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MANAGERS AND PROJECTIONISTS 648
lished fact that a three-winged shutter having three blades of equal width with each other, and of equal width with the (ight openings, produces the best effect in that a flickerless picture can be had at a lower speed of projection than with any other available shutter.
A shutter of this kind cannot be used except under certain circumstances which will be explained under "intermittent speed and the shutter." It has been very thoroughly demononstrated that a three-wing shutter with the flicker blades of different width than the master blade, and with the light openings of different widths, produces an excellent effect, but this unbalancing may only be done in a certain prescribed way, one feature of which is that on either side of the master blade there shall be a comparatively wide opening, and between the two flicker blades an opening of relatively narrow width.
INTERMITTENT SPEED AND ITS EFFECT ON THE SHUTTER. — To the uninitiated it may seem a very simple matter to have a 50-50 shutter, i.e., a shutter in which all blades are of equal width with each other and with the openings, but as a matter of fact, it is not. As has already been pointed out, the master blade of the shutter must be wide enough to cover about three-quarters of the lens opening, or to cut about three-quarters of the light beam when the intermittent sprocket starts to move, and still have about three-quarters of the beam covered when it comes to rest. Remembering that the shutter revolves once to every cycle of movement of the intermittent, it will be seen that the longer the time consumed by the intermittent in movement, the wider must be the master blade of the shutter.
In other words, the shorter the period of time the intermittent is in movement with relation to the time it is at rest, the less width of master shutter blade is necessary. Conversely, the slower the movement the wider the master blade must be.
The correct way of describing the speed of the intermittent movement is in degrees. The driving member (cam) of the intermittent movement is circular, and revolves continuously. Every circle is divided into 360 degrees. If the driven member (star) is engaged with the driving member (cam) and in movement during 60 degrees of the revolution of the cam, then the movement is a "60 degree movement" corresponding to a five-to-one movement, because the total cycle of movement is divided into six periods, one of which (60 de