Instrumental Errors

(a) The focusing tube might be faulty causing some tilting in line of sight although focusing.

(b) The bubble might be sluggish or insensitive. It can remain in central position even when bubble tube is not horizontal.

(c) More common and serious instrumental error is maladjustment of level. The bubble tube line and collimation line do not remain parallel. Even while the bubble tube is horizontal, the collimation line might remain inclined.

(d)  The staff graduations might not be accurate giving wrong results.