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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 sufficiently 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 example, 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