Transactions of the Society of Motion Picture Engineers (1919)

remain stationary during the other three-quarters of the cam revolution. This is called a 90-degree cam action. Place a sprocket on the shaft of the star, with a diameter and teeth to fit four successive pictures to its circumference and you have the common ''three-to-one" projector intermittent movement. As the movement of the film must be masked by the shutter, it is evident that, if the film could be moved more quickly, its stationary image could be correspondingly prolonged on the screen, thus increasing the proportion of the illumination and enhancing the realism of the picture. Obviously, this can be accomplished by increasing the diameter of the cam and thus decreasing the number of degrees during which the pin is turning the star. Fig. 2. This is shown in Fig. 2 which illustrates one form of a "fiveto-one" intermittent movement with a 60° cam action. But note that the pin no longer enters the star slot on a tangent to its travel. Instead, it strikes the side of the slot violently, at an angle of about 15°, which starts the star (and with it the sprocket and film) with a jerk, destructive alike to the mechanism of the machine and the perforations of the film. Such an intermittent movement is subject to rapid wear, besides wearing out the film quickly. Now, suppose that instead of making the slots in the star radial, they be inclined obliquely in such a manner that the axis of the slot coincides with the tangent to the course of the cam pin at the instant of its entrance. Fig. 3 illustrates such an eccentric star intermittent movement having a cam action of 60°, or another form of a "five-to-one" movement. 71