CH. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 181 



wanted very good results can be obtained by using a white back- 

 ground. 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 dis- 

 tance 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 



