_Magazine racks._ Reconnaissance and mapping call for a number of exposures much greater than the capacity of one 12, 18, or 24 plate magazine. Additional magazines must therefore be carried. These should be in racks convenient to the observer (Fig. 36), securely held yet capable of quick removal and insertion. In the rack designed to carry two of the metal magazines for the American L Camera, the magazines slide into loose grooves formed by a metal lip. They are prevented from slipping out by a spring catch, past which they slide when inserted but which is instantly thrown aside by pressure of the thumb as the hand grasps the magazine handle for removal. CHAPTER VII HAND-HELD CAMERAS FOR AERIAL WORK =Field of Use.=—The first cameras to be used for aerial photography were hand-held ones of ordinary commercial types. Indeed the idea is still prevalent that to obtain aerial photographs the aviator merely leans over the side with the folding pocket camera of the department store show window and presses the button. But the Great War had not lasted long before the ordinary bellows focussing hand camera was replaced by the rigid-body fixed-focus form, equipped with handles or pistol grip for better holding in the high wind made by the plane's progress through the air. Even this phase of aerial photography was comparatively short-lived. The need for cameras of great focal length, and the need for apparatus demanding the minimum of the pilot's or observer's attention, both tended to relegate hand-held cameras to second place, so that they were comparatively little used in the later periods of the war. Yet for certain purposes they have great value. They can be used in any plane for taking oblique views, and for taking verticals, in any plane in which an opening for unobstructed view can be made in the floor of the observer's cockpit. They can be quickly pointed in any desired direction, thus reducing to a minimum the necessary maneuvering of the plane, a real advantage when under attack by “Archies” or in working under adverse weather conditions. For peace-time mapping work the hand-held camera, when equipped with spirit-levels on top, and when worked by a skilful operator, possesses some advantages over anything short of an automatically stabilized camera. For experimental testing of plates, filters and various accessories, the ready accessibility of all its parts makes the hand-held camera the easiest and most satisfactory of instruments. The limitations of the hand-held camera lie in its necessary restriction to small plate sizes and short focal lengths, and in the fact that it must occupy the entire attention of the observer while pictures are being taken—the latter a serious objection only in war-time.