SCIENCE 
IS 
KNOWLEDGE. 
KNOWLEDGE 
IS 
POWER. 
^ J PRACTICAL JOURNAL OF 
• HOME ARTS. 
Vol. IV. 
NEW YOKK, JUNE, 1881. 
No. 6. 
Common Telescopes —11. 
HE test of excel- 
lence in separa- 
ting power has 
been fixed by 
Dawes at the qiio- 
t i e n t , expressed 
in seconds, of 4'56 
divided by aper- 
ture in inches. 
Thus a 10-inch 
object-glass or 
speculum ought 
to separate double 
stars at 0"-456 of 
distance between their centres. This 
value practically concurs with those 
given by Dallmeyer and Alvan Clark. 
Keflectors somewhat surpass achromatics 
in this respect, as theoretically they ought 
to do ; but they are apt to be more troubled 
by rings and flare, and scattered light. 
The best telescopes of either kind will 
bear a power of 100 per inch of aperture 
on stars : for planets, or the moon, half 
that i:>ower will usually more than suffice. 
An object-glass of inferior definition 
may sometimes be improved by stopping 
oul defects, or contracting the aperture. 
Streaks or specks of unequal density are 
very injurious ; they may be detected by 
turning the telescope to a bright light, 
taking out the eye-piece, and placing the 
eye in the focus ; every irregularity will 
then be visible in the illumination which 
overspreads the object-glass; and, if of 
small extent, may be stopped out by a bit 
of sticking-plaster. If the performance 
is not thus improved, try a contracted 
aperture : make a cap of pasteboard fit- 
ting over the object-glass like the usual 
brass cap, but with a circular opening a 
little less than the clear aperture — if the 
indistinctness is thus diminished, but not 
removed, try several discs of pasteboard 
placed successively within this cap with 
progressively contracted openings, till 
distinct vision is obtained ; there we must 
stop, or valuable light will be^ lost. An 
eccentric opening in the pasteboard disc 
may sometimes be serviceable, being 
turned round the axis so as to expose dif- 
ferent parts of the glass or mirror, till the 
best effect is produced ; in other cases, a 
central pasteboard disc, supported by nar- 
