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240 RICHARDSON'S BLUEBOOK OF PROJECTION
reel is threaded in, unless only old, well-seasoned films are used.
(119) The film must pass through the projector and sound mechanism in a perfectly straight line. (120) It is guided past the aperture of both the projector mechanism and sound head by lateral guides, which should never be disturbed except to be replaced. (121) Assuming these guides to be rightly placed, the correctness of the path of the film elsewhere may be judged by examining the loops (see Fig. 53), which should set perfectly "square." If either or both of the loops are at all distorted (crooked) it is — unless caused by the presence of a poor film splice in the loop — evidence that something requires immediate attention.
If it becomes necessary to remove and replace a lateral guide follow the manufacturer's instruction book, or if this is unavailable secure detailed instructions by addressing the projector manufacturer. Correct placement of the lateral guides is necessary to good projection.
The Automatic Fire Shutter
Projectors are all equipped with an automatic fire shutter (122) consisting of a mechanically controlled sheet of metal that drops down, automatically, between the cooling plate and aperture, cutting off all light from the film and the screen the instant projection speed is reduced below the safe limit which is the moment the film is in danger of ignition because of the slowness of its passage through the intense heat of the spot.
(123) The mechanical operation of the fire shutter varies widely in different makes of projector, but in none of them does it require much attention beyond lubrication. (124) The fire shutter should not rise until projection speed has fallen to at least sixty (60) feet of film per minute. It should drop automatically when projection speed falls below sixty feet of film per minute. (125) Projectionists should check the rising and falling speeds of the shutter at least twice a week to make sure the device is operating properly. Improper action of the