(190.) 
Nebulae, 
(191.) 
Classifica- 
tion of ne- 
bule ; 
(192.) 
resolvable 
and unre- 
solvable. 
842 
Greater precision has been in somerespects attained, but 
little, if anything, added to the theory of double stars. 
II. On Nebula and the (so-called) Nebular Hy- 
pothesis—The cloudy patches of mild light which 
are visible in clear but moonless nights in various 
parts of the sky are called nebula from their resem- 
blance to light vapours moderately illuminated. 
Some regions of the sky abound more with them than 
others. The Milky Way presents to the naked eye 
a nebulous appearance extending over large tracts, 
and nebule more or less dense are frequent within and 
near its borders. These facts were well known from 
an early period. Some conspicuous nebule were 
described in the sixteenth century; that in Andro- 
meda by Marius and the conspicuous nebula in 
Orion by Huygens. Halley added several, but the 
first descriptive catalogue of these objects was pub- 
lished by Messier in 1783 (in the Connaissance des 
Tems), and it forms the basis of the modern obser- 
vations. 
Sir W. Herschel took up the subject with his cus- 
tomary zeal. In 1786 he published a catalogue of 
1000 nebule, which he had inereased by 1802 to 
2500. The first step to some appreciation of the 
nature of these extraordinary appearances was of 
course to classify them, and this Herschel did with 
such success that it may be affirmed that his classes 
remain the best at the present time, as generally re- 
presenting the phenomena. Without entering into 
all his details, the following division embraces the 
main facts :— 
A. Clusters of stars, 
B. Nebule proper. 
a, Having a certain regularity, as 1. Circular or 
planetary nebule; 2. elliptic; 3, annular. 
8. Wholly irregular, as the nebula of Orion, 
the Magellanic clouds, &e. 
C. Nebulous stars. 
Remarks. A number of faint stars produces a 
nebulous haze. This is a physiological fact. Parts 
of the Milky Way which present this appearance to 
the naked eye are at once shown by a telescope of 
small power to be composed of stars.4. Patches which 
appear nebulous in such a telescope are shown by a 
better one to be composed of small stars. This is 
called the Resolution of a nebula. Such resolutions 
become more and more numerous as the apertures of 
telescopes are enlarged, Lord Rosse’s telescope has 
resolved several which withstood the highest power 
of Herschel’s. One inference might therefore be that 
all nebule are clustered stars, at a greater or less 
distance. Such an opinion is very general, and very 
plausible. It may on the other hand be observed 
that Herschel abandoned this his earlier inference, 
ASTRONOMY.—SIR WILLIAM HERSCHEL. 
[Diss. VI. 
whilst, at the same time, he proceeded diligently with 
the resolution of successive nebule, by means of his 
powerful telescopes. He maintained on the contrary to 
the last that nebule belong to two classes, resolvable 
and irresolyable. The latter he believed to be composed 
of diffuse self-luminous matter, more or less con- 
densed, whilst the milky light of the former is occa- 
sioned by the blended gleam of numerous close stars. 
With this difference, we find his views regarding the 
changes in the sidereal system and the progress of 
the condensation and breaking up of nebule, nearly 
the same in his earlier and later papers, particularly 
those of 1785 and 1811. 
The opinions of Herschel on these changes were 
the following :—Having observed that the more ; 409 
regular nebule (whether with a central star or their pro- 
not) presented almost invariably an appearance of gressive 
gradually increasing brilliancy towards its centre, o™4e0s% 
he was led to conclude that this is due to the aggre- ~— 
gation of the self-luminous parts in virtue of their 
mutual gravitation; and the conclusion would so 
far be the same, whether the parts thus coalescing 
were in fact numerous distinct masses, or were like 
a vapour diffused through space. This was the pro- 
cess of condensation, which he inferred on the one 
hand from the law of gravity, on the other from the 
generally symmetrical arrangement of the luminous 
matter in nebule of the first class. In order to de- 
tect, by actual observation, the progress of condensa- 
tion, he invented a simple and highly expressive 
terminology, indicating the general brightness, the 
amount of its gradation, and the law of its gradation 
from the circumference towards the centre. In 
whatever degree Lord Rosse’s late observations may 
be considered to deprive nebule of the character of 
symmetry which Herschel had conferred on them, 
the general fact of an apparent condensation in a 
great majority of instances cannot be denied. , 
The breaking up of nebulz (also described in his _ (194.) 
paper of 1785, as well as in his later ones) is likewise = eae 
an. inferred change anticipated in large irregular 45%. cel 
nebule, which usually exhibit bright spots or centres, 
with irregular vacant spaces, which sometimes seem 
like portions of a black and distant profundity seen 
through the rents of a nebulous veil. Herschelsup- 
poses that the luminous centres are also places of 
greatest attraction, and must gradually draw away 
the star-like substance from the rarer spaces. 
It is curious that these speculations of Herschel (195,) 
(which are substantially common to his papers of Early anti- 
(193.) 
Herschel 
1785 and 1811) had been at least imagined at a very ‘a byaee - 
early period. Arago has pointed out that Tycho thesis’ 
and Kepler conceived the New Stars of 1572 and 
1604 to have resulted from a sudden condensation 
1 The group of the Pleiades presents a peculiar instance, When the eye is turned full upon it, no more than six stars (as is 
well known) can be counted, but when the eye is turned aside through 10° or 15°, by a peculiarity of indirect vision by means of 
the lateral parts of the retina, the group appears a mass of nebulous light. This arises from an immense crowd of stars just 
below visibility, which, probably from the greater sensitiveness of this part of the eye, make a sensible though somewhat indis- 
tinct impression. 
This observation as to the Pleiades, and the inference which explains by it the seeming countlessness of the 
stars when viewed cursorily, were published by the present writer in Brewster’s Journal for 1826, 
