his work is correct, the highest criterion--commercial
expediency--bears witness; for in spite of the fact that the courts have
somewhat narrowed the broad claims of Edison's patents by reason of the
investigations of earlier experimenters, practically all the immense
amount of commercial work that is done in the motion-picture field
to-day is accomplished through the use of apparatus and methods licensed
under the Edison patents.

The philosophy of this invention having already been described in
Chapter XXI, it will be unnecessary to repeat it here. Suffice it to say
by way of reminder that it is founded upon the physiological phenomenon
known as the persistence of vision, through which a series of sequential
photographic pictures of animate motion projected upon a screen in rapid
succession will reproduce to the eye all the appearance of the original
movements.

Edison's work in this direction comprised the invention not only of a
special form of camera for making original photographic exposures from a
single point of view with very great rapidity, and of a machine adapted
to effect the reproduction of such pictures in somewhat similar manner
but also of the conception and invention of a continuous uniform, and
evenly spaced tape-like film, so absolutely essential for both the above
objects.

The mechanism of such a camera, as now used, consists of many
parts assembled in such contiguous proximity to each other that an
illustration from an actual machine would not help to clearness of
explanation to the general reader. Hence a diagram showing a sectional
view of a simple form of such a camera is presented below.

In this diagram, A represents an outer light-tight box containing a
lens, C, and the other necessary mechanism for making the photographic
exposures, H<1S> and H<2S> being cases for holding reels of film before
and after exposure, F the long, tape-like film, G a sprocket whose teeth
engage in perforations on the edges of the film, such sprocket being
adapted to be revolved with an intermittent or step-by-step movement
by hand or by motor, and B a revolving shutter having an opening and
connected by gears with G, and arranged to expose the film during the
periods of rest. A full view of this shutter is also represented, with
its opening, D, in the small illustration to the right.

In practice, the operation would be somewhat as follows, generally
speaking: The lens would first be focussed on the animate s