In these magazines the center of gravity changes as the exposed plates are moved over, and only half the inside space is occupied with plates. These objections are overcome in the Chassel form, where both compartments are always full. Transfer of the bottom exposed plate from one compartment to the other is compensated for by the simultaneous shift of the top plate in the receiving compartment, to the feeding side. In a modification of this idea by Ruttan the exposing position is when the plates are half-way through the shifting process, whereby the magazine may be symmetrically mounted on the camera body. [Illustration: FIG. 35.—Apparatus for straightening plate sheaths.] Other more complicated magazines have been designed, some of which are shown in the diagrammatic _ensembles_ of Figs. 32 and 48. In the Jacquelin, the main body of plates is raised while the bottom (exposed) plate is folded against the side. The main body of plates then drops back to place, the exposed plate is carried on upward and folds down on the back of the pile. The Bellieni magazine system uses three, a central feeding one and two below for receiving, one on each side of the camera body. These were made solely for attachment to captured German cameras. In the Fournieux magazine the plates are carried in an interior rotating box. The plate to be exposed is dropped off the front of the pile, down to the focal plane, and after exposure is picked up and placed at the back of the pile, which has turned over in the meanwhile. The deRam rotating magazine is described in connection with the camera of which it is an essential part (Fig. 52). [Illustration: FIG. 36.—Training plane equipped for photography, showing “L” camera in floor mount and magazine rack forward of the observer.] For the protection of the plates during their manipulation, and in the camera, all plate magazines thus far developed carry them in thin _metal sheaths_. These add greatly both to the weight and to the time necessary to handle the plates, but no means have as yet been found for dispensing with them. Fig. 35 shows a representative sheath or septum, as used in the L camera. On three sides the edge is bent up and turned over, forming a ledge for the plate to press against. The fourth side is left open for inserting the plate, which is then held in by a small upward projecting lip, and kept close against the ledges by narrow springs at the sides. To insert or remove the plate the projecting lip is depressed, either by springing the sheath by pressure from the sides or by using an appropriate tool. _Care of sheaths._ Unless systematically taken care of, plate sheaths become bent or dented. They are then a menace to camera operation, catching or jamming in the plate changing process, breaking plates and damaging camera mechanisms. In order to maintain them flat and true, steel forms are necessary on which the sheaths may be hammered to shape with a mallet (Fig. 35).