Brahe had tried to explain the blaze of
the star of 1572 as due to a sudden concentration of nebulous material
in the Milky Way, even pointing out the space left dark and void by the
withdrawal of the luminous stuff; and Kepler, theorising on a similar
stellar apparition in 1604, followed nearly in the same track. But under
Herschel's treatment the nebular origin of stars first acquired the
consistency of a formal theory. He meditated upon it long and earnestly,
and in two elaborate treatises, published respectively in 1811 and 1814,
he at length set forth the arguments in its favour. These rested
entirely upon the "principle of continuity." Between the successive
classes of his assortment of developing objects there was, as he said,
"perhaps not so much difference as would be in an annual description of
the human figure, were it given from the birth of a child till he comes
to be a man in his prime."[53] From diffused nebulosity, barely visible
in the most powerful light-gathering instruments, but which he estimated
to cover nearly 152 square degrees of the heavens,[54] to planetary
nebulae, supposed to be already centrally solid, instances were alleged
of every stage and phase of condensation. The validity of his reasoning,
however, was evidently impaired by his confessed inability to
distinguish between the dim rays of remote clusters and the milky light
of true gaseous nebulae.

It may be said that such speculations are futile in themselves, and
necessarily barren of results. But they gratify an inherent tendency of
the human mind, and, if pursued in a becoming spirit, should be neither
reproved nor disdained. Herschel's theory still holds the field, the
testimony of recent discoveries with regard to it having proved strongly
confirmatory of its principle, although not of its details. Strangely
enough, it seems to have been propounded in complete independence of
Laplace's nebular hypothesis as to the origin of the solar system.
Indeed, it dated, as we have seen, in its first inception, from 1791,
while the French geometrician's view was not advanced until 1796.

We may now briefly sum up the chief results of Herschel's long years of
"watching the heavens." The apparent motions of the stars had been
disentangled; one portion being clearly shown to be due to a translation
towards a point in the constellation Hercules of the sun and his
attendant planets; while a large balance of displacement was left to be
accounted