is on the drum. Then with the 42.50 D. calibration ball on the unit, focus and align the mires properly on it. Holding the drum in this position (do not let it rotate), loosen the second set screw. The measuring drum will now rotate on the axis without the mires images changing. Rotate the drum so that it reads 42.50 D. Look through the eyepiece and be sure the mires are still properly aligned. If they are, tighten the set screws being careful not to rotate the drum. Once tightened, recheck to see that the mires are still properly aligned and the drum reads 42.50 D.
Repeat for the vertical meridian if it did not give the proper reading on the calibration ball.