In a properly adjusted instrument, the line of sight is perpendicular to the horizontal axis.
Test : In order to test it., first the instrument is temporarily adjusted on a station and sight a point say, X (Figure 21.4) about 150 m away from the station point with telescope in the direct position. Keeping main screws of both the upper and lower plates of the instrument clamped, plunge the telescope and set another point Y in the opposite direction of the transit. Open the lower plate clamp screw and swing the instrument about the vertical axis to bring X again on the vertical cross hair. Tighten the lower plate clamp screw and plunge the telescope as before; if Y appears on the vertical cross hair, the line of sight is perpendicular to the horizontal axis.
Adjustment : If Y does not appear on the vertical cross hair, set a point Z on the line of sight of the instrument. Mark another point P, one-fourth of the distance from Y to Z, and adjust the cross-hair ring (by means of the two opposite horizontal screws) until the line of sight passes through P. Tighten the diaphragm screws, keeping the vertical hair in that position. Repeat the test and adjustment till X and Y points, before and after plunging, is in the same line of sight of the telescope. The points sighted should be at about the same elevation as the transit.