CH. XII] WHITE IMAGE SCREENS 457 



painting leave the cloth flexible, and without liability of cracking 

 and peeling. They are mounted on heavy spring rollers as 

 ordinary window curtains are so commonly mounted, and can be 

 rolled up when not in use. 



627. White cloth screens without paint or other facing. 



White cloth such as a bed sheet has always been and still is used. 

 The cloth should be as white as possible, and of good thickness. 

 It is also advantageous to have the screen of one piece without 

 seams. Bed sheets may be obtained in large dry goods houses 

 about 3 meters square (10 ft. sq.) without seams. These make 

 very good curtains when the folds are ironed out, and the sheet 

 stretched to hold it flat. It is not easy to stretch a sheet so evenly 

 that there will be no folds or wrinkles. Fortunately, a slight 

 unevenness is not noticeable in the screen image. A screen which 

 appears quite uneven to the naked eye in daylight may give very 

 good screen images and appear perfectly smooth, when giving an 

 exhibition. 



Cloth screens have the disadvantage that they are not suffi- 

 ciently opaque. If one goes behind the screen the image is almost 

 as well seen as in looking at the face of the screen. This means 

 that almost as much light traverses the screen as is reflected from 

 the face. Naturally, it takes much more light for a brilliant screen 

 image than with an opaque screen ( 632). 



For some purposes it is advantageous to be able to see the image 

 on the back, then assistants behind the screen can make the 

 appropriate noises to make the scene seem more real. For exam- 

 ple, in a moving picture scene, sounds can be made to imitate 

 the breaking of the waves on the shore, the clatter of horses hoofs 

 on a pavement, etc., etc. Unless the assistant could see the 

 image it would not be possible to suit the sound so accurately to 

 the scene. 



Sometimes so large a screen is needed that strips of white cloth 

 are sewed together. If this must be done the seams should be very 

 smooth. On such screens the seams show like lighter streaks on 

 the image, as more light is reflected from the double thickness of 



