be completely ruined.

The observer should not leave to the precious hours of the night the
study of the bearing and position of the objects he proposes to examine.
This should be done by day--an arrangement which has a twofold
advantage,--the time available for observation is lengthened, and the
eyes are spared sudden changes from darkness to light, and _vice versâ_.
Besides, the eye is ill-fitted to examine difficult objects, after
searching by candle-light amongst the minute details recorded in maps or
globes. Of the effect of rest to the eye we have an instance in Sir J.
Herschel's rediscovery of the satellites of Uranus, which he effected
after keeping his eyes in darkness for a quarter of an hour. Kitchener,
indeed, goes so far as to recommend (with a _crede experto_) an
_interval of sleep_ in the darkness of the observing-room before
commencing operations. I have never tried the experiment, but I should
expect it to have a bad rather than a good effect on the eyesight, as
one commonly sees the eyes of a person who has been sleeping in his
day-clothes look heavy and bloodshot.

The object or the part of an object to be observed should be brought as
nearly as possible to the centre of the field of view. When there is no
apparatus for keeping the telescope pointed upon an object, the best
plan is so to direct the telescope by means of the finder, that the
object shall be just out of the field of view, and be brought (by the
earth's motion) across the centre of the field. Thus the vibrations
which always follow the adjustment of the tube will have subsided before
the object appears. The object should then be intently watched during
the whole interval of its passage across the field of view.

It is important that the student should recognise the fact that the