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LENSES AND MIRRORS
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the lens and observe the beam diameter that way if you prefer.
(Ill) If a plano-convex condenser is used the beam will be as in Fig. 39. It may or may not all enter the lens. (112) If it does not, a lens of larger diameter is needed although other considerations may make it impractical. A longer focal length converging condenser lens may increase the distance between the condenser and the aperture, and thus reduce the spread of the beam beyond the aperture. Increasing the focal length of the condenser will automatically increase the distance of the light source frorn the collector lens, and thus set up a loss which may neutralize any gain, but any reduction of the spread of the beam between the aperture and lens will tend to make the screen illumination smoother even if the beam is not all entering the lens.
In Fig. 40, a photograph with light made visible by smoke, we see how the light beam overlaps the lens. (113) Any projectionist may ascertain for himself by means of a simple, convincing experiment whether this sets up uneven light distribution on the screen. Secure a small sheet of stove pipe or other thin flat metal. Cut out a strip about three inches long the width of a film. On its surface lay out two frame lines the same shape and size as those on film. Lay this on a freshly sawed block of wood, end-of -grain side up, and with a punch filed to a sharp point, punch a small hole exactly in the center of one of the "frames'' and near one (either) corner of the other. Now place the metal strip over the aperture with the center hole in tincenter of the aperture. Centering the hole is important, though if it is Figure 40 within 1/16 of an inch of the center