paratus as early as 1853 to throw
these pictures of the stroboscopic disks on the wall. Horner followed
with the daedaleum, in which the disk was replaced by a hollow cylinder
which had the pictures on the inside and holes to watch them from
without while the cylinder was in rotation. From this was developed the
popular toy which as the zooetrope or bioscope became familiar
everywhere. It was a revolving black cylinder with vertical slits, on
the inside of which paper strips with pictures of moving objects in
successive phases were placed. The clowns sprang through the hoop and
repeated this whole movement with every new revolution of the cylinder.
In more complex instruments three sets of slits were arranged above one
another. One set corresponded exactly to the distances of the pictures
and the result was that the moving object appeared to remain on the
same spot. The second brought the slits nearer together; then the
pictures necessarily produced an effect as if the man were really moving
forward while he performed his tricks. In the third set the slits were
further distant from one another than the pictures, and the result was
that the picture moved backward.

The scientific principle which controls the moving picture world of
today was established with these early devices. Isolated pictures
presented to the eye in rapid succession but separated by interruptions
are perceived not as single impressions of different positions, but as a
continuous movement. But the pictures of movements used so far were
drawn by the pen of the artist. Life showed to him everywhere continuous
movements; his imagination had to resolve them into various
instantaneous positions. He drew the horse race for the zooetrope, but
while the horses moved forward, nobody was able to say whether the
various pictures of their legs really corresponded to the stages of the
actual movements. Thus a true development of the stroboscopic effects
appeared dependent upon the fixation of the successive stages. This was
secured in the early seventies, but to make this progress possible the
whole wonderful unfolding of the photographer's art was needed, from the
early daguerreotype, which presupposed hours of exposure, to the
instantaneous photograph which fixes the picture of the outer world in a
small fraction of a second. We are not concerned here with this
technical advance, with the perfection of the sensitive surface of the
photographic plate. In 1872 the