THE STEREOSCOPE : ITS HISTORY. 



41 



Subjectively, therefore, our condition is not so very different from 

 that of the famous Cyclops. We have the advantage of being able to 

 see double, by adjusting conditions properly ; but, if sensation is to be 

 trusted, the object is duplicated while the eye is single, although by 

 other means we learn that the object remains single, and is only viewed 

 from two different stand-points at the same moment, while the sepa- 



Fig. 2. Wheatstone's Stereoscope. (Front View.) 



1 







Fig. 3. Wheatstone's Stereoscope. (Ground Plan.) 



rate lines of direction for the two eyes meet elsewhere. By appropri- 

 ate muscular training the eyes may be directed, each slightly outward, 

 so that these "lines meet behind the observer's head while the object, 

 apparently duplicated, is seen still in front. The recognition of the 

 subjective fusion of the two eyes into a Cyclopean, or central binocular 

 eye, is a fundamental prerequisite for the explanation of vision in the 

 stereoscope. In consequence of this, if two similar pictures are placed 

 close in front of the eyes, the distance between their centers being 

 equal to the distance between the pupils, they at once appear to coa- 

 lesce into a single picture. In this way an objective existence may 

 appear to be given to the binocular eye by approaching a mirror until 

 the nose touches the glass, and avoiding the convergence of visual 

 lines that would otherwise be natural. A narrow face is seen, possess- 



