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THE POPULAR SCIENCE MONTHLY. 



faces are nearly parallel. The rays are hence but little deviated in 

 transmission, and the condition is the same as that in the ordinary 

 stereoscope. Vision is then as comfortable as can ever be expected, 

 when the stereographic interval is less than three inches. If it exceed 

 this limit, the pain produced by the muscular strain of optic diver- 

 gence, which would now be necessary, is prevented by giving a few 



Fig. 14. The Adjustable Stereoscope. Adjustment tor Natural Perspective. 



leftward turns to each screw. The semi-lenses are at once pressed 

 farther apart by the springs, the rays pass through at points where the 

 opposite surfaces are more inclined to each other, and they are hence 

 more deviated, so as to enter the eyes still without imposing the ne- 

 cessity of divergence. Indeed, if the stereographic interval be small, 

 and free play be given to the springs, uncomfortable convergence may 

 be induced at will. Under this condition a stereograph may be em- 

 ployed on which the interval is as great as four inches. If, while 

 viewing the combined image, the semi-lenses be screwed closer to- 

 gether, the eyes will continue to adapt themselves, while fusion of im- 

 ages is retained, and any degree of divergence is thus induced that 

 the observer may be disposed to endure. If the stereograph has been 

 properly selected to illustrate the effects of muscle-reading, the image 

 will appear to increase in depth as the visual lines diverge. 



In front of the partition between the lens-cases are a pair of fold- 

 ing metal screens, of such width that when pressed flat against the 

 wood they will hide from each eye the picture on the side belonging 

 to the other, but when folded, as shown in Fig. 15 s, the whole stereo- 

 graph becomes visible to each eye. On a movable cross-bar there is 



