803. Translucent Screens. A screen for pro jection by transparence may be a sheet of glass matted with a sand-blast, in the case of a fixed installation of large size, or a fine metal gauze or muslin, stretched on a frame and serving as a support for a translucent coating (gelatine, or cellulose varnish, holding in suspension a very finely-divided white substance such as alumina, barium sulphate, etc.).
An excellent screen is obtained by using tracing paper, or, better still, tracing cloth, sup plied in rolls of various widths up to about 5 ft.
Plain (uncoated) calico may be used, but it must be wetted to increase its transparency. For this purpose a florist's syringe is generally used. To prevent the evaporation of the water during the lecture, which would entail re spraying, the water should contain about no per cent of glycerine, or, cheaper, about no per cent of the very hvgroscopic calcium chloride.
With too transparent a screen (ground glass, thin fabric tissues such as cambric or lawn, etc.)
the projection lens is seen through the screen in the form of a bright spot which greatly inter feres with the viewing of the picture. Also the angle within which the spectators may be placed is then very narrow.
804. Daylight Screens. Various arrangements enable pictures to be shown in full light on a translucent screen.
We must particularly mention the use, between the screen proper and the spectators, of a translucent curtain, either black or dark coloured, so that no light other than that of the lantern reaches the screen except the light filtered by the curtain (Isnardou, 1911). It is also possible to employ screens of translucent plastic material of somewhat dark colour, e.g. greenish-grey), the rear surface of which (turned towards the lantern) is flat, whereas the surface turned to the spectators is ribbed (J. F. R. Trocger, The screen is placed so that it receives only a little stray light on its rear surface, at least in the direction of the spectators.