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THE PROJECTOR 315
jecting white light to the screen and passing a sheet of opaque materia], preferably metal, up or down through
the light ^beam at different distances from the lens until a point is found where two shadows appear simultaneously at top and bottom of screen, meeting at its center. (8>7) It is important that the shutter be located at this point, both to obtain maximum screen illumination, and for (88) the dissolving effect produced. 1 lowever (89) this latter effect will be the only gain unless the shutter blade width be reduced to the minimum width that will not produce travel ghost.
If the shutter shaft be too short to permit locating the shutter at the point of aerial image, then locate it as near there as possible and reduce blade width as above indicated.
'(90) The projectionist may test for the width of his shutter blade in the following manner : remove the shutter from the shaft and the blade from its hub. Lay the blade on a piece of thin, stiff cardboard about 12 inches square and with a pencil trace the outline of the blade. Cut the cardboard pattern out and clamp it into the shutter hub, just as if it were the .metal blade, and put the shutter back on the projector in its proper place. Don't bother to connect the outer edges of the paper blades; if the cardboard is strong it will run perfectly for several days without such connection.
Having set the cardboard blade properly, project a black and white title upon the screen and continue to trim off small but equal amounts from each edge of the master blade, after each trial projection, until a faint travel ghost begins to creep up and down upon the screen. Now the blade is just a bit too narrow on both sides. Remove it and trim the metal master blade to almost the same proportions, but a little wider to kill all traces of the travel ghost. Additional width, to be determined by the projectionist, must be provided or travel ghost will appear as soon as the mechanism gears and bearings begin to wear. (91) Trim the other blade to the same width as the master blade, always taking off an equal amount from each side. The shutter is now