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454 WHITE IMAGE SCREENS [Cn. XII
usually in the general exhibition room, and there is no special boxing or enclosure of the apparatus. But in moving picture theaters, where there is some danger from the inflammability of the picture films, both the fire underwriters and the municipal regulations usually require some form of fire-proof operating room.
IMAGE SCREEN
§ 621. Next in importance to a suitable room for exhibitions with projection apparatus is a good screen upon which to project the image.
No one has ever more briefly and clearly stated the qualities of a good image screen than Goring & Pritchard: "It should reflect the greatest -possible quantity of light and absorb the least." "Every care should be taken to render the surface as smooth, white and opaque as it can be made" . . . "inasmuch as the brilliancy and perfectness of the picture will greatly depend on the whiteness, and the sharpness of its outline upon the smoothness of the screen." The screen should be dull white, never shiny.
§ 622. Screens of plaster paris upon the wall. — A screen fulfilling all the requirements just given is a wall coated with a smooth finish of pure, fine plaster of Paris.
§ 623. Painted wall screen. — While a plaster of Paris wall screen is perhaps the best, a smoothly plastered wall, if properly painted, gives almost as good results and is much cheaper. The wall, as stated, should be finished as smoothly as possible by the plasterers, then it is coated with pure linseed oil if porous, or with a mixture of equal parts of linseed oil and turpentine if the wall is hard and non-porous. When this is dry, the wall is painted with either white lead ground in oil and thinned with turpentine, or with "sanitary paint" thinned with turpentine. The sanitary paint has the advantage that it does not turn yellow with age, and that it is more easily cleaned with soap and water.
When the paint is properly thinned it should be strained through one or two layers of gauze (cheese cloth) to get out any lumps or coarse particles.