A series of such observations will show the confidence which is due to the observer's eye in bisecting an object, and also in reading the verniers; and as the first direction gave him some measure of the latter, he may, in a great measure, appreciate his skill in the former.

He should also, when he finds a deviation in the reading, return to the telescope, and satisfy himself if he has made the bisection as complete as he can.

In general, the student should practise each adjustment separately, and write down the results wherever he can measure its deviations.
Having thus practised the adjustments, the next step is to make an observation; but in order to try both himself and the instrument, let him take the altitude of some fixed object, a terrestrial one, and having registered the result, let him derange the adjustment, and repeat the process fifty or a hundred times.

This will not merely afford him excellent practice, but enable him to judge of his own skill.
The first step in the use of every instrument, is to find the limits within which its employer can measure the SAME OBJECT UNDER THE SAME CIRCUMSTANCES.

It is only from a knowledge of this, that he can have confidence in his measures of the SAME OBJECT UNDER DIFFERENT CIRCUMSTANCES, and after that, of DIFFERENT OBJECTS UNDER DIFFERENT CIRCUMSTANCES.
These principles are applicable to almost all instruments.