an orbit as
results from its own atomic density, and the resultant action of both
the acting forces. From the law of density of the ether, the coma ought
to be brightest and the radiating stream of the comet's nucleus
strongest on the side of least pressure: from this cause, and the fact
that the body of the comet affords a certain protection to the particles
immediately behind it, there will be an interval between the comet and
the tail less luminous, as is almost invariably observed. We thus have
an explanation of the fact noticed by Sir John Herschel, "that the
structure of a comet, as seen in section in the direction of its length,
must be that of a hollow envelope of a parabolic form, enclosing near
its vertex the nucleus or head." We have, also, a satisfactory
explanation of the rapid formation of the tail; of its being wider and
fainter at its extremity; of its occasional curvature; and of its
greater length after perihelion than before. But, more especially may we
point to the explanation which this theory gives of the fact, that,
_ceteris paribus_, the long-period comets, when their perihelion
distances are small, have tails of such exaggerated dimensions.

A comet, whose mean distance is considerable, is supposed by the theory
to be composed of elements less dense, and, during its long sojourn at
its aphelion, it may be also supposed that it there receives continual
accessions to its volume from the diffused siftings of the system, and
from the scattered debris of other comets. On approaching the
perihelion, the rapidity of the change in the density of the ether in a
given time, depends on the eccentricity of the orbit, and so does the
change of temperature; so that, from both causes, both the length of the
tail and the brilliancy of the comet measurably depends on the magnitude
of the period and of the eccentricity.

If the nuclei of comets be gaseous as we suppose, and that the smallest
stars are visible through them, it is an outrage on common sense, to
refer that light, which renders a comet visible at noon-day, within six
minutes of space of the sun itself, to the reflected light of the sun.
When a small star has been seen through the nucleus of a comet, without
any perceptible diminution of light, it indicates perfect transparency;
but there can be no reflection from a perfectly transparent body, and
therefore, a comet does not shine by reflected light. It is true that
Arago discovered traces of polariz