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creates this impression?
    No one knows the full answer. Many people have speculated that the effect results from “persistence of vision,” the tendency of an image to linger briefly on our retina. Yet if this were the cause, we’d see a bewildering blur of superimposed stills instead of smooth action. At present, researchers believe that two psychological processes are involved in cinematic motion: critical flicker fusion and apparent motion.
    If you flash a light faster and faster, at a certain point (around 50 flashes per second), you see not a pulsating light but a continuous beam. A film is usually shot and projected at 24 still frames per second. The projector shutter breaks the light beam once as a new image is slid into place and once while it is held in place. Thus each frame is actually projected on the screen twice. This raises the number of flashes to the threshold of what is called
critical flicker fusion.
Early silent films were shot at a lower rate (often 16 or 20 images per second), and projectors broke the beam only once per image. The picture had a pronounced flicker—hence an early slang term for movies, “flickers,” which survives today when people call a film a “flick.”
    Apparent motion
is a second factor in creating cinema’s illusion. If a visual display is changed rapidly enough, our eye can be fooled into seeing movement. Neon advertising signs often seem to show a thrusting arrow, but that illusion is created simply by static lights flashing on and off at a particular rate. Certain cells in our eyes and brain are devoted to analyzing motion, and any stimulus resembling movement apparently tricks those cells into sending the wrong message.
    Apparent motion and critical flicker fusion are quirks in our visual system, and technology can exploit those quirks to produce illusions. Some moving-image machines predate the invention of film ( 1.10 , 1.11 ). Film as we know it came into being when photographic images were first imprinted on strips of flexible celluloid.
     
    1.10 The Zoetrope, which dates back to 1834, spun its images on a strip of paper in a rotating drum.
     
     
     
    1.11 The Mutoscope, an early-20th-century entertainment, displayed images by flipping a row of cards in front of a peephole.
     
     
Machines That Use Film
    At all stages of a film’s life, machines move the film strip one frame at a time past a light source. First, there is the
camera
( 1.12 ). In a light-tight chamber, a drive mechanism feeds the unexposed motion picture film from a reel (a) past a lens (b) and aperture (c) to a take-up reel (d). The lens focuses light reflected from a scene onto each frame of film (e). The mechanism moves the film intermittently, with a brief pause while each frame is held in the aperture. A shutter (f) admits light through the lens only when each frame is unmoving and ready for exposure. The standard shooting rate for sound film is 24 frames per second (fps).
     
    1.12 The camera.
     
     
    The projector is basically an inverted camera, with the light source inside the machine rather than in the world outside ( 1.13 ). A drive mechanism feeds the film from a reel (a) past a lens (b) and aperture (c) to a take-up reel (d). Light is beamed through the images (e) and magnified by the lens for projection on a screen. Again, a mechanism moves the film intermittently past the aperture, while a shutter (f) admits light only when each frame is pausing. As we’ve seen, the standard projection rate for sound film is 24 fps, and the shutter blocks and reveals each frame twice in order to reduce the flicker effect on the screen.
     
    1.13 The projector.
     
     
    A feature-length film is a very long ribbon of images, about two miles for a two-hour movie. In most theaters, the projector carries the film at the rate of 90 feet per minute. In the typical theater, the film is mounted on one big platter, with another platter underneath to take it up after it has passed through the projector (