A knowledge of the workings of the motion-picture camera is essential to a clear understanding of the chapters that follow, so it might be well to include that here. You have seen the large box camera employed in the ordinary photographer’s gallery. The motion-picture camera appears very much like an enlarged box camera, with the addition of a crank on the side and a dial to measure the amount of film used. It is mounted on a tripod which is also movable, either laterally or horizontally, by means of cranks. In all essential points motion-picture photography is really continuous snap-shot photography. Celluloid film is used, that for motion-picture purposes being one and three-eighth inches wide, and supplied in long strips, the average length being two hundred feet. By turning the crank at the side the motion photographer is able to get continuous photographs of a person or object in motion, instead of the single picture that the snap-shot photographer gets. As each of these photographs is only three-quarters of an inch high, it will be seen that the cinematographer can take sixteen separate photographs on each foot of film in his camera. This “sixteen” is a cabalistic figure in motion pictures. There are sixteen photographs on each foot of film. For average work the camera-man photographs the pictures at the rate of sixteen to the second, and they are shown on the screen in the picture theater at the same speed. When the camera-man turns the crank at the side of his camera two independent mechanisms are affected by the operation, the shutter and the device for feeding the film. The shutter is opened to allow the brief exposure of the film necessary to take a single picture, and it is then closed while another three-quarter inch of film is moved into position ready for the next picture. The camera-man continues to turn his crank, thus repeating the operation over and over again until the entire scene is photographed. The shutter in motion-picture cameras is a revolving disk, in which a “V”-shaped opening is cut, or also the aperture may be formed by two disks superimposed, in which case the operator is able to vary the size of the opening. The operator’s crank is connected by gears to the shutter, which is placed between the lens and the film. The action of the shutter has been explained in the paragraph above. It might be stated here that the exposure of the film is a trifle longer than that which would be allowed by the ordinary snap-shot photographer, since the blurring which results on the picture is indistinguishable, owing to the rapidity with which the photographs follow each other on the screen.