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.