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616 HANDBOOK OF PROJECTION FOR
grees) represents the time during which the star and intermittent sprocket will be in motion. A true 60 degree movement should allow us to have a 50-50 three-wing shutter because there are three wings and three light openings, or in other words, 6 divisions, and since the time of movement itself is equal to one of the six periods, the shutter blades and the shutter openings may all be equal with each other, always provided there be no lost motion in the mechanism.
NOTE: This is not strictly correct except in theory because of the fact that the lens has considerable diameter the greater part of which must be covered at opening and closing.
In considering the speed of the intermittent we have only to determine whether it is a "six-to-one," a "five-to-one" or whatever it may be, and then divide 360 by the number of cycles in the movement (a five-to-one is, for this purpose, a six-cycle movement, a four-to-one a five-cycle movement and so forth) in order to reduce the matter to degrees. For instance, with a four-to-one we have 4-f-l=5, and 360-^-5=72, therefore the four-to-one is a 72-degree movement. With such a movement the intermittent would be in action 72 degrees of the entire cycle, and the master blade of the revolving shutter would have to be 72 degrees wide if the light beam had no diameter and if there was no lost motion in the gearing between the intermittent and the shutter. As a matter of fact, however, a sufficient width would have to be added to the master blade to cover about three-quarters of the diameter of the light beam, and to allow for lost motion, so that a four-toone movement would mean a very bad optically balanced revolving shutter, which would set up tendency to flicker at low speeds, and cut off a very great percentage of the light. The width of such a blade is easily calculated, as follows: Measure distance center of shutter shaft to center of diameter of projection lens. Multiply this measurement by two. Multiply that result by 3.1416 and divide that result by five, or the total number of cycles in the movement, meaning that if it be a six-to-one movement, then the divisor would be seven. The result will be the necessary width of master blade in inches at center of light beam, measured on the arc of a circle, if the light beam had no diameter.
It must be remembered that whereas we may not hope to have a revolving shutter cut much less than fifty per cent, of the light, still we may have it cut decidedly more than fifty per cent., and the nearer we can approach to the best