Richardson's handbook of projection (1930)

MANAGERS AND PROJECTIONISTS 1267 Another feature of this assembly is that, not only will the lens always be exactly the correct distance from the film when inserted, but also it will always be in precisely the same position rotationally. This is provided for very simply. Examining Fig. 450 we see that the ring (F in Fig. 450) we have been talking about has a hole drilled through it at G. Turning to Fig. 449 we see rod M, the front end of which passes through this hole in the ring. This may also be examined in Fig. 447, where the end of the shaft is seen at M. Considering this matter, you will see that since shaft M must pass through the hole in the lens ring, the lens barrel, to which the ring is clamped, will always, perforce, be in precisely the same rotational position, and since the lens barrel is shoved back until the ring encounters the lens clamp, the lens must be always the same distance from the film, with which double check focus cannot possibly be in any degree interfered with by the removal of one lens and the insertion of another, provided the ring has been properly installed on both lenses, and that the lens barrel be shoved in as far as it can go. NOTE. — There is considerable repetition in the foregoing, but I thought it well to set the matter forth in more than one way. I might add that in case shaft F, Fig. 449, does not project far enough to enter the lens ring, you have but to loosen screws O, Fig. 449, and shove the shaft ahead, being certain, however, to retighten screws O. Having previously set ring L, Fig. 449, correctly, to change lenses you have only to loosen thumbscrews F and J, Fig. 449, which releases the lens barrel from its