MOVING PICTURES are produced by lantern projection on a screen, of a series of pictures in such rapid succession that the eye sees them as one continuous representation. The possibility of such pictures depends upon what might be considered a d,:fect of the human eye, known as the of vision"— the technical name for the lingering on the retina of an impression received, after the excitation which produced the impression has passed. This lingering of the impression is sufficiently long to tide over the gap of one sixtieth to one-eightieth of a second during which there is no picture on the screen. So that if the mechanism of human vision was per fect the illusion of the moving nicture would be impossible.
It has been determined by experiment that the duration of the persistence of vision aver ages one-fiftieth of a second and it was cal culated that 14 pictures per second would be needed to produce a continuous impression upon the eye — assuming each 'picture to occupy the screen for three-fourths of its allotted share of the second, the other fourth being the gap of darkness while the film was moving for ward to the next picture. With the improve ment of the exhibiting mechanism, however, the number of pictures per second was in creased to 16, the length of time each is on the screen being one-twentieth of a second, and the dark gap between being reduced to one eightieth of a second. In this way the presen tation has been made much smoother and the former unpleasant flickering eliminated.
The fact that moving pictures are projected on a screen by means of an optical lantern (q.v.) has already been referred to. This lantern with the film moving mechanism is called the kinetoscope or kinetograph (and nu merous other names). The lantern part is of the same general type as the magic lantern, being equipped with a brilliant light (usually the electric arc) and a projection lens by which the image is enlarged upon a screen. The moving-picture lantern, however, is a very highly complicated apparatus as compared with the magic lantern, for, instead of the simple lantern-slide pushed into place in the field of the lens of the latter and left there as long as desired, the kinetograph mechanism must bring its long series of thousands of pictures one by one into place with the most exquisite accuracy, opening the shutter when the picture is ready and closing it while another is being moved into place, and all at a speed so rapid that the human eye is unable to follow the motion.
One of the requirements arising from this great rapidity of movement is that the indi vidual pictures must be very small— a large picture could not be moved fast enough with out the destruction of the film on which the pictures are carried. These physical conditions have resulted in fixing the workable size of the kinematograph pictures at one inch in length by three-fourths of an inch in height. The magnification of this little picture on the screen is therefore about 10,000 times its original area — which suggests the extreme perfection in the picture itself as well as in the projection appa ratus. These little pictures are arranged con tinuously one above another on a strip of per fectly transparent horny material resembling celluloid. The strip is one and three-sixteenths of an inch in width and may be any length de sired — generally ranging from 12') feet to 1,000 feet. The pictures occupy the centre of the
strip and in the margins on both sides are square-cut perforations by which the movement of the strip in the lantern is accomplished through sprocket wheels and hooks.
The pictures themselves are produced by photography, on a coating of sensitized gelatine spread upon the strip of film, and the pictures are photographic positives printed from nega tives on a similar strip made in a special mov ing-picture camera which will he briefly de scribed farther on in this article. For exhibi tion with the lantern the film is rolled on a spool or reel and fed in a loose loop by a series of sprocket wheels and idlers to the °gate," as it is called, through which the light passes to the screen. The gate carries a mask which cuts down the size of the picture actu ally illuminated and projected to fifteen sixteenths of an inch in length and eleven sixteenths of an inch in width. The film is pressed against this mask firmly but gently by delicately acting springs. The movement of the film ic downward through the gate and is accomplished by a pair of hooks just below the gate, framed together so that they will accurately engage the perforations in the margin of the film, one hook of the pair on each edge of the film. These hooks are oper ated by a revolving cam which causes them to move forward into the perforations; then to move downward, pulling the film with them just the height of one picture; then to move backward out of the perforations, and then up ward to their first position. The cam is so fixed that the pull downward on the film oc cupies about one-fifth of the complete cycle of the movement of the hooks. During that pe riod of one-fifth of the time allotted to one picture — one-sixteenth of a second — the shutter which revolves in front of the projec tion lens shuts off the light from the screen for the eightieth of a second; when, the next picture being in position, the shutter allows the light to pass to the screen for four-eightieths of a second — the entire operation being re peated for the next picture. And thus the operation goes on, 16 times per second until the film has all been run. As the pictures run 16 to the foot of film, a short film of 120 feet, containing 1,920 individual pictures, will be run off in two minutes. A 1,000-foot film, with 16,000 individual pictures, will occupy nearly 17 minutes. As the film which has been shown passes the hooks it is gathered by sprocket wheels and passed to a receiving reel on which it is wound for storage and from •which it must be rewound before it is in proper order for another exhibition. The entire mechanism of the lantern is operated by the handle turned by the operator. In some instances electricity has been used in place of hand power, but the latter is relied on in the great majority of cases. Fig. 1 shows the relative arrangement of the carbons, the condensing system of lenses, the gate with the film passing downward through it from the full reel box to the empty one, pro pelled by the sprocket wheels. The projecting lens is shown between the reels. The drawing is skeletonized in order to show simply the essentials of the moving picture lantern.