ower as the rest; and the
part which is defective can be determined by applying in different
positions a cap which hides half the object-glass. If the irradiation is
double, it will probably be found that the object-glass has been too
tightly screwed, and the defect will disappear when the glass is freed
from such undue pressure.

If the object-glass is not quite at right angles to the axis of the
tube, or if the eye-tube is at all inclined, a like irradiation will
appear when a bright star is in the field. The former defect is not
easily detected or remedied; nor is it commonly met with in the work of
a careful optician. The latter defect may be detected by cutting out
three circular cards of suitable size with a small aperture at the
centre of each, and inserting one at each end of the eye-tube, and one
over the object-glass. If the tube is rightly placed the apertures will
of course lie in a right line, so that it will be possible to look
through all three at once. If not, it will be easy to determine towards
what part of the object-glass the eye-tube is directed, and to correct
the position of the tube accordingly.

The best tests for determining the defining power of a telescope are
close double or multiple stars, the components of which are not very
unequal. The illuminating power should be tested by directing the
telescope towards double or multiple stars having one or more minute
components. Many of the nebulae serve as tests both for illumination and
defining power. As we proceed we shall meet with proper objects for
testing different telescopes. For the present, let the following list
suffice. It is selected from Admiral Smyth's tests, obtained by
diminishing the aperture of a 6-in. telescope having a focal length of
8-1/2 feet:

A two-inch aperture, with powers of from 60 to 100, should exhibit

[alpha] Piscium (3".5). | [delta] Cassiopeiae (9".5),
| mag. (4 and 7-1/2)
[gamma] Leonis (3".2). | Polaris (18".6), mag. (2-1/2
| and 9-1/2)

A four-inch, powers 80 to 120, should exhibit

[xi] Ursae Majoris (2".4). | [sigma] Cassiopeiae (3".1),
| mag. (6 and 8).
[gamma] Ceti (2".6). | [delta] Geminorum (7".1),
| mag. (4 and 9).

The tests in the first column are for definition, those in the second
for illumination. It will be noticed that, though in the case of Polaris
the smaller a