=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_. [Illustration: FIG. 154.—Schweissguth stereoscope, used for selecting portions of prints to be mounted.] [Illustration: FIG. 155.—Method of taking stereoscopic pictures.] In order to secure this condition it is necessary, first, that each element of the stereoscopic pair be correct in its perspective. This is fortunately an old photographic problem, already well understood. Its solution is to view the photograph from a distance exactly equal to the focal length of the camera lens. Since the normal viewing distance is not less than 25 centimeters, lenses of this focal length at least are requisite for correct perspective. Secondly, it is necessary for correct relief that the two views be taken with a separation equal, on the plane of the plate, to the separation of the eyes, or 65 millimeters. If _d_ is the interocular distance, a the viewing distance, identical with the focal length of the lens used, and _A_ the altitude, then _D_, the distance between exposures, is given by the relation— _d_ _D_ ——— = ——— _a_ _A_ For _a_ = 25 centimeters, _D/A_ = 6.5/25, approximately ¼, or the interval between exposures must be a quarter the altitude. With a 50 centimeter lens this becomes ⅛, and so on. These figures show the fallacy of the suggestion sometimes made that we take stereoscopic pictures by two cameras placed one at the extremity of each wing.