Two light-tight boxes are contained in the camera, one at the top to hold the raw film, and the lower one to receive the film after it has been exposed. Perforations have been made along the edges of the film before it is placed in the camera, the holes being oblong in shape, one-eighth of an inch wide and one-sixteenth of an inch in height. The film-feeding device, which is either of the sprocket-wheel type, which drives the film through, or the claw-hammer type, which pulls it, engages in these holes. The perforations serve a similar purpose in the camera, the projection-machine, and in the process of developing the negative and printing the positives. It will be seen that the perforating, which is done by machinery, must be accurate to the one-hundredth of an inch. The working of the projection-machine, the apparatus which throws the image on the film onto the screen, is in many ways similar to that of the camera. Or, a simpler comparison, the projection-machine is really the familiar stereopticon, or magic lantern, with the addition of mechanism for feeding the film rapidly before the light and the lenses. In the first place, there is a “lamp-house,” a small cabinet which contains the light, supplied by means of an arc-light, and the condensing lens. The light created by the carbons of the arc, while strong, is diffused in the lamp-house, and it is the purpose of the condensing lenses to concentrate the rays before throwing them on the screen. The pictures are outlined on the screen because of the fact that the figures on the film obstruct light in proportion to their density. From the lamp-house and the condensing lenses we naturally pass to consideration of the film, and the devices by which it is fed before the rays of light with such accuracy that the picture is always in place on the screen, and with such rapidity and uniformity of speed that the spectator is not aware of the fact that he is really looking at sixteen separate pictures each second. The principle of the operation is similar to that used in the camera. The film passes from a fire-proof magazine above, being pulled out by a sprocket-wheel that engages in the perforations. After passing from these sprockets there is a brief respite for the film, a loop being provided to relieve it so that the pull will not be too great when passing through the film-gate which holds the image in place before the rays of the condenser. It is necessary that the gate hold the film perfectly taut and in alignment, for it can be seen that the slightest fractional deviation would appear great when the image is magnified by the objective lenses onto the screen.