M. Struve of Pulkowa has compared Sir William Herschel’s opinion
on this subject, as maintained in 1785, with that to which he was
subsequently led; and arrives at the conclusion that, according to Sir
W. Herschel himself, the visible extent of the Milky Way increases with
the penetrating power of the telescopes employed; that it is impossible
to discover by his instruments the termination of the Milky Way (as an
independent cluster of stars); and that even his gigantic telescope of
forty feet focal length does not enable him to extend our knowledge
of the Milky Way, which is incapable of being sounded. Sir William
Herschel’s _Theory of the Milky Way_ was as follows: He considered
our solar system, and all the stars which we can see with the eye, as
placed within, and constituting a part of, the nebula of the Milky Way,
a congeries of many millions of stars, so that the projection of these
stars must form a luminous track on the concavity of the sky; and by
estimating or counting the number of stars in different directions, he
was able to form a rude judgment of the probable form of the nebula,
and of the probable position of the solar system within it.
This remarkable belt has maintained from the earliest ages the same
relative situation among the stars; and, when examined through powerful
telescopes, is found (wonderful to relate!) _to consist entirely of
stars scattered by millions_, like glittering dust, on the black ground
of the general heavens.