the 9th magnitude, a larger aperture is required to show the 8th
magnitude component of [sigma] Cassiopeiæ, on account of the greater
closeness of this double.

In favourable weather the following is a good general test of the
performance of a telescope:--A star of the 3rd or 4th magnitude at a
considerable elevation above the horizon should exhibit a small well
defined disc, surrounded by two or three fine rings of light.

A telescope should not be mounted within doors, if it can be
conveniently erected on solid ground, as every movement in the house
will cause the instrument to vibrate unpleasantly. Further, if the
telescope is placed in a warm room, currents of cold air from without
will render observed objects hazy and indistinct. In fact, Sir W.
Herschel considered that a telescope should not even be erected near a
house or elevation of any kind round which currents of air are likely to
be produced. 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