ers. If the divisions are so fine that the coincidence is
frequently doubtful, the best plan will be for the learner to get some
acquaintance who is skilled in the use of instruments, and having set
the instrument at hazard, to write down the readings of the verniers,
and then request his friend to do the same; whenever there is any
difference, he should carefully examine the doubtful one, and ask his
friend to point out the minute peculiarities on which he founds his
decision. This should be repeated frequently; and after some practice,
he should note how many times in a hundred his reading differs from his
friend's, and also how many divisions they usually differ.

The next point is, to ascertain the precision with which the learner
can bisect an object with the wires of the telescope. This can be done
without assistance. It is not necessary even to adjust the instrument,
but merely to point it to a distant object. When it bisects any
remarkable point, read off the verniers, and write down the result; then
displace the telescope a little, and adjust it again. 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 instru