LINE OF, the line of sight in astronomical and geodetical instruments, as mural circles, transit instruments, sextants, quadrants, theodolites, &c. Thus, in a telescope, the line of collimation is the straight line passing through the centre of the object-glass, at the point where the fine wires or spider-webs intersect each other in its focus. The term is derived from the Latin collimo, to aim at.

Collimation, Error of, the deviation of the actual line of sight, in a telescope, from the centre of the object glass, where it ought to be perpendicular to the horizontal axis. The amount of this deviation must be precisely ascertained and corrected, or allowed for, in order to insure accuracy in the result of the observations.

Among other contrivances for this purpose, the collimator of Captain Kater is used for determining the error of collimation in any principal instrument, by which the necessity of the reversal of the instrument itself is obviated.