Assuming the camera and mounting supplied, the next step—a very difficult one—is to insure uniformity in the structures to be built into the planes for the purpose of supporting the camera mountings. With this uniformity must, however, be combined the greatest possible flexibility to provide for various designs of cameras. In the English service the standard camera installation consists of wooden uprights with cross bars athwart the plane, adjustable as to height (Fig. 95). A distance between the cross bars of 13¼ inches has been standardized, and all camera cradles and mountings are notched or otherwise spaced to fit this dimension. The installation adopted in the American planes is similar, but with a distance of 16 inches between cross bars. These uprights and cross bars are ordinarily situated in the bay behind the observer, but can be placed in any available space. Fig. 83 shows, in a model bay, the arrangement of uprights and cross bars in the American DH 4, with the L camera in place in its cradle. It is just possible to introduce camera and cradle separately from the observer's cockpit through the tension wires, and, by uncomfortable reaching, magazines may be changed. [Illustration: FIG. 95.—“LB” camera with 20-inch lens, mounted on bell-crank suspension in skeleton fuselage. Stream-lined hood below to cover projecting end of lens cylinder. Propeller and Bowden release in place.] A step in advance is made when the top tension wires and superstructure are replaced by a rigid frame with an opening large enough to admit the entire camera and mounting. When this is done considerably larger cameras may be accommodated in the same sized bay, as shown in Fig. 96. A further advance, from the standpoint of accessibility and convenience of installation, follows when the tension wires between observer's and camera bay are replaced by a ply-wood ring, as shown in Fig. 97. Here the only serious limitations are those due to the vertical height of the camera, and of course its weight. Openings for the lens to point through are simply provided in the fabric covered aircraft, by cutting through the canvas and stiffening the edge of the hole by wire. Tension wires are often in the way. They may either be disregarded, since they merely cut off a little light, or replaced in part by metal rings, as shown in Fig. 96. In veneer covered fuselages the hole must of course go through the wood. This may be undesirable, since the veneer is depended on to furnish structural strength, a point which further emphasizes the importance of the photographic requirements being thoroughly considered while the plane is being designed.