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 photographer's camera had reached a
stage at which it was possible to take snapshot pictures. But this alone
would not have allowed the photographing of a real movement with one
camera, as the plates could not have been exchanged quickly enough to
catch the various phases of a short motion.

Here the work of Muybridge sets in. He had a black horse trot or gallop
or walk before a white wall, passing twenty-four cameras. On the path of
the horse were twenty-four threads which the horse broke one after
another and each one released the spring which opened the shutter of an
instrument. The movement of the horse was thus analyzed into twenty-four
pictures of successive phases; and for the first time the human eye saw
the actual positions of a horse's legs during the gallop or trot. It is
not surprising that these pictures of Muybridge interested the French
painters when he came to Paris, but fascinated still more the great
student of animal movements, the physiologist Marey. He had contributed
to science many an intricate apparatus for the registration of movement
processes. "Marey's tambour" is still the most useful instrument in
every physiological and psychological laboratory, whenever slight
delicate movements are to be recorded. The movement of a bird's wings
interested him especially, and at his suggestion Muybridge turned to the
study of the flight of birds. Flying pigeons were photographed in
different positions, each picture taken in a five-hundredth part of a
second.

But Marey himself improved the method. He made use of an idea which the
astronomer Jannsen had applied to the photographing of astronomical
processes. Jannsen photographed, for instance, the transit of the planet