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THE PROJECTION ROOM 337
ing and, with the projector anchored permanently in its place and the light properly centered on the (use
the magnascope lens if one is provided), project the light beam upon part A nearest the lens. With a pencil trace the beam outline on the part, remove the whole device to the bench, take out Part A, cut a hole therein just a little larger than the actual beam, reassemble, replace in the wall opening and do the same thing with every one of the obstructing sheets of material.
When you have finished you will have a port just a trifle bigger than the light beam, with a sound baffle that should make a glass cover, except possibly in very thin walls, unnecessary. Cement around the edges, both inside and out, with half good, clean sand and half cement or plaster of paris.
Three baffle plates should be sufficient, but in case of very thick walls four might be used to more advantage.
(29) Lens ports and openings other than observation ports should be no larger than is actually necessary but (30) observation ports should be large enough to afford a clear view of the screen from any normal working position beside the projector.
(31) Authorities who compel reduction of observation ports to less than twelve inches square gain nothing in added safety, but do reduce the efficiency of the projection crew. They forget that ports are now invariably glass covered and a fire shutter will drop and cover a twelve-inch opening just as quickly as it will cover one of smaller dimensions.
(32) Observation ports should be fourteen inches high by twelve inches wide, covered with glass. (33) The bottom of these observation ports should be so located that a line drawn from a point opposite the light source, and 60 inches away from the floor, to the lower edge of the screen image will just miss the port bottom. This makes certain a full view of the screen image from working position regardless of the projection angle. (34) The wall surrounding each port should be painted in a non-gloss black color for a foot and a half in all directions.