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ject, although the eyes are looking at a distant object, and are therefore unadjusted for a near one. The lenses also enlarge the images, acting like a perspective glass, and thus complete the illusion of a natural scene or object.
It is difficult to convince many persons that there is in the stereoscope any doubling of points in the foreground when the background is regarded, and vice versa. But such is really always the fact; and if we do not observe it, it is because we have not carefully analyzed our visual impressions. It is best observed in skeleton diagrams of geometrical figures, such as are commonly used to explain the principles of stereoscopy. In ordinary stereoscopic pictures it is also easily observed in those cases where points in the extreme foreground and background are in the same range; as, for example, when a column far in front is projected against a building. In such a case, when we look at the building the column is distinctly double, and vice versa. self, I never look at a stereoscopic card, whether in a stereoscope or by naked-eye combination, without distinctly observing this doubling. For example: I now combine in a stercoscope the stereoscopic pictures of a skeleton polyhedron. The illusion of a polyhedral space inclosed by white lines is perfect. Now, when I look at the farther inclosing lines I see the nearer double, and vice versa. Moreover, I perceive that this doubling is absolutely necessary to the stereoscopic effect, for it is exactly like what would take place if I were looking at an actual skeleton polyhedron.
Inverse Perspective.—Pseudoscopy. I have heard a few persons declare that they saw no superiority of a stereoscope over an ordinary enlarging or perspective glass; that they saw just as well while looking through
the stereoscope if they shut one eye as with both eyes open. Such persons evidently do not combine prop
erly the two pictures, and they lose a real enjoyment. That the binocular is a real perspective, entirely different from other kinds, inay be clearly demonstrated by the phenomena of inverse perspective now about to be described.
If stereoscopic diagrams suitably mounted for viewing in a stereoscope be combined with the naked eye by squinting (crossing the optic axes), as in Fig. 55 (page 153), or if such diagrams properly mounted for combination by squinting be viewed in the stereoscope, the perspective is completely reversed, the background becoming the foreground, and vice versa. For example, Fig. 56 represents a stereoscopic card. When the two pictures are combined with a stereoscope, the result is a jelly-mold with the small end toward the observer; but if the same be combined with the naked eye by squinting, we have now beautifully shown the same jelly-mold reversed, and we are looking into the hollow. If there should be other forms of perspective strongly marked in the pictures, these may even be overborne by the inverse binocular perspective. For example, in the stereoscopic picture Fig. 57, representing the interior of a bridgeway, the diminishing size of the arches and the converging lines, even without the stereoscope, at once by mathematical perspective suggest the interior of a long archway. This impression is greatly strengthened by viewing it in the stereoscope; for the binocular perspective and the mathematical perspective strengthen each other, and the illusion is complete. But if we combine these with the naked eyes by squinting, we see with perfect distinctness, not a long hollow archway, the small arch representing the farther end, but a short conical solid, with the small end toward the observer. Thus the binocular perspective entirely overbears the mathematical.
The cause of this reversal of the natural perspective
b and W in the background are nearer together than the points a and a' in the foreground. By combining these in a stereoscope, the background is seen nearer the observer at B, and the foreground thrown farther back to A. In Fig. 59 the pictures are mounted suitably for viewing in the stereoscope, but are combined by the naked eye. Here also the perspective is reversed,
for the background is seen at a nearer point B, and the foreground at a farther point A.
This inverse perspective is easily brought out, not only in stereoscopic diagrams, but in nearly all stereoscopic pictures, even in those representing extensive and complex views. In these, of course, when viewed in the stereoscope, the binocular is in harmony with other forms of perspective, and each enhances the effect of the other. But if we combine with the naked eyes by squinting, or if we reverse the mounting and view again