Let F1 be the distance at which the edge of the hole (or a rectangle marked on the lens) appears the size of the camera field (if the hole is the size of the plate, F1 is the focal length of the camera lens). Let F2 be the distance from the floor to the observer's eye. What is desired is a concave lens which will diverge the rays from their normal meeting point at F1 to a new meeting point, F2. The focal length of lens required is given at once by the simple lens formula—
| 1 | 1 | 1 | ||
| - | = | |||
| F1 | F2 | F |
Thus if F1 is 12 inches, and F2 is 36 inches, F will be 18 inches. The lens is to be marked with a rectangle showing the shape and size of the camera field, and a central mark such as a cross. An upper rectangle, or a bead, or a pair of cross wires a few inches below the lens, may be used for the other sight. For precision work the sight above or below the lens should be adjustable in position, especially where the camera suspension permits the camera to be adjusted for the angle of incidence of the plane.
A negative lens sight should be placed in the observer's cockpit, if he takes the pictures, and also in the forward cockpit, so that the pilot may be accurately guided in his part of the task. In addition, it is advisable to place a negative lens well forward in the pilot's cockpit, to enable him to see the country some distance ahead. The lenses should be planoconcave with the flat side upward; otherwise, all the loose dirt in the airplane settles in the middle of the concave depression. A negative lens sight in a metal frame forming a completely self-contained unit ready for mounting in the plane is shown in Figs. [72] and [73].
Devices for Recording Data on Plates.—Numbering devices. The number of the camera is impressed on negatives taken with the American L camera through the agency of a small transparent corner of celluloid. It would be entirely possible to incorporate a rotating disc which should turn by the operation of plate changing and carry a series of numbers, so that each exposure could be numbered serially. Numbering of individual plates is more commonly done by holes, notches, or even numerals, in the turned over portion of the sheaths, which are then recorded photographically when a picture is taken (Fig. [75]). The chief objection to this method is the difficulty of keeping the sheaths together in sets, especially as individual ones become damaged or lost. In practice there is also danger of the sheaths being carelessly loaded in wrong order.
Fig. 72.—Negative lens and mount, viewed from above.
The more ambitious idea of recording on the plate all the information given by the instrument board of the plane occurs independently and spontaneously to all aerial photographic map makers. These ideas vary from attempts to photograph the actual instrument board on every plate—a difficult task indeed with the instruments and camera placed as they are in the ordinary plane—to the incorporation of compass, altimeter, and inclinometer in the camera itself.
Figure 58 shows the plan adopted in the English F type film mapping camera already described, for photographing a compass and an altimeter on the film. Here the combined compass and altimeter dial is above the camera, and is mounted in a cell with a glass bottom. Below it is a lens focussing the needles and compass points on the plane of the film. The light for photography is furnished by a diffusely reflecting white surface on top of the camera, illuminated by the sky. (The camera was carried outboard.) In Fig. [56] is shown a picture with the compass image impressed upon it.
