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396 PRINCIPLES OF CINEMATOGRAPHY
4 minutes. (4) A mechanism should be provided to enable the screen picture to be accurately framed within the projecting aperture and without moving the picture with relation to the boundaries of the screen when such adjustments are being made. (5) The projection lens should preferably be so mounted that both a coarse and fine adjustment is provided to obtain critically sharp image definition. (6) The projection lens should have a minimum resolving power of 90-lines per millimetre in the centre of the field and no less than 40-lines per millimeter at the four corners of the field. (7) the shutter should only interrupt the light beam twice during the projection of each frame and the duration of the light and dark periods should be carefully balanced. (8) the diameter of the shutter should be as great as possible and the aperture should be as far from the centre of the shutter as possible. (9) The screen picture should remain substantially free from unsteadiness and, when such unsteadiness can be traced to the mechanism, it should not exceed 0-3 % of the picture width. (10) Any mechanism should be capable of projecting a new film the perforation pitch being no less than 0-3 % below the recommended standard for at least 1,000 showings without causing any damage sufficiently serious either to show on the screen or to cause defects in the sound reproduction.
Turning now to the requirements of sound reproduction, the following points should be carefully checked when any machine is being inspected. (1) The mechanical noise created by the projector gearing, both with and without film passing through the machine, should be at a minimum and, preferably, some efficient 'blimp' should be provided to enclose most of the machine during operation. (2) The film should travel past the scanning point with such uniformity that *wowing' or flutter no greater than 0-35% is produced with a constant frequency sound track of a 3,000 cycle signal. (3) The sound scanning beam should measure 0-070-inches by 0-0005-inches (1 -778-mm by 0-0127-mm) at the film surface or, if a track magnifying system is used, proportional measurements should exist at the scanning aperture. (4) The intensity of the scanning beam should remain uniform throughout the scanning aperture. (5) The film should run past the scanning point with such accuracy that lateral movement greater than plus or minus 0-002-inch (0-050-mm) cannot occur, and should also be guided so that the track is symmetrically positioned about the scanning aperture. (6) The optical system used to form the scanning slit at the film surface should be so adjustable that critical focus may be obtained on either the front or rear surface of the film. (7) The