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THE PROJECTOR 289
This is the secret of moving pictures. The intervals of darkness are not seen by the audience; the screen appears
to be continuously illuminated.
Threading the Film
(17) In every mechanism there is some provision whereby the idlers can be pushed back or hinged back from the sprockets, and the "gate" can be opened, so that the projectionist (having pulled down a few feet of film from the upper magazine) can insert the celluloid between each sprocket and its idler, and between the polished steel tension pads and runners of the gate. As he sets the film properly at each sprocket, accurately engaging the perforations with the sprocket teeth, the projectionist snaps the idler back into .position to hold the film; as he sets it properly in the gate, he snaps the gate closed. He takes care to leave the correct length of loop or slack above the gate and below the intermittent sprocket, and he makes sure that one photograph is squarely framed in the aperture, not parts of two photographs. This entire process is called threading the film.
The Take-up
(18) The process of threading includes engaging a few inches of film on the hub of the reel in the lower magazine. When the motor is started, and film begins running downward through the projector, the reel in the lower magazine revolves, driven by the take-up, and begins winding up the film. Now the hub of the reel may be — to take an even figure — nine inches around ; and as the speed of the film through the projector is 18 inches per second, the hub will have to revolve twice in one second, to take up 18 inches of film. But as more and more film is wound up on the reel, adding to the size of the "hub", a time will come when the "hub", including film already reeled up, is 18 inches around, and the lower reel then will have to revolve once per second. In other words, the speed of the lower reel must be variable, not constant, and therefore it cannot be driven by a simple