THE STEREOSCOPE : ITS THEORY. 



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another folding-screen of wood, which is shown pressed down in Fig. 

 14, and raised in Fig. 15. In the former condition it does not obstruct 

 any part of the field of view, but in the latter it hides from each eye 

 the half of the stereograph on its own side, and permits that on the 

 other side to be seen through the opening at the middle. By now lift- 

 ing the cover of the cases containing the semi-lenses, these glasses may 

 be removed, and their places supplied with a pair of wedge-shaped 

 prisms, which are introduced with their bases, instead of their sharp 



Fig. 15. The Adjustable Stereoscope. Adjustment for Reversed Perspective. 



edges, against the springs, while the screens are arranged as in Fig. 

 15. Pushing the cross-bar, intended to hold the picture, out to the 

 farther end, a stereograph is put upon it that has been specially selected 

 to show the effects of binocular perspective. Any stereograph in which 

 mathematical perspective is not strong may be employed that of the 

 moon is excellent. Looking at this now through the prisms, instead 

 of appearing convex it presents the aspect of a lustrous hollow hemi- 

 sphere of crystal, through which on its farther side are seen the famil- 

 iar dead sea-bottoms and jagged volcanic ridges. Our prisms and 

 windowed screen have apparently turned the moon into a cup by bring- 

 ing into each eye the picture originally intended for the other. On 

 folding down the windowed screen, two extra moons spring into view. 

 Comparing the middle concave image with the flat ones upon the two 

 sides, it appears smaller and nearer, and this disparity is increased by 

 pulling the stereograph nearer. As it approaches it grows shallower 

 and slightly elliptic, the horizontal diameter becoming shorter ; for, as 

 the card is brought nearer, its plane becomes more oblique to the di- 

 rection of the rays, which leave it to be refracted by the prisms before 

 entering the eye. > To the combined Cyclopean eye, while each circle 



