ADJUSTMENT OF THE DUMPY LEVEL

               The temporary adjustments have been made at every instrument setting and preparatory to taking observations with the instrument.

The following steps have been followed:-

1.      Setting up the level: - This operation includes the fixing the instrument on the tripod and levelling the instrument approximately by the leg adjustment.

2.      Levelling up: - In this step, accurate levelling has been done with the help of foot screws and with reference to the plate levels. The purpose is to make the vertical axis truly vertical or perpendicular to the line of sight. The manner of levelling the instrument with three levelling screw are following: -

a)      Loose the clamp. Turn the instrument until the longitudinal axis of the plate level is roughly parallel to a line joining any two of the levelling screws.

b)      Hold these two levelling screws (‘A’ and ‘B’) between the thumb and first finger of each hand and turn them simultaneously so that the thumbs move either towards each other or away from each other until the bubble comes in central area. (The bubble will move in the direction of movement of the left thumb.)

c)     Turn the instrument by 90^o and then turn the third levelling screw ‘C’. The third levelling screw ‘C’ should be turn in such a way that the bubble comes in the centre.

d)     Turn the instrument again and repeat the above procedure. The instrument must be level in any direction means the bubble must be in centre. 

Fig. 2.6 Levelling-up with three foot screws

      3. Elimination of parallax: - Parallax is a condition arising when the image formed by the objective is              not in the plane of the cross hairs. It has been done by rotating the eye-piece for clear cross hairs and            by rotating focusing screw provided for objective focus.