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AMATEUR MOVIE MAKING 239
proper relation. The two plano-convex lenses form the most common condensor system.
The condensor system has a focal point and it is important that the condensor be properly focussed. This is easily done by projecting a beam of light upon the screen without any film being in the projector.
The screen will either be intensely illuminated or it will have purplish blue shadows upon its surface. The position of these shadows indicate the error in relationship between condensor and lamp. If the shadow is annular or circular it indicates that the lamp is too near the condensor or too far from it. Move the lamp backward and forward until the shadoAvs disappear.
If the shadows are semi-annular, the lamp is displaced, laterally or vertically. If the shadows are at the side of the screen the lamp is displaced laterally, if they are at top or bottom the lamp is displaced vertically and if they are near the corners of the screen, the lamp is displaced obliquely. The remedy is to move the lamp in the direction indicated.
The condensor is almost certain to collect a thin film of oil from the mechanism and this in turn collects dust and dirt. Soon the surfaces of the lenses become so clouded that they lose much of their efficiency, perhaps forty or fifty percent. This in turn cuts down the screen illumination enormously. It is quite essential therefore that the condensor lenses be kept clean, and that they be polished periodically.
Film : The subject of the care of the film is too broad to be covered here and will be discussed later in the chapter. In actual projection the film may be displaced in the aperture showing the frame line and parts of two pictures. This is remedied by moving the "framing lever" with which every projector is equipped. Moreover, the film often breaks during projection. When this occurs, the broken end is tucked under the corresponding broken end which is wound upon the take-up reel, and projection continued. The repair will be taken care of later.
Projection Lens : The projection lens must be moved back and forth until the image of the film is shown plainly