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THE CINEMATOGRAPH CAMERA 55
required. Naturally such a practice eliminates the possibility of using film which may be supplied on a spool modern practice is always to supply film on a 2-inch (50.79-mm) diameter central core, without flanges, unless requests to the contrary are made when the order is placed with the manufacturer.
The magazine cover may be either a single door, hinged at a line where the magazine joins the camera body, or it may take the form of two circular discs which screw into suitable openings in the magazine castings. The latter method has now become most popular as it ensures absolute safety from light fog and does not permit errors in locating lock fasteners. Some magazines are fitted with automatically operated light traps, (through which the film is led to the camera mechanism), arranged to open only after the magazine has been located on the camera body and clamped into position. In all cases provision is made to thread the magazine before it is attached to the camera and to leave an adequate loop between the inlet to each spool chamber. A corresponding aperture is provided in the top of the camera body through which this loop may be passed as the magazine is being located in position. If automatic light traps are not used it is usual to find close fitting felt pads fitted to the slots through which the film enters and leaves the magazines. However, this method frequently leads to scratches on the negative, due to an accumulation of dust on the surface of the pads and, in consequence, has been discontinued in modern apparatus. Two guide rollers, 'G' and 'L', Figure 18, are provided in the magazine to lead the film from the central cores and into the camera body.
The camera body, usually of cast aluminium, contains all the essential mechanism although, by comparison with early models, it is greatly reduced in size. It is almost universal practice to use a central sprocket as seen at *F\ to act both as a feed and take-up control. The film gate may be mounted in an accurately machined horizontal slide to enable the film channel to be moved laterally away from the optical axis of the camera and, at the same time, to introduce a focusing screen in precisely that plane which was previously occupied by the film. Some cameras have mirrorsurfaced shutters at 45° to the optical axis. By this means an accurately framed image of the scene is reflected into the viewfinder whilst the camera is actually recording the scene.
To simplify this illustration only a simple link claw, indicated at *N\ has been shown and is merely to indicate the approximate position of this mechanism. The central sprocket 'F' is surrounded by four guide rollers, 'H' and T, which are usually mounted on