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314 RICHARDSON'S BLUEBOOK OF PROJECTION
tion with the propeller ends, sets up air currents and draws them in through the opening of the shutter housing at the cooling plate, and forces them out 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.
Simplex Double Shutter
(84) The Simplex E-7 projector mechanism is equipped with both a rear and a front shutter blade, each of the ordinary disc type, both mounted on the same shutter shaft. Both rotate in the same direction. But they nevertheless produce the optical effect of cutting the light beam from opposite directions (as does the Motiograph cylindrical shutter) because (85) the projection lens is located between them and that lens reverses the beam so that its bottom rays on the lamphouse side are its top rays on the screen side. Therefore the action is exactly the same, in effect, as it would be were two blades, both located either in front of or at the rear of the projection lens, cutting the beam from opposite directions. Hence, the time required to cut light off the screen and restore it is reduced by one half as compared with the single shutter, and the screen receives proportionally increased total illumination. The two shutters are adjusted with relation to each other and to the beam by means of a special device supplied with each projector mechanism.
Correct Position for Front Shutters
(86) If an opaque sheet of material be held in front of a projection lens, an image of the converging condenser or mirror surface will be impressed thereon at a distance from the lens equal to the focal length of the lens. This point is known as the "point of aerial image.'' Its exact location may be ascertained by pro