silk is exceedingly objectionable in
texture,--old silk, perfectly soft to the touch, is perhaps as good as
anything. If the dust which has fallen on the glass is at all gritty,
the glass will suffer by the method of cleaning commonly adopted, in
which the dust is _gathered up_ by pressure. The proper method is to
clean a small space near the edge of the glass, and to _sweep_ from that
space as centre. In this way the dust is _pushed before_ the silk or
wash-leather, and does not cut the glass. It is well always to suspect
the presence of gritty dust, and adopt this cautious method of cleaning.

The two glasses should on no account be separated.

In examining an eye-piece, the quality of the glass should be noted, and
care taken that both glasses (but especially the field-glass) are free
from the least speck, scratch, or blemish of any kind, for these defects
will be exhibited in a magnified state in the field of view. Hence the
eye-pieces require to be as carefully preserved from damp and dust as
the object-glass, and to be more frequently cleaned.

The tube of the telescope should be light, but strong, and free from
vibration. Its quality in the last respect can be tested by lightly
striking it when mounted; the sound given out should be dead or
non-resonant. The inside of the tube must absorb extraneous light, and
should therefore be coloured a dull black; and stops of varying radius
should be placed along its length with the same object. Sliding tubes,
rack-work, etc., should work closely, yet easily.

The telescope should be well balanced for vision with the small
astronomical eye-pieces. But as there is often occasion to use
appliances which disturb the balance, it is well to have the means of at
once restoring equilibrium. A cord ring running round the tube (pretty
tightly, so as to rest still when the tube is inclined), and bearing a
small weight, will be all that is required for this purpose; it must be
slipped along the tube until the tube is found to be perfectly balanced.
Nothing is more annoying than, after getting a star well in the field,
to see the tube shift its position through defective balance, and thus
to have to search again for the star. Even with such an arrangement as
is shown in fig. 8, though the tube cannot readily shift its position,
it is better to have it well balanced.

The quality of the stand has a very important influence on the
performance of a telescope. In fact, a moderately go