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June, 1930] LENS WHEELS FOR PROJECTORS 635
means of the index ring, the index stop pins are inserted and No. 2 lens sector is loosely clamped in position by eye. Since the telescope tube is correctly aligned it is only necessary to shift the lens sector in the clamp till the image of the target is centered on the cross hairs and set up the flange sector bolt. The motor is operated again to check the setting and if no movement of the image of the target with respect to the cross hairs is visible this sector is correctly positioned. In like manner the remaining sectors are positioned, and, as a final check, No. 1 lens sector is indexed into position on the telescope tube again and checked to see that there has been no shifting of the lens wheel hub with respect to the index ring during the above operations. If No. 1 sector, on being rotated, shows no movement of the image at the cross hairs, the lens sectors are all correctly positioned and the lens wheel is removed from the lens setter.
With a little practice in using the instrument illustrated, a 16 sector lens wheel can be accurately assembled and checked in a couple of hours, and, when the operation is completed, the optical center of each lens sector will be less than 0.001 in. from its theoretically correct position. This accuracy is easily secured because the apparent movement is proportional to twice the actual displacement. In ten years' experience with lens wheels thus constructed I have never found it necessary to reposition a lens sector and therefore it is not extravagant to state that once a lens wheel is assembled its accuracy is established and it will not change while in service. Barring accidental breakage, a lens wheel is good indefinitely.
Although much time, money, and thought have been required for the development of the apparatus described, the solution of the three critical problems in lens wheel manufacture has worked out very simply and the desired accuracy can be built into these essential parts of this type projector at a very moderate cost. Hence the greatest barrier to the introduction and commercial manufacture of this projector has been successfully removed.