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THE PROJECTOR 233
blades; if the cardboard is strong it will run perfectly for several days without such connection.
Having set the cardboard blade properly, project a black and white title upon the screen and continue to trim off small but equal amounts from each edge of the master blade, after each trial projection, until a faint travel ghost begins to creep up and down upon the screen. Now the blade is just a bit too narrow on both sides. Remove it and trim the metal master blade to almost the same proportions, but a little wider to kill all traces of the travel ghost. Additional width, to be determined by the projectionist, must be provided or travel ghost will appear as soon as the mechanism gears and bearings begin to wear. (91) Trim the other blade to the same width as the master blade, always taking off an equal amount from each side. The shutter is now ready for efficient operation allowing maximum light on the screen.
Trimming the Rear Shutter Blades
Since the rear shutter is located a fixed, unchangeable distance from the projector aperture, (92) the projectionist is only concerned with the width of the blade and the air movement it creates. He need only ascertain whether or not the shutter is keeping too much light from the screen. He can do this easily if the shutter is an adjustable one, as most disc type shutter blades are: reduce the blade until travel ghost appears, and then increase the width slightly until it again disappears. (93) If the shutter has non-adjustable blades it is possible to make a test along the lines suggested for the front shutter, only an experimental blade of sheet metal, not of paper, must be employed for trimming. Because of the extremely hot light beam paper would be dangerous. This test can be made with the disc type shutter only.
(94) Since some theatres use a wide-diameter, closeup converging element, while others use a small diame ter, far-away element, there is a decided difference in the beam depth to be cut by the shutter blades. (95)