d, with his
usual care, to have a diameter of 428 miles. From the total absence of
phases, as well as from the vivacity of its radiance, he confidently
inferred that its light was not borrowed, but inherent.[269]

This remarkable apparition formed the subject of a memoir by
Olbers,[270] the striking yet steadily reasoned out suggestions
contained in which there was at that time no means of following up with
profit. Only of late has the "electrical theory," of which Zoellner[271]
regarded Olbers as the founder, assumed a definite and measurable form,
capable of being tested by the touchstone of fact, as knowledge makes
its slow inroads on the fundamental mystery of the physical universe.

The paraboloidal shape of the bright envelope separated by a dark
interval from the head of the great comet of 1811, and constituting, as
it were, the _root_ of its tail, seemed to the astronomer of Bremen to
reveal the presence of a double repulsion; the expelled vapours
accumulating where the two forces, solar and cometary, balanced each
other, and being then swept backwards in a huge train. He accordingly
distinguished three classes of these bodies:--First, comets which
develop _no_ matter subject to solar repulsion. These have no tails, and
are probably mere nebulosities, without solid nuclei. Secondly, comets
which are acted upon by solar repulsion _only_, and consequently throw
out no emanations _towards_ the sun. Of this kind was a bright comet
visible in 1807.[272] Thirdly, comets like that of 1811, giving evidence
of action of both kinds. These are distinguished by a dark _hoop_
encompassing the head and dividing it from the luminous envelope, as
well as by an obscure caudal axis, resulting from the hollow, cone-like
structure of the tail.

Again, the ingenious view subsequently propounded by M. Bredikhin as to
the connection between the _form_ of these appendages and the _kind_ of
matter composing them, was very clearly anticipated by Olbers. The
amount of tail-curvature, he pointed out, depends in each case upon the
proportion borne by the velocity of the ascending particles to that of
the comet in its orbit; the swifter the outrush, the straighter the
resulting tail. But the velocity of the ascending particles varies with
the energy of their repulsion by the sun, and this again, it may be
presumed, with their quality. Thus multiple tails are developed when the
same comet throws off, as it approaches perihelion, specificall