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52 G. L. DIMMICK AND H. BELAR [J. S. M. P. E. of sound track are shown in Fig. 4. A beam of light 0.084 inch long and 0.001 inch wide is projected on the film 44 by means of a standard reproducing optical system 47 and a 10-volt, 5-ampere lamp 45. A cylindrical lens 48 forms an image of the width of the sound track upon the two cylindrical lenses 49 and 50. In the other plane, light from the reproducing optical system is allowed to expand to a height of about an inch. Lenses 49 and 50 each form an image of lens 48 upon the two cathodes 52 and 53, of the photoelectric cell 46. By means of such an arrangement the two images of lens 48 are sepa- rated, each containing the light transmitted through half the track, since the center of the track coincides with the dividing line between FIG. 5. A 9000-cycle negative made with the push-pull system. the two adjacent cylindrical lenses. The system produces variations in the intensity of the light striking the cathodes, corresponding to variations in the width of the clear portion of the sound track. As shown in Fig. 4, the cathodes are connected to the primary terminals of a transformer. The anode is connected through a battery to the center of the transformer. The secondary is connected to the repro- ducing amplifier. It is obvious that the two halves of the sound waves, which were recorded 180 degrees out of phase, are recombined in the proper phase by the push-pull transformer. In addition to its inherent freedom from ground noise, the push- pull sound track has other advantages of equal importance. The