sitiveness of a photographic plate), and gradually became familiar
to everyone in the exhibitions known as the "biograph" or
"cinematograph," the actual position of the legs in a galloping horse
at any given fraction of a second was unknown. Anyone who has tried to
"see" their position will agree that it cannot be done. Attempts had
been made to make out what the movements and positions of the legs
"must" be, by studying the hoof-marks in a soft track laid for the
purpose. But the result was not satisfactory.

As everyone knows, the so-called "biograph" pictures are produced by
an enormous series of consecutive instantaneous photographs taken on a
continuous transparent flexible film or ribbon. The camera has a
mechanism attached to it by which the sensitive film is jerked along
so as to expose a length of two inches (the size of the picture given
by the camera) for, say, one-thirtieth of a second without movement.
The film is then jerked on and a second bit of two inches is brought
into place for a thirtieth of a second and so on until a ribbon of
some thousand pictures is obtained. The interval between each picture
is usually also about one-thirtieth of a second, so that at least
fifteen pictures are taken in every second of time, and according to
the requirements of illumination and the rapidity of the movements of
the men or animals photographed this number may be greatly increased.
The film is developed, printed and fixed on a similar rolling
mechanism and the pictures are thrown one by one by a powerful
lantern on to a screen, and are jerked along at the same rate as that
at which they were taken, and are magnified enormously. Animals and
men in rapid movement, railway trains, the waves of the sea are thus
photographed, and when the serial pictures are thrown successively on
the screen the result is that the eye detects no interval between the
successive pictures--the figures appear as continuous moving objects.
This is due to the fact that whilst the impression produced on the
retina of the eye by each picture lasts for a tenth of a second (less
with brighter light), the interval between the successive pictures is
only one-thirtieth of a second, and accordingly the retinal impression
has not gone or ceased before the next is there; hence there is no
break in the series of retinal impressions, but continuity.[1]

[Illustration: Plate I.--Figs. 1 to 11, drawings from Muybridge's
photographs of consecutive poses of