Fan. 27, 1870] 
NATURE 
ARE ANY OF THE NEBUL& STAR-SVSTEMS? 
HIS may seem a bold question, for it is commonly 
believed that Sir William and Sir John Herschel—the 
Ajax and the Achilles of the astronomical host—have 
long since proved that many of the nebule are star- | 
systems. If we inquire, however, into what the Herschels 
have done and said, we shall find that not only have they 
not proved this point, but that the younger Herschel, at 
any rate, has expressed an opinion rather unfavourable 
than otherwise to the theory that the nebula are galaxies 
in any sense resembling our own sidereal system. : 
Sir William Herschel, by his noble plan of star-gauging, 
proved that the stars aggregate along a certain zone, 
which in one direction is double. He argued, therefore, 
that presuming a general equality to exist among the stars 
and among the distances separating them from each 
other, the figure of the sidereal system resembles that of 
a cloven disc. 
And as the only 
system from 
which he could 
form a probable 
judgment—I 
mean the plane- 
tary system— 
presented to him 
a number of 
bodies, widely 
separated from 
each other and 
each a globe of 
considerable im- 
portance, he rea- 
soned from ana- 
logy that similar 
relations exist in 
the sidereal 
spaces. This 
being so, his clo- 
ven disc theory 
of the sidereal 
system seemed 
satisfactorily es- 
tablished. 
Then, of course, 
those nebulex 
which exhibit a 
multitude of mi- 
nute points of 
light very close to- 
gether, and those 
other nebule 
which, while not 
thus resolvable 
into minute points, yet in other respects resemble those 
which are, came naturally to be looked upon as distinct | 
from the sidereal system. The analogy of this system, in 
fact, pointed to them as external star-systems, resembling 
it in all important respects. 
Then there were certain other objects, which seemed to 
present no analogy either to the sidereal system or to 
separate stars. These objects Sir Wm. Herschel con- 
sidered to belong to our sidereal system ; for he could 
not put them outside its range without looking on them 
as objects suz generis, which would have been to abandon 
the argument from analogy. In order to explain their ap- 
pearance, he suggested that they might be gaseous bodies, 
by whose condensation stars would one day be 
formed. 
The value of Sir Wm. Herschel’s work is not in the 
least affected even if science have to reject every one of 
these opinions. He himself held them with a light hand; 
he had once held other opinions ; and he was gradually 
| modifying these. Had he seen one sound reason for 
| rejecting any or all of them he would have done so in- 
| stantly. For it belonged to the strength of his character 
| that he was never fettered by his own opinions, as weak 
| men commonly are. 
| Sir John Herschel did for the southern heavens what 
| his father had done for the northern. He completely sur- 
veyed and gauged them. It is commonly believed that 
the results of his labours fully confirmed the opinions 
which his father had looked upon as probable. 
| Let us see if this is so. 
| Sir W. Herschel thought the Milky Way indicated that 
the sidereal system has the figure of a cloven disc; Sir 
John Herschel judges rather that the sidereal system has 
the figure of a flattened ring. Sir Wm. Herschel thought 
the stellar nebulz are probably external galaxies; Sir John 
gives reasons for believing that they lie within our system, 
and Whewell considered that these reasons amount to 
absolute proof. 
It has been 
further believed 
and stated that 
the researches of 
the elder Struve 
go far to confirm 
the opinions put 
forward by Sir 
W. Herschel as 
probable. 
Let us inquire 
how far this is 
true, 
Struve found 
that the numbers 
of stars of given 
magnitudes  ex- 
hibit nearly the 
same proportion 
in different direc- 
tions. Thus sup- 
posing that in a 
given directicn 
there are three 
times as many 
stars of a certain 
magnitude as 
there are of the 
next highest mag- 
then 
nitude, in 
other directions, 
also, the same 
relation is  ob- 
served. This is 
a very striking 
law ; but tomake 
it serve as a proof of the opinion which Sir Wilham 
Herschel had put forward as probable, it would be neces- 
sary that another law should be exhibited, For clearly, if 
that opinion were just, it would be easy to calculate what 
the relation should be between stars of different mag- 
nitudes. Had Struve been able to show that the numbers 
actually seen corresponded to the relations thus calculated, 
he would have gone far to render that view certain which 
Herschel always spoke of as merely an assumption. 
But Struve found no such law of stellar distribution. 
On the contrary, he found a law so different, that in order 
to force the facts into agreement with Sir William Her- 
schel’s views about the sidereal system, he had to invent 
his famous theory of the extinction of light in traversing 
| space. Now, according to this theory, we cannot see to 
the limits of our sidereal system, even though we could 
increase the powers of our telescopes a million-fold ; so 
that if the theory is true, the question which heads this 
| paper is at once disposed of, Obviously, we cannot see 
