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MANAGERS AND PROJECTIONISTS 999
that an opening .002 of an inch high by .280 of an inch wide is formed.
The light from the lamp being concentrated upon this opening, it of course follows that a beam of the above dimensions will emerge upon the opposite side. This beam is then passed through a tiny objective lens which optically still further reduces its dimensions to .0005 of an inch high by .070 of an inch wTide when it finally reaches the sound track of the film.
When the oscillatory galvanometer is at rest — no current reaching it from the microphone — the light beam covers only half the width of the sound track. When the microphone current passes through the molybdenum loop, however, it, the loop, starts to vibrate, and of course the mirror, being cemented to it, vibrates with it, in synchronism with the frequency of the sound waves, with the result that the light beam is moved sidewise across the film sound track, the action "exposing" a portion of it to the light thereof, which exposed portion constitutes the sound record. It is shown you in Fig. 385.
It will be understood that since the film moves at the rate of exactly ninety feet per minute, which is steady and unvaried, if the mirror vibrates rapidly the base of the "peaks" (Fig. 385) will not.be so broad as will they if the vibration be more slow. Also if the vibration be slight, the height, or more properly speaking, the width, since they lay in horizontal position, will not be so great as they will be if the vibration of the mirror be heavier.
The width of the "peaks" represents volume. The higher or wider the peak, the greater the volume of sound it represents. The thickness of the "peaks" represents the sound frequency. The thinner the peak the