Motography (Jan-Jun 1913)

April 5, 1913 MOTOGRAPHY 227 shown by the arrows, the blade A starts to cut off the upper rays C-C, and the blade B cuts upward through the lower rays D-D, thus cutting the beam at double the peripheral speed of the drum. When the edges of A and B are opposite one another on a horizontal line, the light is completely interrupted. This action occurs twice per revolution, making it possible to run the shutter at half the speed of the film feed or at the rate of one revolution for two pictures. As the blades work from both sides of the ray at the same time, the cutting action is very rapid, being twice that of a single edge that passes through the ray in the ordinary manner. When the drum is revolved at the same speed as the film shift mechanism, the light is admitted to the lens and cut-off twice per picture, which makes the interruption less apparent and reduces the flicker that is in evidence at the lower speed of one interruption per picture. Because l f the high cutting speed that results in a small cylinder diameter, the barrel type of shutter is placed inside of the motion head casing between the film and the objective lens, and centered on the apex of the converging light rays. The disc shutter, the most commonly used type, is simply a circular sheet metal disc in which two or more sector shaped windows or openings are cut, and unlike the barrel type, its edges enter one side of the beam only, and from there pass entirely across the beam. As the cutting speed of a single opening is only half that of the barrel type shutter, the disc is necessarily of larger diameter and must contain more openings in order to keep the rate of opening and closing above the flicker point. When a disc shutter has but one blade that acts only during the fifth of the total period when the film is being changed, four-fifths of the light reaches the screen. As this period of exposure is comparatively long, the single blade is not desirable. With a two-blade shutter that is arranged so that each blade covers the lens during one fifth of the total picture shift, twice as many impulses are obtained, but the light is reduced by one fifth more, making the screen illumination only threefifths of the maximum. The number of interruptions Fig. 11. — The Geneva Movement Connected to the Sprocket Wheel as it Appears on the Machine. given by a two-blade disc shutter are equal to those of the barrel type running at the same speed. In practice the disc shutter is usually supplied with three openings and blades, each opening being approximately one-sixth of the total area of the disc. . Fig. 9 shows the arrangement of the three bladed disc. Double discs have been used in some cases to obtain the quick the discs being revolved in opposite directions so that the opening and closing characteristics of the barrel shutter, light beam is cut in two places at the same time. This of course doubles the cutting speed and materially reduces the flicker incident to a single blade traveling at a low speed. INTERMITTENT MOVEMENTS. The intermittent motion required for shifting the film through the gate converts the continuous rotary mo Fig. 12. — Film Sprocket. tion of the crank into a series of short rectilinear movements, each of which is equal to the height of the picture on the film. There are many devices by which this result may be accomplished, but as there are only two of these movements in extended use, we will confine ourselves to a description of these types. The "Geneva movement," which is by far the most commonly used type on projectors, possesses nearly all of the desirable qualities of a film feeding mechanism. It starts the film slowly, brings it up to speed without strain, it then brings it to rest at a gradually decreasing rate. During the interval at which the film is at rest in the gate, the device holds it firmly in place without danger of slack or vibration, either of which would cause the image to flicker on the screen. The movement consists of two parts : the "star," which is fastened to the sprocket shaft, and the "pin" wheel that revolves continuously with the operating crank, the latter element being the driving member. These parts are shown in elevation by Fig. 10, in which A is the star wheel, and B is the wheel carrying the pin. When the wheel B is revolved in the direction shown by the arrow, the pin C engages with the slot F and turns the cross A through one-quarter of a revolution, the point of the cross passing through the opening G in the retaining ring D. After turning through this quarter revolution, the slot arrives at the point H and is held rigidly in position by the ring D that fits into the concave face / of the star wheel. As the wheel B continues to turn, the ring D holds the star wheel in position so that it cannot move until the pin C completes another revolution, and enters the next slot of the star wheel. In this way the star wheel makes one quarter of a revolution for every complete revolution of the pin wheel B, or one revolution for four of the wheel B. As will be seen from the figure, the starting of the movement is slow, as the pin enters the slot in a direction nearly parallel to the groove. As the pin approaches the center line of the wheels, the speed of the star wheel is increased rapidly but smoothly as the effective radius of the pin increases at the expense of that of the star wheel. From this point on, the rapidity of movement gradually decreases until the pin finally leaves the slot in a direction parallel to the edges. At this point, of course, the star comes to a stop, and the ring comes into contact with the concave face, holding it firmly in position. The shaft R connects with the sprocket wheels, and the shaft