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SOUND TECHNIQUE 223
as follows : instead of varying the intensity of the light by means of electric currents emanating from the microphone, these same microphone currents are made to vary the area of film upon which the light falls. Instead of superimposing the microphone currents directly upon the electric light current, the microphone currents are connected with an apparatus which varies the size of the slot through which the light falls upon the recording film. The brightness of the light is kept constant. The slot through which the light shines is varied in size by the microphone currents. Thus, the area of sensitized film which is developed by the light beam varies in accordance with the sounds registered in the microphone. When the finished positive film record is shown, therefore, we find light bars of equal brightness, but of varying lengths, on the sound track.
The first method of photographing sound is known as the method of variable density, and the second is known as the variable area method.
The Fox Movietone sound recording apparatus uses the variable density method, while the R. C. A. Photophone uses the variable area method. The latter method is much more reliable than the density method of recording, because variations in density may be caused, not only by the original light variations, but by irregularities and uncontrollable variations in the developing of the film negative, as well as in the printing of positive films from the negative. In other words, when the variable density method of photographing sound is used, there is considerable likelihood that the record will not be an accurate one, because the accuracy of the original record may be upset in the process of developing and printing the film. With