Stereo obliques can of course only be made with any facility by laterally pointing cameras. From the calculations already given it appears that a “correct” stereo oblique of an object 500 meters away will mean exposures only two or three seconds apart, too short an interval for any of the ordinary plate-changing and shutter-setting mechanisms; and the case is even worse should less relief be desired. One solution of this problem has been the use, already mentioned, of two cameras mounted together, either side by side or one over the other, with separate shutter releases. Both releases may be controlled by the observer, using a sight, or else pilot and observer may work in harmony as has been recommended in the English service, where the pilot releases one shutter and the observer counts time from the instant he sees the first shutter unwind and releases the second. [Illustration: 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.