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CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 181
wanted very good results can be obtained by using a white background. They will appear like silhouettes, but almost no details will show.
§ 286. Screens for opaque projection. — On the whole no screen is so satisfactory as a white one of the best quality (see § 621).
If the room is narrow, so that all the spectators are included in about 30 degrees, the metallic screen answers fairly well. If the room is wide, those on the sides near the screen will get only a very dim screen image from the metallic screen. With the white screen it is practically as good in one place as in another, for the reflection is about equal throughout the entire 180 degrees (§ 622, 630).
For darkening the room see § 280 and § 608.
§ 287. Magnification of the picture and size of screen image. —
For lantern slides the magnification can be 30 to 60, with resulting brilliant pictures; but with opaque projection one can rarely magnify more than six to ten times and get good results.
If the area to be shown is relatively small and the illuminating beam is made converging and a powerful radiant (50 amperes) is used, the magnification may be carried up to 25 or 37 diameters (Zeiss, p. 6) or perhaps more.
The screen image should not exceed 2 x 2, or 3 x 3 meters (8 x 10 feet), (Zeiss, p. 6).
§ 288. Screen distance. — In opaque projection, the screen images are usually not magnified so much as lantern-slide images and the screen distance is usually from three to ten meters. The correct magnification (six to ten) is obtained by using an objective of the proper focal length, i. e., for a magnification of six and a screen distance of three meters there should be an objective of 50 cm. or 20 in. If the magnification is to be 10 and the screen distance three meters then the objective should have a focus of 30 cm. or 12 inches. For the discussion relating to magnification, screen distance, and focus of the objective see § 3Q2a.
Sometimes it is necessary to project at a screen distance of 1 5 to 20 meters (50 to 70 feet) . As the magnification of the screen image must not usually exceed six to ten, a very long focus projection