Condition- To make the horizontal axis perpendicular to vertical axis.

Necessity- By mean of the 2nd and 3rd adjustment, we ensure that line of sight will revolve in vertical plane. The adjustment become essential in all work necessitating motion of the telescope in altitude.

Spire test-

1. Set up the instrument over high building or other object on which there is defined point at a considerable altitude such as flag pole , lighting etc.
2. And mark a well defines point A at a considerable height.
3. Level the instrument accurately thus making vertical axis truly vertical.
4. Sight the point as shown in fig and horizontal motion clamped depress the telescope and set a point P on or near the ground.
5. Unclamped and transit the telescope and swing through 1800 with telescope inverted again sight on P.
6. Depress the telescope as before, if the light of sight fall on P, the horizontal axis is perpendicular to vertical axis.

Adjustment

1. It not marks another point Q in the line of sight on the wall at the same level as P.
2. Mark point R midway between P and Q sight on point R.
3. Clamp the upper motion.
4. Raise the telescope.
5. The line of sight will now strike the point A.