332 
NATURE 
[Fan. 27, 1870 
galaxies beyond the sidereal system if we cannot see to 
the limits of that system. And I may note in passing 
that (independently of Struve’s theory) the most powerful 
telescopes cannot render visible the most distant stars of 
our sidereal scheme ; so that if the nebulz are really ex- 
ternal galaxies, the stars we see in them must be 
enormously greater than those in our galaxies, suppos- 
ing Herschel was right in thinking these tolerably uniform 
in magnitude. 
Before proceeding to exhibit the evidence which has led 
me to the conviction that the nebula belong to our sidereal 
system, I may mention some reasons for believing that if 
Sir William Herschel’s labours in the sidereal heavens 
were to be begun now, not only would he not have been 
led to adopt as probable the view on which he formed his 
opinions, but he would haye rejected it as opposed to 
known analogies. 
He had argued that because the planetary system ex- 
hibits a definite number of bodies separated by wide 
distances, therefore analogy should lead us to regard the 
sidereal system as similarly constituted, though on a 
much larger scale. This was perfectly just. Despite the 
various differences which no one recognised more clearly 
than he did, this view was the only one he could safely 
adopt for his guidance, ninety years ago. 
But would not he have been the first to reject that view 
if he had known what we now know of the solar system? 
If he had known that besides the primary planets, there 
are hundreds of minute bodies forming a zone between 
the orbits of Mars and Jupiter ; that the rings of Saturn 
are formed of a multitude of minute satellites ; that innu- 
merable meteor-systems circle in orbits of every conceiv- 
able degree of eccentricity; that near the sun these 
systems grow denser and denser; that the comets of the 
solar system must be counted by millions on millions ; 
that, in fine, every conceivable form of matter, every con- 
ceivable degree of aggregation, and every conceivable 
variety of size, exists within the limits of the solar system, 
—would he, then, have been led by analogy to recognise 
in the sidereal system only discrete stars and masses 
forming into stars ? 
From a careful study of all that Sir William Herschel 
has written, I feel certain, that in the case I have ima- 
gined, he would have been prepared, even before com- 
mencing his labours, to expect precisely that variety of 
matter, size, and aggregation, which modern observations, 
rightly understood, prove actually to exist within the range 
of the sidereal system. 
The Herschels, father and son, discovered about 4,500 
nebulz. Other observers have brought up the number to 
about 5,400. When these are divided into classes, it 
appears that some 4,500 must be looked on as irresolvable 
into discrete points of light. But of these the greater 
proportion so far resemble resolvable nebulz as to lead 
to the belief that increase of optical power alone is want- 
ing to resolve them. 
Taking these irresolvable nebulz, however, as we find 
them, and marking down their places over the celestial 
sphere, we recognise certain peculiarities in their arrange- 
ment. In the northern heavens they gather into a clus- 
tering group as far as possible from the Milky Way. In 
the southern heavens they form into streams, which run 
out from a region nearly opposite the northern cluster of 
nebulz ; but the ex¢vemztzes of the streams are the region 
where nebulze are most closely crowded. The Milky 
Way is almost clear of nebula, 
This withdrawal of the nebule from the Milky Way has 
been accepted by many as clearly indicating that there | 
is 70 association between them and the sidereal system. 
The opinion of the Herschels, if they had been led to 
pronounce definitively on this point, would have been dif- 
ferent, however; for the younger Herschel quotes (as 
agreeing with it) a remark of his father’s to the effect that 
the peculiar position of the northern nebular group is not 
accidental. If not accidental, it can only be due to some 
association between the nebular group and the galaxy. 
Every other conceivable explanation will be found to make 
the relation merely apparent—that is, accidental, which 
neither of the Herschels admit. 
But yet stronger evidence of association exists ; evidence 
which I do not hesitate to speak of as incontrovertible. 
Space will only permit me to treat it very briefly. 
There is a certain well-marked stream of nebulz in the 
southern heavens leading toa well-marked clusterof nebulz. 
There is an equally well-marked stream of stars leading to 
an equally well-marked cluster of stars. The nebular stream 
agrees in position with the star-stream, and the proba- 
bility is small that this coincidence is accidental. The 
nebular cluster agrees in position with the star-cluster, 
and the probability is still smaller that this second coin- 
cidence is accidental. Such are the separate chances. It 
will be seen at once, therefere, how small the chance is 
that both coincidences are accidental. 
The cluster here referred to is the greater of the cele- 
brated Magellanic Clouds. When it is added that the 
evidence is repeated point for point in the case of the 
lesser Magellanic Cloud, the indications of association 
appear overwhelmingly convincing. If the nebulz really 
are associated in this manner with fixed stars, the question 
which heads this paper is disposed of at once. : 
But there is yet further evidence. 
The nebulz pass by insensible gradations from clusters 
less and less easily resolvable, to nebulz properly so 
called, but still resolvable, and so to irresolvable nebulz. 
Now clusters are found not only to aggregate in a general 
manner near the Milky Way, but in some cases (on which 
Sir John Herschel has dwelt with particular force) to 
exhibit the clearest possible signs of belonging to that 
zone. If they then belong to the Milky Way, can any 
good reason be given for believing that the various other 
classes of nebulz# are not associated with the sidereal 
scheme? Where should the line be drawn ? 
Again, some of thenebulaare gaseous, and allthe gaseous 
nebulz exhibit the same spectrum. Now, two classes of 
gaseous nebulz, the planetary and the irregular nebula, 
exhibit a marked preference for the Milky Way, and 
therefore we must admit the probability that they, at any 
rate, belong to the sidereal scheme. But then a large 
proportion of the irresolvable nebule are also gaseous, 
and as they are formed of the same gases, we see good 
reason for believing that they also must belong to our 
galaxy. This, however, brings in all the nebulz, since the 
recent detection by Lieut. Herschel of the same bright 
lines in or rather 07 the continuous spectrum of a star- 
cluster, shows the great probability which exists that with 
more powerful spectroscopes all the nebula may be 
found to exhibit these bright lines, that is, to contain 
these particular gases. I pass over the facts, that many 
nebulz are found to be closely associated with stars, and 
that if any doubt could remain as to the association being 
real and not apparent, it would be removed by a picture of 
the nebula M 17, as seen in Mr. Lassell’s great reflector at 
Malta. The reader will be more interested by the follow- 
ing quotation, which I extract (by permission) from a 
letter of Sir John Herschel’s :— 
“A remark which the structure of Magellanic Clouds 
has often suggested to me has been strongly recalled by 
what you say of the inclusion of every variety of nebulous 
or clustering forms within the galaxy, viz., that if such be 
the case—z.e. if these forms belong to, and form part and 
parcel of the Galactic system—then that system includes 
within itself miniatures of itself on an almost infinitely 
reduced scale; and what evidence, then, have we that 
there exists a universe beyond—unless a sort of argument 
from analogy, that the Galaxy with all its contents may 
be but one of these miniatures of a more vast universe, 
and that there may, in that universe of other systems on 
a scale as vast as our galaxy, be the analogues of those 
