_Sights to indicate the size of the field_ are usually less needed on hand cameras than on fixed vertical cameras. Yet certain circumstances make them most desirable, for instance in naval work where a complete convoy must be included on the plate. A sight of this kind can be made up of two wire or stamped metal rectangles, a large one in front and a smaller one behind, of such relative sizes and separations that the true camera field is outlined when the eye is placed in position to see the two rectangles just cover each other. The dimensions should be so chosen that the correct position of the eye is approximately its natural location with respect to the camera when this is held in the hands in the plane. It is usual to provide the rectangular sights with cross-wires to indicate the center of the field. Alternative rear sights are simple beads or peep-holes—the use of the bead assuming that the camera is held at about the right distance from the eye for the rectangle to indicate the field. The peep-sight is not a desirable form, as it is hard to hold the camera as near the face as is necessary. These various types of rectangle sights are well illustrated in the cameras shown in Figs. 38, 40 and 186. They are all made so as to fold down flat on the camera and to snap quickly open when needed. The springs to support the sights must be fairly strong, and the surface presented to the wind as small as possible. Wire frames give very little from the pressure of the wind, but flat metal frames are apt to be bent back. The position of the sight on the camera is important. If the observer can stand, or if he sits up well above the edge of the cockpit, the conventional position of the sight on a pistol, namely, on top, is unobjectionable. But if the observer sits very low, as he usually does, then the sight should be on the bottom of the camera, thereby avoiding any need for the observer to raise his head unduly into the slip stream. Similarly, if the camera is used over the side for verticals, as it is in flying boats, a sight on the top is impractical, since it requires the observer to lean out dangerously far (Fig. 185).