hundreds of feet of film being developed at once.

A negative, however, is of no use unless a positive or print of some
kind is made from it. If shown through a stereopticon, for instance, a
negative would make all the shadows on the screen appear lights, and
vice versa. A positive, therefore, is made by running a fresh film, with
the negative, through a machine very much like the moving-picture
camera. The unexposed surface is behind that of the negative, and at the
proper intervals the shutter is opened and the admitted light prints the
image of the negative on the unexposed film, just as a lantern slide is
made, in fact, or a print on sensitised paper. The positives are made by
this machine at the rate of a score or so in a second. Of course, the
positive is developed in the same manner as the negative.

Therefore, in order to show the people in the theatre the Suburban, five
hundred feet of film was exposed, developed, fixed, and dried, and
nearly ten thousand separate and complete pictures were produced, in the
space of two hours and fifteen minutes, including the time occupied in
taking the films to and from the track, factory, and theatre.

Originally, successive pictures of moving objects were taken for
scientific purposes. A French scientist who was studying aerial
navigation set up a number of cameras and took successive pictures of a
bird's flight. Doctor Muybridge, of Philadelphia, photographed trotting
horses with a camera of his own invention that made exposures in rapid
succession, in order to learn the different positions of the legs of
animals while in rapid motion.

A Frenchman also--M. Mach--photographed a plant of rapid growth twice a
day from exactly the same position for fifty consecutive days. When the