angles of the maps, and comparing these either with those in the centre
of the map, or with the known figures and dimensions of the
corresponding spaces on a globe.

We may now proceed to discuss the different tests which the intending
purchaser of a telescope should apply to the instrument.

The excellence of an object-glass can be satisfactorily determined only
by testing the performance of the telescope in the manner presently to
be described. But it is well to examine the quality of the glass as
respects transparency and uniformity of texture. Bubbles, scratches, and
other such defects, are not very important, since they do not affect the
distinctness of the field as they would in a Galilean Telescope,--a
little light is lost, and that is all. The same remark applies to dust
upon the glass. The glass should be kept as free as possible from dirt,
damp, or dust, but it is not advisable to remove every speck which,
despite such precaution, may accidentally fall upon the object-glass.
When it becomes necessary to clean the glass, it is to be noted that the
substance used should be soft, perfectly dry, and free from dust. Silk
is often recommended, but some 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.