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OBJECTIVE LENS:
The objective lens used in the projection of pictures by means of incandescent lamps should be of wide aperture to pick up as much as possible of the cone of light which is passed through the film. In most of the projectors, the diameter of the lens holder is about 2", and the lens which is g^reater in diameter than %" must therefore be held in a special adapter or mounted in a jacket on the face plate of the machine. In any event, the cone of light which passes to the lens cannot be greater in diameter than the tube or opening in which the lens is mounted. Increasing the diameter of the lens beyond about 2^", would therefore seem to be of doubtful value for existing: equipment.
SCREEN RESULTS:
A number of tests of the illumination delivered to the screen, show that an incandescent lamp can be depended upon to deliver fully 600 lumens on the screen. This means that the averasre illumination on a screen sixteen feet in width, will be in the neighborhood of three foot candles. Our tests have shown that this illumination will produce satisfactory pictures. In fact, the pictures, when compared with those produced by a 40 ampere direct current arc, were their equal in every way, and had the added advantage of being perfectly steady.
The average illumination produced by the arc in a test case was found to be 4.8 foot candles. This average was determined by taking readings every half minute during the period of ten minutes, while the operator held the arc as steady as possible. The variation in the arc was from about 2 foot candles to 9 foot candles, the variation seemingly being caused by the wandering of the arc and by the necessity for trimming it occasionally. The measurments in both cases were taken with the shutter open. The projection distance was 112 feet.