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980 HANDBOOK OF PROJECTION FOR
brilliancy of the light reaching it during exposure and the length of exposure. If the light be brilliant, or the exposure long, then the resultant discoloration will be relatively great. If the light be dim, or the exposure short, then the discoloration will be relatively slight, these values being exactly reversed in the printing of the positive.
That is just plain photographic lore now, I think, fairly familiar to all. It then follows that when the negative film has been developed, the sound band will vary in density constantly and, since the time of exposure is fixed and constant, in exact proportion to the brilliancy of the light to which it has been exposed ; in other words, in exact proportion to the brilliance of the Aeo lamp at any instant of time.
This being true, then, since the sound wave vibrations control the variations in filament brilliancy, it follows that what we have on the sound band after development, is an exact photographic representation of the sound wave frequencies, or, in other words, of the sound itself.
In Fig. 383 we have a section of the Movietone sound band magnified thirty (30) times. Note the shadings from dark to light, and vice versa. Each difference in density means a difference in sound. In considering this photograph, remember that it represents a section of sound band only about three-sixteenths (3/16) of an inch in length, so that you see these wide "lines" actually occupy less than one-sixtyfourth (1/64) of an inch of space, up and down, on the sound band.
Fig. 384 is a section cut out of the magnified sound band shown in Fig. 383, together with a graph showing the percentages of total transmission of light, or in other words the percentages of total light the various shadings