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230 RICHARDSON'S BLUEBOOK OF PROJECTION
(78) All modern shutters have two blades of equal width — hence either may be used as a master blade.
All rear shutters cause movement of the air around them. (79) In the Simplex disc type shutter, one side of each blade has a vane fixed at an angle to the line of rotation. This vane is approximately half an inch wide by about four inches long. When the blade is rotated at high speed the vane forces a strong current of air outward (toward the lamphouse), dispersing a considerable portion of the heat from the light source; the vane also draws air into and around the mechanism and helps to keep both the mechanism and the film at low temperature, without causing any great amount of dust to circulate from deposits at the aperture. All shutter blades are adjustable for width.
The Horizontal, Cylindrical Type Shutter
(80) This type of shutter is radically different in design and operation from the disc type. Essentially it is a hollow cylinder of metal, rotating in an enclosed housing.
(81) The Motiograph horizontal shutter is a light aluminum casting rotating on ball bearings and enclosed in a small metal housing. The shutter is positioned transversely (horizontally). In its cylindric surface are two openings through which the light beam passes; the rest of the cylinder serves exactly the same purpose as the blades of the disc shutter. Its cylindrical diameter is 3.25 inches. Its axis is 4 inches from the projector aperture.
(82) What is true of the speed and width of the disc type master blade edges does not apply to this shutter, and for two reasons : it is located closer to the projector aperture and consequently has a smaller beam diameter to cut; the shutter is positioned horizontally, or transversely of the beam, the light passing through its center or cylinder so that when one blade is cutting down through the beam the other is cutting upward, the two meeting at the beam center. Only half the time is required to cut off the light that would be necessary