by Bob Thrun » Apr 6, 2009 9:48 pm
The hairline and the dial markings are not the same distance from the user's eye. The purpose of the slit is to put the user's eye, the hairline, and the dial markings all near the centerline of the instrument. You can turn the hairline horizontally, but you should be careful to take your readings when the hairline is in the center of the eyepiece. Do not let the hairline wander near the edge of the field of view.