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Types of Visual Aids and Their Uses 81
ward in the lamp house as necessary. It should be adjusted to the point where the clearest image appears on the screen. Sometimes it is advisable to adjust the light against a small, white card as the variations can be noted here more quickly than on the screen. Usually the lamps are set at the proper place when the projector leaves the factory and it is not advisable to make any change. In the case of older projectors, which have been adjusted frequently, it may be necessary to move the lamp to the proper position.
Selection of a Projector.* The selection of a glass-slide projector for any situation can be determined only by giving consideration to the situation. Projectors which would be entirely satisfactory in one room or building might not prove to be desirable in another. This does not mean that one make of projector has any great superiority over any of the others, but that the equipment itself should be selected with some degree of care. For example, if the rooms in a building for which the projector is to be purchased are extremely long and narrow, it is quite probable that a projector will be needed which has a lens of comparatively long focal length. If the room is short, a shorter lens will be needed in order to get a large enough picture to be clear to all in the room. In ordering a projector, therefore, it is necessary that the approximate distance from the projector to the screen be given, as well as the approximate size of the picture desired on the screen. It is then possible for the manufacturer or distributor to select the proper length of lens for that situation. The majority of the classroom projectors for 3*4" x 4" glass slides have become somewhat standardized in the use of 150 or 120-volt, 500-watt lamps, so it is hardly necessary to give any consideration to the illuminant.
If a lantern is desired for use in an extremely large auditorium, it will be necessary to secure one which has a lamp stronger than a 500-watt lamp, as well as an objective lens of extremely long focus. This increase of the illumination naturally increases the amount of the heat generated by the lamp and makes it necessary to use a protector for the slides. In some cases a motor-driven fan propels a current of fresh air past the slide during projection to protect it from excessive heat. In other cases a water cell is inserted between the condenser lens assembly and the slide for the same purpose.
Those who may be interested in securing a lecture lantern for personal use and may want to secure the most convenient equipment will find the overhead lantern to be highly satisfactory. With this lantern
*See list of manufacturers and distributors of glass-slide projectors, page 218.