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374 RICHARDSON'S BLUEBOOK OF PROJECTION
the right distance from the film plane at the projector aperture — not from the cooling plate, but from the film itself.
Even so small an error as the fraction of a millimeter in the distance of the light source from the mirror may reduce the screen illumination, though a small error in the distance of the mirror from the film plane will do no serious harm. It is best to have all measurements exactly right. The lamp manufacturer will supply the exactly correct distances.
To obtain the right distance follow these simple instructions : First, before inserting the carbons, cut a stiff wire exactly the length of the distance the lamp manufacturer recommends between mirror and film plane. Stretch this wire through the carbon clamp openings and with it locate the mirror at exactly the right distance— which will be the length of the wire from the film plane to the surface of the mirror beside the hole in its center, or to the center of the mirror if there is no hole.
Next, cut small copper wires, say about a No. 10, as long as the distance that is recommended between the light source to mirror Install two carbons and burn them until a normal crater floor has been established. Then, after the parts have cooled down, locate the crater where the mirror surface, about 2l/2 " from its center, just contacts the end of the wire all the way around when the other wire end is in contact with the crater center.
By this method the mirror will be very close to correct position, though not exactly. For one thing, the center of the crater floor being the carbon core, it is likely to be depressed below the rest of the crater floor ; also the wire end may not be precisely at the crater center.
Next, having set the projector rotating shutter at open position and blocked up the automatic fire shutter, project the white light to the screen (the projector not running), and move the mirror very slightly toward and a'way from the light source, watch the results upon the screen closely. Make all possible experimental mirror adjustments very slowly and cautiously, until the point