ed light in the comet of 1819, and
also in more recent comets, but they are mere traces, and Arago himself
admits, that they do not permit "the conclusion decidedly that these
stars shine only with a borrowed light." But it still does not follow
that a comet (even if independent of reflected light) is in an
incandescent state. The auroral light is not polarized, nor any other
electric light, neither is it owing to a state of incandescence, yet it
is luminous. The intense light of a comet at perihelion is analogous to
the charcoal points of a galvanic battery, caused by a rapid current of
ether from the nucleus, and assisted by the radial stream of the vortex.
This will account for the phenomenon in all its shades of intensity, as
well as for the absence of any perceptible phase. It will also account
for the non-combustion of such comets as those of the years 1680 and
1843. We shall also be at no loss to understand, why there is no
refraction when a ray of light from a star passes through the nebulosity
of a comet; and if, as we may reasonably suppose, the gaseous matter
composing the nucleus be very attenuated, instruments are yet too
imperfect to determine whether these also have any refracting power. On
this point, however, it is safest to suspend our judgment, as there may
be comets not belonging to our system, with even liquid or solid nuclei,
or of matter widely different to those elements composing the members of
the solar system.

In addition to what has been already advanced on this subject of a
comet's light, we may appeal to the well-known fact that the visibility
of a comet is not reciprocally as the squares of the distances from the
earth and sun as it ought to be, if shining by reflected light. In
Mr. Hind's late work on comets, the fact is stated that "Dr. Olbers
found that the comet of 1780 attained its greatest brightness on the 8th
of November, thirteen days subsequent to its discovery, whereas
according to the law of reflected light, it should have become gradually
fainter from the day of its discovery; and supposing the comet
self-luminous, the intensity of light should have increased each day
until November 26th; yet in the interval between the 8th and 26th of
that month, it grew rapidly less." Now this theory teaches, that a comet
is neither self-luminous nor dependent on the sun, but on its distance
from the axis of the vortex, and a certain amount of elapsed time from
the perihelion, varying somew