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THE PROJECTOR 231
were only one blade in active operation. (83) Of the two blades each acts as a master and cut-off blade.
(84) The Motiograph horizontal rear shutter is shaped at each end of the cylinder to act like a fan or air propeller. From each end air currents are directed toward the center of the cylinder. Each of the two blades of the cylinder has a transverse vane, which, in combination with the propeller ends, sets up air currents and draws them in through the opening of the shutter housing at the cooling plate, (85) and forces them out
Fig. 67. — Motiograph Horizontal type rear shutter.
of the shutter housing toward the lamphouse. This draws air over the metal of the projector mechanism around the film gate, keeping it and the film at relatively low temperature and preventing deposits of dust.
The cylinder shutter, like the disc type, may be adjusted manually and may be partially adjusted while the projector is in motion.
Regulating the Front Shutter
Front shutters have been abandoned almost entirely for theatre projectors. Their use subjects both the film and projector mechanism to excessively high temperature and they should be replaced by the newer type shutter located between the light source and the aperture. Some are still used, however, therefore the following information concerning them is of value.
(86) Were a screen held in front of a projection lens at a distance corresponding to the focal length of the lens, an image of the converging condenser lens or