ch of which
showed a difference in the position of the legs of a man running, for
instance, was passed quickly before the eye so that the space between
the pictures would be screened, the figure would apparently move. The
eyes retain the image they see for a fraction of a second, and if a new
image carrying the movement a little farther along is presented in the
same place, the eyes are deceived so that the object apparently actually
moves. An ingenious toy called the zoltrope, which was based on this
optical illusion, was made long before Edison invented the vitascope,
Herman Caster the biograph and mutoscope, or the Lumiere brothers in
France devised the cinematograph. All these different moving-picture
machines work on the same principle, differing only in their mechanism.

A moving-picture machine is really a rapid-fire repeating camera
provided with a lens allowing of a very quick exposure. Internal
mechanism, operated by a hand-crank or electric motor, moves the
unexposed film into position behind the lens and also opens and closes
the shutter at just the proper moment. The same machinery feeds down a
fresh section of the ribbon-like film into position and coils the
exposed portion in a dark box, just as the film of a kodak is rolled off
one spool and, after exposure, is wound up on another. The film used in
the biograph when taking the Suburban was two and three-fourth inches
wide and several hundred feet long; about forty exposures were made per
second, and for each exposure the film had to come to a dead stop before
the lens and then the shutter was opened, the light admitted for about
one three-hundredth of a second, the shutter closed, and a new section
of film moved into place, while the exposed portion was wound upon a
spool in a light-tight box. The long, flexible film is perforated along
both edges, and these perforations fit over toothed wheels which guide
it down to the lens; the holes in the celluloid strip are also used by
the feeding mechanism. In order that the interval between the pictures
shall always be the same, the film must be held firmly in each position
in turn; the perforations and toothed mechanism accomplish this
perfectly.

In taking the picture of the Suburban race almost five thousand
separate negatives (all on one strip of film, however) were made during
the two minutes five and three-fifths seconds the race was being run.
Each negative was perfectly clear, and each was different, th