The commencement of our Planetary System, including the sun, must,
according to Kant and Laplace, be regarded as an immense nebulous mass
filling the portion of space which is now occupied by our system far
beyond the limits of Neptune, our most distant planet. Even now we
perhaps see similar masses in the distant regions of the firmament, as
patches of nebulæ, and nebulous stars; within our system also, comets,
the zodiacal light, the corona of the sun during a total eclipse,
exhibit resemblances of a nebulous substance, which is so thin that the
light of the stars passes through it unenfeebled and unrefracted. If we
calculate the density of the mass of our planetary system, according
to the above assumption, for the time when it was a nebulous sphere
which reached to the path of the outmost planet, we should find that
it would require several cubic miles of such matter to weigh a single
grain.--_Professor Helmholtz._
A quarter of a century ago, Sir John Herschel expressed his opinion
that those nebulæ which were not resolved into individual stars by the
highest powers then used, might be hereafter completely resolved by a
further increase of optical power:
In fact, this probability has almost been converted into a
certainty by the magnificent reflecting telescope constructed by
Lord Rosse, of 6 feet in aperture, which has resolved, or rendered
resolvable, multitudes of nebulæ which had resisted all inferior
powers. The sublimity of the spectacle afforded by that instrument
of some of the larger globular and other clusters is declared by
all who have witnessed it to be such as no words can express.[23]
Although, therefore, nebulæ do exist, which even in this powerful
telescope appear as nebulæ, without any sign of resolution, it may
very reasonably be doubted whether there be really any essential
physical distinction between nebulæ and clusters of stars, at least
in the nature of the matter of which they consist; and whether the
distinction between such nebulæ as are easily resolved, barely
resolvable with excellent telescopes, and altogether irresolvable
with the best, be any thing else than one of degree, arising merely
from the excessive minuteness and multitude of the stars of which
the latter, as compared with the former, consist.--_Outlines of
Astronomy_, 5th edit. 1858.
It should be added, that Sir John Herschel considers the “nebular
hypothesis” and the above theory of sidereal aggregation to stand quite
independent of each other.