tus in which a series of views
representing closely successive phases of a moving object are exhibited
in rapid sequence, giving a picture which, owing to persistence of
vision, appears to the observer to be in continuous motion. It is a
development of the zoetrope or "wheel of life," described by W.G. Horner
about 1833, which consists of a hollow cylinder turning on a vertical
axis and having its surface pierced with a number of slots. Round the
interior is arranged a series of pictures representing successive stages
of such a subject as a galloping horse, and when the cylinder is rotated
an observer looking through one of the slots sees the horse apparently
in motion. The pictures were at first drawn by hand, but photography was
afterwards applied to their production. E. Muybridge about 1877 obtained
successive pictures of a running horse by employing a row of cameras,
the shutters of which were opened and closed electrically by the passage
of the horse in front of them, and in 1883 E.J. Marey of Paris
established a studio for investigating the motion of animals by similar
photographic methods.

The modern cinematograph was rendered possible by the invention of the
celluloid roll film (employed by Marey in 1890), on which the serial
pictures are impressed by instantaneous photography, a long sensitized
film being moved across the focal plane of a camera and exposed
intermittently. In one apparatus for making the exposures a cam jerks
the film across the field once for each picture, the slack being
gathered in on a drum at a constant rate. In another four lenses are
rotated so as to give four images for each rotation, the film travelling
so as to present a new portion in the field as each lens comes in place.
Sixteen to fifty pictures may be taken per second. The films are
developed on large drums, within which a ruby electric light may be
fixed to enable the process to be watched. A positive is made from the
negative thus obtained, and is passed through an optical lantern, the
images being thus successively projected through an objective lens upon
a distant screen. For an hour's exhibition 50,000 to 165,000 pictures
are needed. To regulate the feed in the lantern a hole is punched in the
film for each picture. These holes must be extremely accurate in
position; when they wear the feed becomes irregular, and the picture
dances or vibrates in an unpleasant manner. Another method of exhibiting
cinematographic effects is