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Types of Visual Aids and Their Uses
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Optical Parts of the Projector. The optical parts of a filmslide projector are essentially the same as those of all projectors, still or motion. At the back of the projector there is a reflector to reflect forward many of the rays from the lamp which would be lost without it.
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Typical Optical Arrangement in Filmslide Projector
Just ahead of the reflector is the lamp, which is the source of illumination for projection. The next unit is the condenser, which is usually composed of two or three lenses in a series. Its purpose is to concentrate the direct and reflected rays of the lamp on the slide or other object to be projected. The slide itself occupies a position in the slide rack or aperture of the projector which is just in front of the condenser lenses. The foremost part of the projector is the objective lens which is used to secure a clear image of the projected picture on the screen at any given distance. This must be moved forward and backward somewhat in accordance with the distance of the projector from the screen, and is mounted on a sliding rack to permit easy adjustment. On the majority of the projectors the space between the objective lens and aperture gate or slide rack is covered by black fabric or leather bellows. The bellows serves several purposes. It avoids the possibility of stray light hitting upon the screen, walls, or other parts of the room, and serves to prevent interference with the light rays themselves. Furthermore, it serves as a protector for the lens and aperture gate, by keeping out dust and other particles.
Care and Operation of the Projector. One of the greatest advantages of the glass-slide projector is that it may be operated by almost anyone. There are certain simple rules which must be observed in handling the instrument, and the best results will not be obtained if those are not observed. In the first place, all parts of the projection