Along the edges of the ribbons are rows of perforations that are most accurately equalized with respect to their size and of the distances between them. It is by means of wheels with teeth that engage with the perforations and the movement of another toothed part of the mechanism that the ribbon or film is carried across the path of light in the projecting machine. The device for moving the film, although not of a very intricate character, is nevertheless of an ingenious type. It is intermittent in action and operates so that one section of film, containing a picture, is held in the path of light for a fraction of a second, moved away and another section, with the next picture, brought into place to be projected in its turn. This way of working, in most of the projectors, is obtained by the use of a mechanical construction known as the Geneva movement. The pattern of its principal part is a wheel shaped somewhat like a Maltese cross. The form shown in the illustration is given as a type; not all are of this pattern, nor are they all four-parted.
FOUR PHASES OF THE ACTION OF THE INTERMITTENT GEARING KNOWN AS THE GENEVA MOVEMENT.
It is obvious that while one picture moves out of the way for the next, there would be a blur on the screen during such a movement if some means were not devised to prevent it. This is found by eclipsing the light during the time of the change from one picture to another. The detail of the projector that effects this is a revolving shutter with a solid part and an open section. (This is the old type of shutter. It is noticed here because the way in which the light rays project the picture is easily explained by using it as an example.) This shutter is so geared with the rest of the mechanism that (1) the solid part passes across the path of light while another picture is moving into place; and that (2) the open section passes across the path of light while a rectangular area containing a picture is at rest and its details are being projected on the screen.
It may be asked, at this point, why the eye is not aware on the screen of the passing shadow of the opaque part of the shutter as it eclipses the light. It would seem that there should be either a blur or a darkened period on the screen. But the mechanism moves so rapidly that the passing of the solid portion of the shutter is not ordinarily perceptible.
A MOTION-PICTURE PROJECTOR.
A. Film. B. Upper magazine. C. Feed reel. D. Lower magazine, containing the take-up reel. E. Crank to operate mechanism by hand. F. Motor. G. Where the film stops intermittently to be projected. H. Lamp-house. I. Port, or window in the fireproof projection booth. J. Rotating shutter. K. Lens. L. Condenser. M. Switches. N. Fire shutter; automatically drops when the film stops or goes too slowly.
One foot of celluloid film contains sixteen separate pictures, and these pass in front of the light in one second. One single tiny picture of the film takes up then one-sixteenth of a second. But not all of this fraction of a second is given to the projection of the picture as some of the time is taken up with moving it into place immediately before projection. The relative apportionment of this period of one-sixteenth of a second is so arranged that about five-sixths of it (five ninety-sixths of a second) is given to the holding of the film at rest and the projection of its picture, and the remaining one-sixth (one ninety-sixth of a second) is given to the movement of a section of the film and the shutting off of the light by the opaque part of the shutter.
