OSCILLOGRAPH RECEIVERS:
The point sources of light used and methods of modulation have been almost as varied as the temperaments of the several workers. One of the first was the employment of a steady light source which, reflected in the tiny mirror of an oscillograph, is caused to vibrate at a high frequency across a minute aperture which in turn is imaged on the film on the cylinder. As the amplitude of vibration of the mirror determines the amount of light passing through the aperture and falling on the film, it will readily be understood that the strength of the incoming electric signals, representing light values of the picture at the distant station, reproduce duplicate values on the exposed film. When this film is developed a copy of the picture on the cylinder of the sending machine is obtained.