Fig. 38.
6. The Opera-Glass Stereoscope.
As the eyes themselves form a stereoscope to those who have the power of quickly converging their axes to points nearer than the object which they contemplate, it might have been expected that the first attempt to make a stereoscope for those who do not possess such a power, would have been to supply them with auxiliary eyeballs capable of combining binocular pictures of different sizes at different distances from the eye. This, however, has not been the case, and the stereoscope for this purpose, which we are about to describe, is one of the latest of its forms.
Fig. 39.
Fig. 40.
In [Fig. 39], MN is a small inverting telescope, consisting of two convex lenses M, N, placed at the sum of their focal distances, and OP another of the same kind. When the two eyes, R, L, look through the two telescopes directly at the dissimilar pictures A, B, they will see them with perfect distinctness; but, by the slightest inclination of the axes of the telescopes, the two images can be combined, and the stereoscopic effect immediately produced. With the dissimilar pictures in the diagram a hollow cone is produced; but if we look at B with the telescope M′N′, as in [Fig. 40], and at A′ with O′P′, a raised cone will be seen. With the usual binocular slides containing portraits or landscapes, the pictures are seen in relief by combining the right-eye one with the left-eye one.
The instrument now described is nothing more than a double opera-glass, which itself forms a good stereoscope. Owing, however, to the use of a concave eye-glass, the field of view is very small, and therefore a convex glass, which gives a larger field, is greatly to be preferred.