Taking up first the oblique mounting of cameras, we find two ways of doing this: longitudinal mounting and lateral mounting. In longitudinal mounting the camera projects forward and downward, usually from the nose of a pusher or bi-motored plane. With this form of mounting (Fig. 147) it is necessary of course to fly directly toward the objective. If this is a portion of enemy trench, which must be photographed from a height of 400 or 500 meters, the plane will be directly on top of its objective a few seconds after the exposure is made, and be a conspicuous target, in imminent danger of destruction. Moreover, only a single short section of the trench would be obtained for each crossing of the line. The one case where resort to this method is practically forced is with the 120-centimeter cameras which simply cannot be slung athwart the plane. There is a slight advantage in this method of carrying in that the motion of the image is less if the objective is approached, instead of being passed at the side, and so longer exposures can be made. The longitudinal mounting has, however, been very generally superseded by the lateral. [Illustration: FIG. 147.—120-centimeter camera mounted obliquely in the fore-and-aft position.] Methods for mounting cameras obliquely for taking pictures through the side of the plane have been discussed in detail in connection with camera mountings and installations (Fig. 93). The chief difficulties are want of space, obstacles at the side such as control wires and longerons, and failure of the camera to function properly at an angle. Even in the broad circular sectioned fuselage of the Salmson plane, quarters are so cramped that the French 50-centimeter camera when obliquely mounted cannot be used with the 12-plate magazine, and recourse is made to thin flat double plate-holders. Holes in the side of the fuselage should clear all wires and should command a view unobstructed by the wings—which often means that the camera must be carried behind the observer's cockpit, irrespective of the suitability of that space from other standpoints. Cameras dependent for their action on gravity, such as the deRam and English L type, are unsuited for oblique suspension. [Illustration: FIG. 148.—Mirror on camera cone for taking oblique views.] For cameras which, because of their method of operation or shape cannot be slung obliquely, the only way to take obliques is to employ mirrors (Fig. 148) or prisms. These must be of the same optical quality as the photographic lens. They are both necessarily of considerable weight because they must be of large area of face to fill the entire aperture of an aerial lens. Mirrors are lighter than prisms, but must be quite thick to prevent distortion of the surface due to any possible strains to their mount. Right angle glass prisms have been used by the English with the 8 and 10 inch L cameras. The prisms were uniformly tilted to an angle of 12½ degrees from the horizontal.