which enter into the formation of this wonderful zone exhibit
in a marvellous degree the amazing profusion in which these orbs exist
in certain regions of space; yet those multitudes of stars perform their
motions in harmonious unison and in orderly array, and by their mutual
attraction sustain the dynamical equilibrium of this stupendous galactic
ring, the diameter of which, according to one authority, is not
traversed by light in less than 13,000 years.

[Illustration: FIG. 8.--A Portion of the Milky Way.]

Sir William Herschel, to whom we are indebted for most of what we know
of the Milky Way, commenced a series of observations in 1785 with the
object of acquiring a knowledge of the structure of the sidereal
heavens. In the accomplishment of this object, to which he devoted a
considerable part of his life, he undertook a systematic survey of that
portion of the Galaxy which is visible in the Northern Hemisphere. By a
method called star-gauging, which consisted in the enumeration of the
stars in each successive telescopic field as the instrument moved slowly
over the region under observation, he found that the depth of the star
strata could be approximately ascertained by counting the stars along
the line of vision; those were most numerous where the visual line
appeared of the greatest length and fewest in number where it was
shortest. Herschel perceived the internal structure of the Galaxy to be
exceedingly intricate and complex, and that it embraced within its
confines an endless variety of systems, clusters, and groups, branches,
sprays, arches, loops, and streaming filaments of stars, all of which
combined to form this luminous zone. 'It is indeed,' says a well-known
astronomer, 'only to the most careless glance, or when viewed through an
atmosphere of imperfect transparency, that the Milky Way seems a
continuous zone. Let the naked eye rest thoughtfully on any part of it,
and, if circumstances be favourable, it will stand out rather as an
accumulation of patches and streams of light of every conceivable
variety of form and brightness, now side by side, now heaped on each
other; again spanning across dark spaces, intertwining and forming a
most curious and complex network; and at other times darting off into
the neighbouring skies in branches of capricious length and shape which
gradually thin away and disappear.' Sir John Herschel, who was occupied
for four years at the Cape of Good Hope in exploring the celesti