If a telescope is used in a room, the temperature of the room should be made as nearly equal as possible to that of the outer air.
When a telescope is used out of doors a 'dew-cap,' that is, a tube of tin or pasteboard, some ten or twelve inches long, should be placed on the end of the instrument, so as to project beyond the object-glass.

For glass is a good radiator of heat, so that dew falls heavily upon it, unless the radiation is in some way checked.

The dew-cap does this effectually.

It should be blackened within, especially if made of metal.
"After use," says old Kitchener, "the telescope should be kept in a warm place long enough for any moisture on the object-glass to evaporate." If damp gets between the glasses it produces a fog (which opticians call a sweat) or even a seaweed-like vegetation, by which a valuable glass may 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 versa_.
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.