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1140 HANDBOOK OF PROJECTION FOR
sound volume, but also it will tend to injure the tonal quality of the sound. However, at the risk of being accused of repeating, I will add that this is because of the fact that the filament illuminates a narrow slit, and since the most powerful illumination will be in a line representing the center of the filament, it follows that should the filament not be perfectly straight, the line representing the highest illumination value would not all fall upon the entire length of the slit, which is itself only .006 of an inch high. I think you will get the idea.
Summed up it amounts to this : you need the very best possible illumination at the slit opening, and upon every part of it, and you cannot possibly have it unless the exciting lamp filament be perfectly straight. Examine the exciting lamp filament every day, discarding any lamp immediately its filament departs in any degree from a perfectly straight line. See Fig. 394.
Clean the outer surface of the exciting lamp every day, just as you would clean a lens. A dusty, dirty globe will decrease the amount of light, and will to some extent tend to diffuse the light which does pass through the glass, all of which makes for general inefficiency and poor sound volume and quality.
BLACKENED GLOBE.— In course of time the interior of an exciting lamp globe will begin to darken. See Fig. 394. This is because of a deposit of matter thrown off by the filament. Discard the lamp as soon as this condition appears. True the lamp may be made to serve longer, but only at the sacrifice of both volume and sound quality. It will be much cheaper to install a new lamp than to get a little more service from the old one at the expense of sound quality and volume.