i 
(196.) 
Herschel’s 
opinions 
on the dis- 
tance of 
nebule ; 
(197.) 
and their 
changes of 
form. 
Cnap, IIT., § 2.] ASTRONOMY.—SIR WILLIAM HERSCHEL. 843 
of diffuse celestial matter, such as the Milky Way ex- 
hibits, and Tycho alleged that the new star was sur- 
rounded by an obscure space, half as large as the 
moon’s disk. The history of Science scarcely pre- 
sents a more curious anticipation. 
One practical difference between the earlier and 
later views of Herschel on the nature of nebula, was 
as to their distance from our system. For ifthe milky 
be due to a confusion of small stars, it is 
assumed that they must be almost incredibly distant. 
In his memoir of 1785, he estimates this distance at 
not less than 6000 or 8000 times that of Sirius. 
Tf, on the other hand, the nebulous appearance be 
due to a diffusion of starry matter, the distance may 
be that of any order of fixed stars ; and this opinion 
derives weight from the evident connection of some 
nebule with stars round which they seem to cluster 
or hang almost like a drapery. Thus in 1811 he 
supposed that the nebula in Orion need not be more 
distant than a star of the 7th or 8th magnitude. 
It appears from his paper of 1802 (page 500), 
that his ideas about nebulz were then wavering. Be- 
tween that time and 1811 he elaborated the nebular 
hypothesis, so far as it is due to him. He seems to 
suppose that starry matter was once in a state of in- 
definite diffusion. That during “ an eternity of past 
duration” (Ph. Tr., 1811, p, 287) it has been “break- 
ing up” by condensation toward centres more or less 
remote. That the Milky Way—or at least the nebu- 
lous parts which it contained—and the dispersed ne- 
bule, are the relics of this former state of things. 
That where condensation has gone on more energeti- 
cally, we have nebule with a gradually or rapidly 
increasing brightness towards the centre ;—if still 
more energetic, a nucleus, or it may be a planetary 
nebula ;—next a nebulous star, which he supposes 
our sun to be, and the zodiacal light a relic of its 
nebula (p. 311);—finally, the completely formed 
stars may be assumed to be merely consolidated ne- 
bulw, (See pp. 284, 285, 299, 310.) This conden- 
sation, he believes, must be accompanied by rotation 
due to the originally irregular distribution of the 
gravitating particles (p. 312-319). 
The proofs on which the whole of this cosmogony 
rests are, 1st, the gradation of appearances above 
described, to be collected from distinct objects in the 
heavens ; and, 2dly, supposed changes observed by 
him in the nebula of Orion and others, during thirty 
years, and even during intervals of afew years. As to 
the last argument, it is admitted, we believe, by the 
best authorities, to be without weight. The changes 
of aspect of such curiously faint and graduated ob- 
jects, even to the same eye using the same telescope, 
are numberless, and may be due to the slightest at- 
mospheric and physiological influences. This test 
(as applied hitherto) is therefore generally rejected. 
Against the first proof much might also be urged, for 
wnicn we cannot afford space, Its idea is essentially 
derived from the Natural History sciences, and I can- 
not help thinking that Herschel must have derived it 
from some one more conversant with these than with 
mechanical physics. This opinion is confirmed by a 
curious illustration which he uses in the same paper 
of 1811. ‘There is perhaps not so much difference 
between them” (viz., the whole group of nebular and 
quasi-nebular phenomena), “if I may use the com- 
parison, as there would be in an annual description 
of the human figure, were it given from the birth of 
achild until he became a man in his prime” (p, 271). 
A theory or hypothesis in some respects similar 199.) 
had been previously expounded by Laplace in the Laplace’s 
concluding chapter of his Systéme du Monde. At era 3 hy- 
least it so far resembles it, that he puts forth Tycho’s ey i ectoa, 
notion of the star of 1572 being a condensation of a with Her- 
widely-spread stellar atmosphere (without, however, *hel’s- 
naming Tycho), and supposes that our solar atmo- 
sphere might also once have extended to the limit of 
the system, and that the planets were thrown off suc- 
cessively in the form of nebulous rings (subsequently 
condensed into spheres) from the equatoreal parts of 
this vast revolving mass during its contraction. He 
ascribes to Saturn’s ring a like origin. It required all 
the authority of a name like that of Laplace to cireu- 
late a theory so bold, if notextravagant, He adhered, 
however (at that time), to the old idea of the consti- 
tution of nebulz, which he considered to be composed 
entirely of stars, and to be at once the most distant 
and the most massive aggregations of matter in the 
universe. Herschel could hardly fail of being ac- 
quainted in 1811 with Laplace’s views, and they pro- 
bably in some degree influenced his own, particularly 
as to the cause of the rotation of his condensing suns, 
But he prudently reserved his hypothesis for sidereal 
objects, and did not deduce from it a planetary cos- 
mogony. In the later editions of the Systéme du 
Monde, Laplace quotes Herschel’s observations in 
confirmation of his views, adopts the notion of a dif- 
fuse nebulous matter, and considers that an hypothesis 
arrived at by a “remarkable coincidence” in oppo- 
site directions, is thereby invested with a great pro- 
bability. 
Even Herschel’s more limited conclusions have 200.) 
been very dubiously received, and notwithstanding Doubts re- 
the weighty adhesion of Humboldt and Arago to Sting it. 
them, it may be affirmed that natural philosophers 
generally are content to leave the solution of this 
cosmical problem to that distant posterity which 
alone can hope to witness unequivocal evidence of pro- 
gressive change in those wonderful objects. 
III. Of the Grouping of the Stars generally m (201.) _ 
Space, and the Signisicance of the Milky Way.—The The consti- 
. : ionof th 
earlier astronomers, and particularly Halley, had procurig he 
the idea that the nebule—which abound so remark- vens, and of 
the Milky 
1 The most intelligible account of the nebular hypothesis, in its least objectionable form, will be found in Sir John Herschel’s Way in par- 
Outlines of Astronomy, Articles 870 to 872. 
ticular. 
