THE YOUNG SCIENTIST. 
63 
despised ; they will show something never 
seen without them. I have a little hand 
telescope, 221 inches long w^hen fully 
drawn out, with an object-glass of about 
14 inches focus, and 1 1-3 inch aperture : 
this, with an astronomical eye-piece, will 
show the existence of the solar spots, the 
mountains in the Moon, Jupiter's satel- 
lites, and Saturn's ring. Achromatics of 
larger dimensions have become much less 
expensive than formerly, and silvered 
specula of very considerable size are now 
comparatively common ; even for these it 
is hoped that this treatise, embodying 
some of the results of the finest instru- 
ments, may not be found an inadequate 
companion as far as it goes. 
In judging of a telescope, we must not 
be led by appearances. Inferior articles 
may be showily got up, and the outside 
must go for nothing. Nor is the clearness 
of the glass, or the polish of the mirror, 
any sign of excellence : these may exist 
with bad figure " {i.e., irregular cur- 
vature), or bad combination of curves, 
and the inevitable consequence, bad per- 
formance. We need not regard bubbles, 
sand-holes, scratches, in object-glass or 
speculum; they merely obstruct a very 
little light. Actual performance is the 
only adequate test. The image should be 
neat and well defined with the highest 
power, and should come in and out of 
focus sharply ; that is, become indistinct 
by a very slight motion on either side of 
it. A proper test-object must be chosen ; 
the Moon is too easy ; Venus too severe, 
except for first-rate glasses; large stars 
have too much glare ; Jupiter or Saturn 
are far better ; a close double star is best 
of all for an experienced eye; but for 
general purposes a moderate-sized star 
will suffice ; its image, in focus, with the 
highest power, should be a very small 
disc, almost a point, accurately round, 
without "wings,' or rays, or mistiness, or 
false images, or appendages, except one 
or two narrow rings of light, regularly 
circular, and concentric with the image ;* 
and in an uniformly dark field ; a slight 
The real diameter of a star in the telescope 
■would be inconceivably small. The apparent or 
"spurious" disc, and rings, result from the un- 
dulatory nature of light. They seem, _ however, 
to be somewhat affected by atmospheric causes. 
Herschel II. speaks of nights of extraordinary 
displacement of the focus either way 
should enlarge the disc into a luminous 
circle. If this circle is irregular in out- 
line, or much brighter or fainter towards 
the centre,* or much better defined on 
one side of the focus than the other, the 
telescope may be serviceable, but is not 
of high excellence. The chances are 
many, however, against any given night 
being fine enough for such a purpose, and 
a fair judgment may be made by day from 
the figures on a watch-face, or a minute 
white circle on a black ground, or the 
image of the sun on a thermometer bulb, 
placed as far off as possible. An achrom- 
atic, notwithstanding the derivation of its 
name, will show color under high powers 
where there is much contrast of light and 
darkness. This " outstanding " or uncor- 
rected color results from the want of a per- 
fect balance between the optical proper- 
ties of the two kinds of glass of which the 
object-glass is constructed ; it cannot be 
entirely remedied, but it ought not to be 
obtrusive. In the best instruments it 
forms a fringe of violet or blue round 
luminous objects in focus under high 
powers, especially Venus in a dark sky: 
A red or yellow border would be bad ; but 
before condemning an instrument from 
such a cause, several eye-pieces should be 
tried, as the fault might lie there, and be 
easily and cheaply remedied. Reflectors 
are delightfully exempt from this defect; 
and as now made, with specula of silvered 
glass, well deserve, from their compara- 
tive cheapness, combined with admirable 
defining i^ower, to regain much of the 
preference which has of late years been 
accorded to achromatics. The horizontal 
view of objects at all altitudes in a New- 
tonian reflector with rotating tube is ex- 
tremely pleasant, when a little experience 
has been gained in finding and following; 
the same advantage, however, attends the 
use of a diagonal eye-piece with the 
distinctness, in which '^tJie rings are hut traces of 
rings, all their light being absorbed into the 
discs." I have entered 1852, March 23, as "a very 
fine night, though the rings and appendages 
around the brighter stars were rather trouble- 
some;" 1852, April 1, "an exceedingly fine night 
at first, with scarcely a trace of rings or append- 
ages." See also the star 70 Ophiuchi. 
* The small 'mirror in a reflector causes a 
central darkness out of the focus, which should 
be nearly the same on either side of it. 
