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232 RICHARDSON'S BLUEBOOK OF PROJECTION
mirror would appear thereon. The point in distance at which it appears is known as the "point of aerial image." The image of the converger may be made visible by means of a screen of dark cardboard.
Since the divergence of the light beam immediately in front of the projection lens differs for different e. f. projection lenses, (87) it is quite important that the shutter be located, as nearly as possible, at the point of aerial image. In some cases this point may be too far out to be reached because the shaft of the shutter is too short.
The exact location of the point of aerial image may be ascertained by projecting light to the screen and slowly passing an opaque object downward through the light beam at different distances from the lens, until a point is found where a shadow starts from both top and bottom of the screen at the same time, meeting in its center. (88) This is the point of aerial image, and here the master blade of the shutter may be narrower than at any other point because in passing through the beam the blade produces a dissolving effect, as is shown by the shadow at the top and bottom when the opaque object cuts across the beam. (89) If, because of the short shaft, the shutter cannot be located at the point of aerial image, the projectionist should put it where, in his judgment, the master blade will operate efficiently at minimum width, remembering that the narrower the blade the more light will pass through to the screen. Merely to locate the shutter at the point of aerial image without reducing the width of the blade is lost motion and energy.
(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