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by reflection from the mirror. The fact that the light source is confined within a bulb eliminates danger of cracking the condenser; hence a short-focus condenser which subtends a large angle can be utilized. Through the design of a special prismatic condenser, Fig. 7, the necessity for a thick and heavy lens with accompanying aberration is obviated. The fact that the light source is absolutely steady and without noticeable flicker, even on 25-cycle current, permits the use of a wide aperture objective lens with an accompanying good utilization of light. Shutters can be designed to operate very effectually with incandescent lamp equipment because of the fact that the filament, being heavy, does not get a chance to cool between current reversals. When arc machines are being adapted to the incandescent lamp, it is of advantage to bear the point in mind that the shutter can usually be improved. It is due solely to the better utilization of light that the incandescent lamp, a source of lower brilliancy and far less energetic consumption than the arc, can be used to project satisfactory motion pictures.
For many years the arc lamp has been practically the only light source used in motion-picture projection work and higher and higher amperages have been employed as the size of theatres has increased and as competition between theatres has become keener. It appears that a reaction from the very high values may be logically expected, for a brilliant picture loses its attractiveness when discomfort and eye strain from glare is experienced. A bright screen viewed against a dark background produces an effect similar to that produced by a bright street light viewed against its background. In the first case the effect is less marked and may not be attributed to contrast in brightness, and for this reason is probably more serious than as if the trouble were recognized at once and corrected. Flicker and arc travel are also more pronounced at high intensities than at low ones, yet while pictures projected by incandescent lamp apparatus are free from these objectionable characteristics too high an intensity with such equipment is nevertheless quite capable of producing glare. We may, therefore, I believe, expect to see the higher intensities gradually be abandoned in favor of intensities of the lower order.
In closing, the subject of projection as treated by others should be of interest to us all. A few of the papers on this subject are:
THE PROJECTION LANTERN, by J. B. Taylor, page 414, No. 3, Vol. XI, I. E. S. Proceeding.
OPTIC PROJECTION, by J. A. Orange, page 768, No. 8, Vol. XI, I. E. S. Proceeding.
OPTIC PROJECTION, by S. P. & H. P. Gage, published by Cornstock Pub. Co., Ithaca, N. Y.
PROJECTION ENGINEERING, by R. B. Chillas, Jr., page 1097, No. 9, Vol. XI, I. E. S. Proceeding.
LIGHT PROJECTION— ITS APPLICATION, by E. J. Edwards and H. H. Magdsick, a lecture delivered at the University of Pennsylvania through the I. E. S. in September, 1916, I. E. S. Proceeding.