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MANAGERS AND PROJECTIONISTS 991
light or, as it is called, the "flashing lamp" method. The latter is the basis of "light value" recording, the action being as follows: The separation between the aforementioned stretched metallic ribbons is ordinarily twothousandths of an inch. By means of a 2 to 1 optical reduction through lenses, a beam of light passing between the ribbons when they have this separation is made to appear on the sound track as a line one-thousandth of an inch wide. Therefore, when the film is passing through the recording machine at its standard speed of 90 feet per minute, or 18 inches per second, each point on it gets an exposure of 1/18,000 of a second. Now suppose that we put the light valve in action recording a sound of a certain intensity, sufficient, let's say, so that when the ribbons vibrate, the opening between them varies from a thousandth minimum to three thousandths maximum. The line of light on the film will then vary in width from y2 to \y2 thousandths. Then when it is at the minimum width the exposure received by a point on the film as it passes will be J/2 divided by 18,000, that is, 1/36,000 second. At maximum width, the exposure will be \y2 divided by 18,000, that is, 1/12,000 second, or three times as much. With the exposure varying from point to point like this, the density will also vary, and hence we will obtain what we require for a photographic sound record, namely, density variations corresponding in number to the sound frequency and in amount or contrast to the loudness.