=Sights Attached to the Plane.=—Any of the sights just described can be attached to cameras fixed in the plane, but they would be useless in the positions ordinarily occupied by the camera. It has therefore become common practice to attach the camera sight to some accessible part of the plane. The most primitive method of sighting is merely to look downward over the side—a method in general use to the very end of the Great War. One step in advance of this is to mark a large inverted V on the side, with its vertex at a point where the observer can place his eye and so see the fore and aft extension of the field of view covered by the camera. This kind of sight was common on the French “photo” planes. On some of the English planes the tube sight was carried on the outside of the cockpit. Any of the sights described can be carried on the inside of the fuselage, provided a hole is cut in the floor. For satisfactory sighting a hole in the floor is really necessary, as it enables the terrain on both sides of the vertical to be seen. One drawback to the simple hole, however, is that it cannot be made large enough to show the whole field from the ordinary height of the observer's eye, thus forcing him to bring his head down near the floor. This difficulty is gotten over in a very beautiful way by the use of the _negative lens sight_ shown diagrammatically in Fig. 71. [Illustration: FIG. 71.—Diagram of negative lens sight.] Let _F_{1}_ 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, _F_{1}_ is the focal length of the camera lens). Let _F_{2}_ 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 _F_{1}_ to a new meeting point, _F_{2}_. The focal length of lens required is given at once by the simple lens formula— 1 1 1 ——————— - ——————— = ——— _F_{1}_ _F_{2}_ _F_ Thus if _F_{1}_ is 12 inches, and _F_{2}_ 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.