They must be able to tell what effect the adjustment of one part will have on other parts of the instrument. They must understand the effect of each adjustment upon the instrument when it is actually used for measurement. An optical instrument equipped with vertical and horizontal cross hairs is out of adjustment if the point of intersection between the cross hairs does not coincide with the optical axis. If the reflected bubble on a Locke or Abney level is centered when the optical axis is other than horizontal, the instrument is out of adjustment. The process of adjustment chiefly involves the steps that are necessary to bring a bubble to center when it should be at center or to bring a cross-hair point of intersection into coincidence with the optical axis.
Instrument manufacturers publish handbooks containing recommended adjustment procedures.
These are usually small pamphlets, obtainable free of charge. The following discussion is intended to give you an idea of general instrument adjustment procedures. For adjusting your particular instruments, however, you should follow the appropriate manufacturer s instructions. General Adjustment Procedures Instruments should be carefully checked periodically to determine whether or not they need adjustment. There is an adage that an instrument should be checked frequently but adjusted rarely.