Fig. 151.—Box stereoscope.
Fig. 152.—Diagram of mirror stereoscope.
Another form of stereoscope, one of the first produced, is the mirror stereoscope (Fig. [152]), now used extensively for viewing stereo X-ray pictures. It consists of two vertical mirrors at right angles to each other, with their edge of contact between the eyes. The two prints to be studied are placed to right and left, an arrangement that permits the use of prints of any size. The convergence point is controlled by the angle between the mirrors. The Pellin stereoscope (Fig. [153]) utilizes two pairs of mirrors in a way to permit the use of large prints. The prints are, however, placed side by side on a horizontal viewing table, which avoids certain difficulties of illumination met with in the simpler mirror form. The box form of stereoscope (Fig. [151]) using either prisms or simple convex lenses, is particularly adapted for viewing transparencies, although the insertion of a door at the top provides illumination for paper prints. The Schweissguth design (Fig. [154]) is intended primarily as an aid to selecting the portions of the prints to be cut out for mounting. The platform on which the pictures rest is composed of two long rectangular blocks, on which are plates of glass raised sufficiently to permit the prints to be slid underneath. The space between the blocks allows the unused portion of the photograph to be turned down out of the way. Prints of any size can thus be moved about until the proper portions for stereo mounting are found. Either block can be moved in its own plane and also to and from the eye, whereby two prints of somewhat different scales can be fused.
Fig. 153.—Pellin double mirror stereoscope.
The Taking of Aerial Stereograms.—The normal separation of the eyes is altogether too small to give an appearance of relief to objects as far away as is the ground from a plane at ordinary flying heights. In order to secure stereoscopic pairs it is therefore necessary to resort to a method originally employed for photographing distant mountains and clouds. This is to take the two pictures from points separated by distances much greater than the interocular separation—by meters instead of millimeters—corresponding to the positions of the eyes on a veritable giant. In the airplane this is accomplished by making successive exposures as the plane flies over the objective, at intervals to be determined by the speed, the altitude and the amount of relief desired (Fig. [155]).
An all important question which arises immediately is: What separation of points of view shall we select? If the exposures are too close together there will be little relief; if too distant the relief will be so great as to be unnatural, even offensive. Obviously we cannot here establish a criterion of natural appearance, since the natural appearance to ordinary human eyes is devoid of relief. We may, however, define correct relief as that obtained when the apparent height of elevated objects is right as compared with their extension or plan.
