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THE STUDIO 215
amplifier is what is known as a " mixing panel" through which circuits from the microphones are combined. The current from the microphone first passes through the panel and directly to the main amplifier, where it receives four additional stages of amplification successively before being passed on to the loudspeakers. By this process the original current is amplified approximately one hundred million times. The current is now connected directly across the oscillograph galvanometer and recorder, which consists of a wire loop through which the amplified microphone current circulates. A small mirror is cemented to this loop. A small lamp supplies the illumination for making the photographic record on the film sound track. The light from this lamp passes through a small condensing lens and is focussed upon the galvanometer mirror. By means of the latter it is reflected through a second condensing lens and focussed upon a slit form through the microscopic adjustment of two sharp edges, so that an opening .002 inches high by .280 long is formed. On the opposite side of the opening there is a light beam of the same dimensions. This tiny beam is then passed through a small projection lens on the sound track of the film, and in the process is optically reduced in dimension until at the film it is only .0005 by .070 of an inch.
When the microphone is in action the galvanometer loop to which the mirror is cemented begins to vibrate, and those vibrations are in exact proportion to the electrical energy that is operative at or during any instant. This energy is itself governed by the vibrations of the microphone diaphragm, the loop and mirror vibrations being an exact duplication of the sound waves which create the current. The latter moves the light beam back and forth across the sound track of the film and in so doing traces on the film what really is a graph of the sound waves, the graph being the line of demarkation between the dark