288 
POPULAR SCIENCE REVIEW. 
The middle rays may, in like manner, be cut off by means of 
a circular piece of card covering the middle of the object-glass, 
and the focal length for distinct vision compared with the focal 
length when the cap is applied. The extent of the spherical 
aberration can thus be determined ; but if the first experi- 
ment gives a satisfactory result, no other is required. 
A star of the first magnitude should next be brought into 
the field of view. If an irradiation from one side is perceived, 
part of the object-glass has a different refractive power than 
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 ; the latter 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 be in a right line, and it will be 
possible to see 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 illuminating and 
defining power. Proper objects of this kind for testing 
telescopes of different sizes will be mentioned in another 
paper.* 
From what has been said of the construction of the astro- 
nomical telescope the following practical rules may be gathered. 
In observing the moon, Yenus, Jupiter, or Mars, with low powers, 
the marginal parts of the object-glass should be covered, or if 
in testing the object-glass any part has been found to be less 
perfect than the rest, that part should be covered. Enough light 
* In favourable weather the following is a good general test of the perform- 
ance 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, sur- 
rounded by two or three fine rings of light. 
