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634 ARTHUR J. HOLMAN [j. s. M. p. E.
on an annular boss projecting from the rear telescope tube bearing housing and does not rotate with the telescope tube. The whole device is mounted in a manner whereby the axis of the telescope tube may be oriented in both the horizontal and vertical planes by means of tangent screws for sighting on a target.
The lens sectors, which are plano-convex, have a flat surface ground on the spherical side parallel to the piano side where they are to be clamped in the lens wheel and paper washers are inserted between the glass and the metallic lens wheel flanges.
The method of assembling the lens sectors on the lens wheel is as follows. The lens wheel hub complete with flange plates is mounted on the index ring shaft, turned to the position where the line joining the axis of the telescope tube and the axis of the index wheel bisects a flange sector, and Iocke4 in position by the wing nut on the index ring shaft. A lens sector with paper washers attached is placed against the solid flange of the lens wheel and the flange sector bolt is tightened sufficiently to hold the lens sector in position. The lens sector is adjusted till its straight edges are radial and its optical center is positioned approximately on the axis of the telescope tube. This approximate positioning is made merely by eye. The lens setter is then sighted at a distant target and adjusted by means of the tangent screws till the image of the center of the target appears at the intersection of the cross hairs. The motor is then started and as the telescope tube slowly rotates, the movement of the image of the target with respect to the cross hairs is noted. The motor is stopped when the lens wheel is 180 degrees from the position it occupied when the device was originally sighted at the target. The displacement of the image of the center of the target from the intersection of the cross hairs represents double the actual displacement of the optical center of the lens sector from the axis of the telescope tube and the device is realigned by means of the tangent screws, till the image of the center of the target moves exactly half way back to the intersection of the cross hairs. The axis of the telescope tube is then correctly aligned with respect to the center of the target and the lens sector is shifted in its clamp till the image of the center of the target falls at the intersection of the cross hairs. The flange sector bolt is then set up and the motor is again operated to check the accuracy of the setting. If the image of the target does not move with respect to the cross hairs as the telescope tube rotates, the lens sector is correctly positioned. The lens wheel is then moved to No. 2 lens sector position by