Fig. 159.—Oblique stereogram made with stereoscopic aerial camera (Fig. [157]). To be viewed by crossing the optic axes (Fig. [158]).
A very satisfactory apparatus for the taking of stereo obliques consists of a 10-inch focus hand-held camera (Fig. [157]), provided with a two-aperture focal-plane shutter. The right-hand half of one curtain aperture is blocked out, the left-hand half of the other. The first pressure on the exposing lever exposes one-half of the plate, the second the other. A stereoscopic sight of the type already described is placed on the bottom. To make an oblique stereo negative the camera is held rigidly by resting the elbows on the top of the fuselage and the first exposure is made when the object comes in line with the rear sight and the leading front sight. The eye is then moved so as to look along the line of the rear sight and the following front sight, and when the object is again in alinement the second pressure is given the exposing lever. Fig. [159] shows a stereo oblique made by this camera. The elements are transposed right and left, and the stereogram may be viewed by crossing the optic axes as shown in Fig. [158], or the two pictures may be cut apart and remounted.
The Mounting of Aerial Stereograms.—The first step in making the printed stereogram is to select two pictures taken on the same scale, but from slightly different positions. These may be two chosen from a collection made for other purposes, or else a pair taken at distances calculated to fit them for stereoscopic use. The next step is to mark the center of each picture, either with easily removed chalk or with a pin point. They are then superposed, and afterward carefully moved apart by a motion parallel to the line joining their centers when superposed. The final step before mounting is to mark out and cut the two elements, their bases being parallel to the line of centers, their horizontal length the distance between the optic axes of the stereoscope (or as near this as the size of the prints will permit). They are then mounted on a card, with their centers separated by approximately 65 millimeters. The right-hand view is the one showing more of the right-hand side of objects, and vice versa. This process of arranging, cutting, and mounting is shown clearly in Fig. [160]. In this case the stereoscopic elements lie symmetrically about the line joining the centers of the original prints. This is not necessary, as they may be selected from above or below this line so long as their bases are parallel to it. A simplification of this method consists in superposing the two prints, laying over them a square of glass of the size to which they are to be cut, then turning it so that a side is parallel to the line of centers, and cutting around it through both prints with a sharp knife. The principle and results are of course the same with both methods.
Fig. 160.—Method of arranging stereoscopic prints for cutting.
If large numbers of stereoscopic prints are required it is necessary, for economy of time, either to photograph a finished stereogram and make prints from this copy negative, or to set up special printing machines. Under the general discussion of printing devices a stereoscopic printer is described (the Richard) in which the two negatives are placed so that stereo prints can be got by two successive printings on one sheet of paper.
Uses of Stereoscopic Aerial Views.—Attention has already been called to the characteristic flatness of the aerial view. Neither the picture on the retina nor that on the photographic plate affords any adequate idea of hills and hollows. Unless shadows are well defined, small local elevations and depressions cannot be distinguished from mere difference in color or marking. Even in the presence of shadows it is often only by close study that differences of contour are noticeable. But with stereoscopic views these features stand out in a striking manner. Taking our illustrations from military sources, we may note the use of stereoscopic pictures to detect undulations of ground in front of trenches (Fig. [161]). They reveal the hillocks, pits, small quarries, streams flowing behind high banks, and other features which make the attack hard or easy. Commanding positions are shown, the boundaries of areas exposed to machine-gun fire, and the defilades where the attackers may pause to reform. Concrete “pill boxes” are located in the midst of shell holes of the same size and outline, and can be differentiated from them.
Railway or road embankments and cuts can be detected and studied to extraordinary advantage in stereoscopic pictures. Thus what appears to be a mine crater on a level road, easily driven around, may be a gap blown in an embankment, a serious obstacle indeed. Bridges, observation towers and other elevated structures jump into view in the stereoscope when often they have entirely eluded notice in the ordinary flat picture. Once presented in relief, camouflaged buildings or gun emplacements, however carefully painted, are ridiculously easy to pick out.
