______ test is conducted for checking the adjustment of a level.

Explanation:

Two peg test:

(i) This method is either for an optical or digital level or a transit being used as a level. If this error is corrected with transit, it also improves the accuracy of its vertical angle readings.

(ii) It is performed to ensure that line of collimation of the telescope is parallel to the bubble tube axis.

Principle:

(i) The basic principle is that since the error in level readings results from the instrument not sighting exactly horizontally, is thus looking up or down at some angle and that this angle is the same whichever way it is sighting: if you place the instrument exactly midway between two-rod sightings, the vertical error reading on the rod is the same for each, thus the difference in reading between the rods will still give you an accurate elevation difference.