Page:Popular Science Monthly Volume 21.djvu/211

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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 lifting 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

PSM V21 D211 Stereoscope adjustable for reversed perspective.jpg
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 hemisphere of crystal, through which on its farther side are seen the familiar dead sea-bottoms and jagged volcanic ridges. Our prisms and windowed screen have apparently turned the moon into a cup by bringing 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 direction of the rays, which leave it to be refracted by the prisms before entering the eye. To the combined Cyclopean eye, while each circle