothetical treasure, just as something was known 

 about the nature of steam before the low-pressure engine was invented, 



