This adjustment includes horizontal hair adjustment and vertical hair adjustment. The purpose of adjusting horizontal hairs is to bring the horizontal hairs of the eyepiece into the horizontal plane through the optical axis. If the horizontal hairs do not lie in the horizontal plane through the optical axis, the vertical angles will be distorted. This is especially required when the instrument is used in leveling operations. It has no effect in measuring horizontal angles.
The purpose of adjusting the vertical hairs is to align the line of collimation to the horizontal axis. This adjustment is necessary for measuring horizontal angles between points at different elevations and for extending lines by making observations on only one face.
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