With the line of sight out of adjustment by a given amount, the effect of the error depends on the vertical angle to the point sighted. For all ordinary cases, the error (E) in direction of an inclined sight is given by
E = e sec q (approx.)
(Figure 22.4) where e is the error in direction for a horizontal sight, where q is the observed vertical angle. The maximum error in horizontal angles due to non adjustment of the line of sight is introduced when the telescope is plunged between backsight and foresight readings and usually appears in the measurement of a deflection angle.
