The most completely worked out photographic plane was probably the model designated P1 by the United States Air Service. This is a modified DeHaviland 4 in which the rear controls have been removed and the cowling raised and at the same time made squarer in cross-section. The space formerly occupied by the rear controls provides ample room for all types of camera. These are carried on uprights at the standard distance apart, 16 inches, with cross-pieces adjustable as to height. The camera space is accessible not only from the observer's cockpit, but from above, upon folding back the metal cover. Doors at the bottom and at each side permit not only vertical but oblique exposures. The latter are not interfered with by the wings, as they would be in some designs of plane if the camera occupied the same position relative to the cockpits. Fig. 91 shows the deRam camera in place, as seen from the rear. Figs. 98 and 99 show the 18 × 24 centimeter film camera, set both for vertical and oblique views. Negative lenses are provided for both pilot and observer, the one for the pilot permitting him to see from a point far ahead to directly underneath, while the observer's is furnished with cross wires below and etched rectangles of the camera field sizes on the upper surface. Windows of non-breakable glass assist in sighting obliques. The accompanying picture (Fig. 100) of the plane showing an oblique camera in position gives an excellent idea of its appearance. Its special features are worthy of copying in peace-time photographic aircraft. =Installation of Auxiliaries.=—It is quite necessary that the camera lens be protected from splashing mud and often from oil spray due to the motor. For this purpose an easily opened and closed door is essential, unless the camera is carried well up in the plane. An alternative, possessing certain advantages, is to incorporate into the camera protecting flaps operating in front of the lens, which open only when the exposure is made. If the camera projects beyond the fuselage, _stream lined hoods_ (Fig. 95) must be provided to protect the camera nose with the minimum of air resistance. [Illustration: FIG. 98.—20-inch focus automatic film camera mounted obliquely in photographic DH-4.] [Illustration: FIG. 99.—20-inch focus automatic film camera mounted vertically in photographic DH-4.] [Illustration: FIG. 100.—DeHaviland 4 re-constructed as a special photographic plane, showing 20-inch camera mounted for oblique photography.]