amateur observer.
Such are the Society's six gnomonic maps, the set of six gnomonic maps
in Johnstone's 'Atlas of Astronomy,' and my own set of twelve gnomonic
maps. The Society's maps are a remarkably good set, containing on the
scale of a ten-inch globe all the stars in the Catalogue of the
Astronomical Society (down to the fifth magnitude). The distortion,
however, is necessarily enormous when the celestial sphere is presented
in only six gnomonic maps. In my maps all the stars of the British
Association Catalogue down to the fifth magnitude are included on the
scale of a six-inch globe. The distortion is scarcely a fourth of that
in the Society's maps. The maps are so arranged that the relative
positions of all the stars in each hemisphere can be readily gathered
from a single view; and black duplicate-maps serve to show the
appearance of the constellations.

It is often convenient to make small maps of a part of the heavens we
may wish to study closely. My 'Handbook of the Stars' has been prepared
to aid the student in the construction of such maps.

In selecting maps it is well to be able to recognise the amount of
distortion and scale-variation. This may be done by examining the spaces
included between successive parallels and meridians, near the edges and
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