NEB 
appear nearly of tlie same size, their real 
magnitudes must be nearly equal. Grant- 
ing, therefore, that these nebula; and clus- 
ters of stars are formed by mutual attrac- 
tion, Dr. Herschel concludes, that we may 
judge of their relative age by the disposition 
of their component parts, those being the 
oldest wliich aie most compressed. He 
supposes, and indeed offers powerful argu- 
ments to prove, that tlie milky way is the 
nebulae of whicli our sun is one of its com- 
ponent parts. 
Dr. Herschel has also discovered other 
phenomena in the heavens, which he calls 
nebulous stars; that is, stars surrounded 
with a faint luminous atmosphere of large 
extent. Those which have been thus stiled 
by other astronomers, he says, ought not to 
have been so called, for on examination 
they have proved to be either mere clusters 
of stars plainly to be distinguished by his 
large telescopes, or such nebulous appear- 
ances as might be occasioned by a multitude 
of stars at a vast distance. The milky way 
consists entirely ot stars ; and he says, “ I 
have been led on by degrees from the most 
evident congeries of stars, to other groups 
in which the lucid points were smaller, but 
still very plainly to be seen ; and from them 
to such wherein they could but barely be 
suspected, until I arrived at last to spots in 
which no trace of a star was to be discern- 
ed. But then the gradation to these latter 
were by such connected steps as left no 
room for doubt but that all these pheno- 
mena were equally occasioned by stars 
variously dispersed in the immense expanse 
of the universe.” 
In the same paper is given an account of 
some nebulous, stars, one of which is thus 
described ; “ Nov. 13, 1790. A most sin- 
gidar phenomenon! A star of the eighth 
magnitude, with a faint luminous atmos- 
phere of a circular form, and of about 3' in 
diameter. The star is perfectly in the cen- 
tre, and the atmosphere is so diluted, faint, 
and equal throughout, that there can be no 
surmise of its consisting of stars, nor can 
there be a doubt of the evident connection 
between the atmosphere and the star. 
Another star, not much less in brightness, 
and in the same field of view with the 
above, was perfectly free from any such 
appearance.” Hence, Dr. Herschel draws 
the following consequences : Granting the 
connection between the star and the sur- 
rounding nebulosity, if it consist of stars 
very remote, which gives the nebulous ap- 
pearance, the central star, which is visible. 
NEC 
must be immensely greater than the rest ; 
or if the central star be no bigger than com- 
mon, how extremely small and compressed 
must be those other luminous points which 
occasion the nebulosity. As, by the former 
supposition, the luminous central point must 
far exceed the standard of what we call a 
star ; so in the latter, the shining matter 
about the centre wilt be too small to come 
under the same denomination ; we there- 
fore, either have a central body which is not 
a star, or a star which is invrflved in a shin- 
ing fluid, of a nature totally unknown to us. 
This last opinion Dr. Herschel adopts. 
Light reflected from the star could not 
be seen at this distance. Besides, the out- 
ward parts are nearly as bright as those 
near tlie star. Moreover, a cluster of stars 
will not so completely account for the 
milkiness, or soft tint of tlie light of these 
iiebute, as a self luminous fluid. “ What a 
field of novelty, says Dr. Herschel, “ is 
here opened to our conceptions. A shining 
fluid, of a brightness sufficient to reach us 
from the regions of a star of the 8th, 9tli, 
10th, 11th, 12th, magnitude ; and of an ex- 
tent so considerable as to take up 3, 4, 5, 
or 6 minutes in diameter.” He conjectures 
that this shining fluid may be composed of 
the light perpetually emitted from millions 
of stars. See Philos. Trans, vol. Ixxxi. p. 1. 
on Nebulous Stars; properly so called. 
NEBULY, or Nebcle'e, in heraldry, U 
when a coat is charged with several little 
figures, in form of words, running within one 
another, or when the outline of a bordure, 
ordinary, &c. is indented or waved. 
NECESSITY, whatever is done by a 
nece.ssaiy cause, or a power that is irresisti- 
ble, in which sense it stands opposed to 
freedom. 
NECESSITY, philosophical. The ad- 
vocates of philosophical necessity maintain 
that the volitions and actions of intelligent 
agents are produced by causes equally 
deciding and resistless as those which are 
admitted to actuate the material system of 
the universe. Wherever the sun shines, or 
the rain descends, it is impossible to con- 
ceive, that in situations precisely simijar to 
those which immediately precede these 
events, the ray should be withheld, or the 
cloud should remain suspended in the at- 
mosphere. The diffused splendour, and the 
falling moisture, are universally cdlowed to 
be in such situations invariably and inevita- 
bly the results. 
The doctrine of necessity extends to the 
mind whatis thus obvious and uncontradicted 
