it introduced the element of precise
knowledge where more or less probable conjecture had previously held
almost undivided sway; and precise knowledge tends to propagate itself
and advance from point to point.

We have now to speak of Herschel's pioneering work in the skies. To
explore with line and plummet the shining zone of the Milky Way, to
delineate its form, measure its dimensions, and search out the
intricacies of its construction, was the primary task of his life, which
he never lost sight of, and to which all his other investigations were
subordinate. He was absolutely alone in this bold endeavour. Unaided, he
had to devise methods, accumulate materials, and sift out results. Yet
it may safely be asserted that all the knowledge we possess on this
sublime subject was prepared, and the greater part of it anticipated, by
him.

The ingenious method of "star-gauging," and its issue in the delineation
of the sidereal system as an irregular stratum of evenly-scattered suns,
is the best-known part of his work. But it was, in truth, only a first
rude approximation, the principle of which maintained its credit in the
literature of astronomy a full half-century after its abandonment by its
author. This principle was the general equality of star distribution. If
equal portions of space really held equal numbers of stars, it is
obvious that the number of stars visible in any particular direction
would be strictly proportional to the range of the system in that
direction, apparent accumulation being produced by real extent. The
process of "gauging the heavens," accordingly, consisted in counting the
stars in successive telescopic fields, and calculating thence the depths
of space necessary to contain them. The result of 3,400 such operations
was the plan of the Galaxy familiar to every reader of an astronomical
text-book. Widely-varying evidence was, as might have been expected,
derived from an examination of different portions of the sky. Some
fields of view were almost blank, while others (in or near the Milky
Way) blazed with the radiance of many hundred stars compressed into an
area about one-fourth that of the full-moon. In the most crowded parts
116,000 were stated to have been passed in review within a quarter of an
hour. Here the "length of his sounding-line" was estimated by Herschel
at about 497 times the distance of Sirius--in other words, the bounding
orb, or farthest sun of the system in that direction, so far