and for each exposure the film had to come to a dead stop before the lens and then the shutter was opened, the light admitted for about one three-hundredth of a second, the shutter closed, and a new section of film moved into place, while the exposed portion was wound upon a spool in a light-tight box.  The long, flexible film is perforated along both edges, and these perforations fit over toothed wheels which guide it down to the lens; the holes in the celluloid strip are also used by the feeding mechanism.  In order that the interval between the pictures shall always be the same, the film must be held firmly in each position in turn; the perforations and toothed mechanism accomplish this perfectly.

In taking the picture of the Suburban race almost five thousand separate negatives (all on one strip of film, however) were made during the two minutes five and three-fifths seconds the race was being run.  Each negative was perfectly clear, and each was different, though if one negative was compared to its neighbour scarcely any variance would be noted.

After the film has been exposed, the light-tight box containing it is taken out of the camera and taken to a gigantic dark-room, where it is wound on a great reel and developed, just as the image on a kodak film is brought out.  The reel is hung by its axle over a great trough containing gallons of developer, so that the film wound upon it is submerged; and as the reel is revolved all of the sensitised surface is exposed to the action of the chemicals and gradually the latent pictures are developed.  After the development has gone far enough, the reel, still carrying the film, is dipped in clean water and washed, and then a dip in a similar bath of clearing-and-fixing solution makes the negatives permanent—­followed by a final washing in clean water.  It is simply developing on a grand scale, thousands of separate pictures on 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.