sly supposed to be. For instance, an equal scattering of the
stars may be admitted in certain calculations; but when we examine the
Milky Way, or the closely compressed clusters of stars of which my
catalogues have recorded so many instances, this supposed equality of
scattering must be given up."[38]

Another assumption, the fallacy of which he had not the means of
detecting since become available, was retained by him to the end of his
life. It was that the brightness of a star afforded an approximate
measure of its distance. Upon this principle he founded in 1817 his
method of "limiting apertures,"[39] by which two stars, brought into
view in two precisely similar telescopes, were "equalised" by covering a
certain portion of the object-glass collecting the more brilliant rays.
The distances of the orbs compared were then taken to be in the ratio of
the reduced to the original apertures of the instruments with which they
were examined. If indeed the absolute lustre of each were the same, the
result might be accepted with confidence; but since we have no warrant
for assuming a "standard star" to facilitate our computations, but much
reason to suppose an indefinite range, not only of size but of intrinsic
brilliancy, in the suns of our firmament, conclusions drawn from such a
comparison are entirely worthless.

In another branch of sidereal science besides that of stellar
aggregation, Herschel may justly be styled a pioneer. He was the first
to bestow serious study on the enigmatical objects known as "nebulae."
The history of the acquaintance of our race with them is comparatively
short. The only one recognised before the invention of the telescope was
that in the girdle of Andromeda, certainly familiar in the middle of the
tenth century to the Persian astronomer Abdurrahman Al-Sufi; and marked
with dots on Spanish and Dutch constellation-charts of the fourteenth
and fifteenth centuries.[40] Yet so little was it noticed that it might
practically be said--as far as Europe is concerned--to have been
discovered in 1612 by Simon Marius (Mayer of Genzenhausen), who aptly
described its appearance as that of a "candle shining through horn." The
first mention of the great Orion nebula is by a Swiss Jesuit named
Cysatus, who succeeded Father Scheiner in the chair of mathematics at
Ingolstadt. He used it, apparently without any suspicion of its novelty,
as a term of comparison for the comet of December 1618.[41] A novelty,
nevert