Page:Popular Science Monthly Volume 21.djvu/210

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



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 necessity 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 employed on which the interval is as great as four inches. If, while viewing the combined image, the semi-lenses be screwed closer together, the eyes will continue to adapt themselves, while fusion of images 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 folding 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