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MANAGERS AND PROJECTIONISTS 652
projection lens will depend upon the distance of the condenser from the projection lens and the focal length of the projection lens itself. As a general proposition the revolving shutter should be set at the point of the aerial image. This does not, however, always hold good.
TO FIND THE IMAGE.— The location of the proper shutter position may be found in three different ways, as follows: (a) place a metal plate, in the center of which is a hole about one-quarter of an inch in diameter, over the face of the converging lens and project the white light to the screen. Blow smoke in front of the projection lens, and you will see that the resultant beam gradually narrows down and then spreads out again. The correct shutter position is at the narrowest point. (b) Project the white light to the screen and slowly pass some opaque object down through the light beam at varying distances in front of the lens. You will find a point at which either two shadows appear simultaneously on the screen, one at the bottom and one at the top, or else the whole screen "dissolves" into darkness. In the first case the point at which the shadows meet exactly in the center of the screen is the correct shutter position. In the second case the point at which the dissolving effect is most perfect is the right place, (c) Project the white light to the screen and hold a piece of black paper or some very dark colored, non-gloss object in the light beam in front of the projection lens, moving it slowly away from the lens until a sharp image of the converging lens of the condenser appears thereon. This test may be made more accurate by first printing some word on the face of the converging lens, using ordinary ink, and then focusing the printing.
The last method locates the aerial image and you can see at a glance whether it is advisable to set the shutter at that point or not, because you can measure the diameter of the beam at that point. If the beam at that point is either more narrow, or even as narrow as it is at any other point, then that is the place for the shutter, but if at a point nearer the lens the beam is more narrow (as is the case under some conditions) then the shutter should be set at the most narrow point of the beam.
DISSOLVING EFFECT.— The reason we say the revolving shutter should be set at the aerial image unless the beam be actually of greater diameter at the image than it is nearer the lens, is because of the fact that at the aerial image there