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July, 1934] IMPROVED NOISELESS RECORDING 51 the single triangular mask by a mask having two triangles. An opti- cal image of two opposing triangles is formed at the slit after being re- flected from the galvanometer mirror, as shown in Fig. 2. The apexes of the triangular images are coincident with the center of the slit and are spaced apart half the length of the slit. When a signal is impressed upon the galvanometer the triangular light beams vibrate in a vertical plane and record two symmetrical tracks, one of which carries the positive half, the other the negative half, of the sound waves. The axes of the two component tracks are located a quarter of the total width of the track from each edge. The purpose of this is to assure proper scanning with low modulation and the proper separation of the two halves of the reproducing light beam in case of either a slight weaving of the film or a slight misalignment of the track. FIG. 4. Reproducing optical system. The axes of the two half-tracks might be made to coincide at the cen- ter of the track, but it would be practically impossible to separate them later. The axes might also be placed at the two outside edges of the track, but then there would be less assurance of proper scanning with low modulation. In practice, the recording system is quite simple to operate and adjust. The points of the two triangles may be made to lie upon a line parallel to the slit by a rotary adjustment of the barrel containing the aperture. The points are brought to the center of the slit by a vernier adjustment of the galvanometer about its horizontal axis. If, for any reason, the latter adjustment were either made incorrectly or accidentally spoiled, no distortion would be introduced. The only effect would be a slight increase of ground noise. Both the optical and the electrical systems for reproducing this type