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214 RICHARDSON'S BLUEBOOK OF PROJECTION
Provision must also be made for side movement of the lamphouse.
(28) The optical axis of each element of the optical train must be centered exactly upon the optical axis of the system considered as a unit. The plane of each element must be perpendicular (at right angles) to the optical axis of the train. This requires means for adjusting the condenser or mirror up, down and sidewise.
In a perfect alignment, a thread stretched tightly from the exact center of the condenser or mirror to the exact center of the front end of projection lens, would be exactly in the center of the rear end of the projection lens and the aperture. If the elements of the optical train are not in alignment then a portion of the light never reaches the screen. Sometimes, too, the bases on which the projector mechanism and the lamphouse rest are not not in proper alignment, affecting the alignment of the lamphouse optical system and of the projector mechanism. These are difficult things to check unless the proper tool is available. Fortunately the SMPE Projection Practice Committee recently developed an instrument for lining up the whole projector optical train. It is inexpensive and every projectionist should secure it, either personally or through his theatre management.
The Intermittent Movement
(29) The intermittent movement and the rotating shutter are so locked together by a train of gears that the shutter must rotate exactly once each time the intermittent movement acts, and by so doing moves the intermittent sprocket precisely one-fourh of a revolution.
(30) This cycle of motion displaces the film photograph from the aperture and substitutes the next succeeding one, the whole cycle lasting one twenty-fourth of a second. (31) In other words, it occurs twentyfour times per second, during which each one of twentyfour photographs is successively projected to the screen. (32) This accuracy of action makes certain that each photograph or "frame" will remain perfectly still and