The art of sound pictures (1930)

222 THE ART OF SOUND PICTURES raphy apparatus. Waves of sound set up by the players’ voices are caught by the microphone. The microphone turns these sound waves into electric currents. Electric currents, in turn, are made to change the brightness of an electric lamp, which is kept constantly lighted. Now it is simply a question of photographing the changes in brightness caused by the flickering light. There are two ways of recording on film the changes in illumination corresponding to the sounds picked up by the microphone. The first method is as follows: the electric light is focused through a slit upon the film sound track. A tiny bar of light, corresponding to the size of the slit, falls constantly upon the film. But the intensity of this light will vary continuously as the sounds, registered in the microphone, vary. Wflien the light is brightest, the sensitive film upon which the light falls will undergo the most complete chemical development. In other words, a very light gray or white bar will be recorded eventually upon the positive film. WTien the light is reduced in intensity by the effect of the sound waves emanating from the microphone, the area of the sound track upon which the light falls will undergo a comparatively slight development. In this case, a much darker bar will ultimately appear on the positive film. According to this system of photographing sound, the voice sounds appear, ultimately, on the positive film sound track as a series of light bars of varying brightness. (We will discuss presently the manner in which this photographic record is translated back again into sound in the projector apparatus. For the moment, we are only concerned with the original photographing of the sound.) The second method of recording the light variations is