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April, 1934] RECORDING SOUND WITH PHOTO-FILM 225 with the picture camera, and the mechanical filter, consisting of a spiral spring and a flywheel, rotates on the vertical shaft ^4 (Fig. 32). Fig. 32 shows the camera. Sound recording is done by a flashing lamp, in the housing C outside the film-drive housing, which il- luminates the drum B (Fig. 31) mounted on the shaft of the flywheel K of the filter. For use in focusing the image and installing the lamp an observing microscope is provided, with two oculars, one on each side of the camera. The objective 0 is focused by the screw R. When sound is recorded for reproduction without a picture on the radio or in the theater, a large portion of the film is not used, and both this film and the chemicals used to develop it are wasted. Various methods have been developed for avoiding this waste by condensing the record on the film, which can be accomplished either by manifold FIG. 33. Multiple-track methods for conserving film and chemicals. recording along the film or across it (Fig. 33). Both methods have been used with rather good results. One method of multiple recording along the film is to record on one edge of the film while it is moving in one direction, then reverse the camera and record on the adjacent strip, etc. Another method is to make a record in the form of a spiral on a closed loop of film. The first method can be accomplished very simply with an ordinary camera by reversing the light modulator and the direction of the film, but this does not produce an uninterrupted record. The second method, producing an uninterrupted record, can be accomplished by using a box permitting continuous movement of the film, as illustrated in Fig. 34. Through the flywheel the motor drives the cylinders of the box which rotate the ring of film. Fig. 35 shows schematically the arrangement of such a box, in which a radial system of cylinders and rollers is driven through conical gears 2 by the central conical gear 3.