Partially fill a vial or a tumbler with water, hold it by the rim,
and move it around a lighted candle placed upon a table. A shadow
surrounding the transmitted light will be cast upon the table. As the
tumbler approaches the light, the shadow follows the tumbler, and when
receding the tumbler follows the shadow; and as the tumbler is moved
around the light, the shadow will swing round from one side to the
other. If the tumbler be held so that a puff of smoke can be blown
into the transmitted rays, the particles of smoke will reflect the
transmitted light, and will illustrate my idea of what constitutes a
comet's tail. A dark band may be observed in this stream of light, as
also in the light cast upon the table.
In these experiments, we see the effects produced by a cause which is
known; the effects are similar to those observed in the tails of comets,
the cause of which we do not know; but is it not reasonable to assume
that the cause is similar?
Assuming now that comets are transparent, can any other phenomena
peculiar to comets be accounted for upon this hypothesis? Next to
the tail itself, the curve is the most noticeable feature, and if we
consider the extraordinary length of these appendages, the astounding
velocity at which comets move in their orbits, and the time that would
elapse before a ray of light, emitted from the nucleus, would reach the
end of the tail, perhaps the curve--which, if I am not deceived in my
observations, always dips toward its orbit--can be accounted for. If a
comet moved in a direct line toward the center of the sun, there would
be no curve to the tail. But taking Donati's comet of 1858 as an
example, the tail of which was said to be about 200,000,000 miles long,
a ray of light traveling at the rate of 192,000 miles per second would
be about twenty minutes in going from the nucleus to the end of the
tail.
But during that time the comet would move in its orbit, say, 50,000
miles, and as light moves in a straight line, and other rays are
constantly emerging from the nucleus as it moves along in its course,
the result is that the tail has a curved appearance.
I have no data at hand regarding this comet, but what I have said will
serve to illustrate my ideas. Again, referring to this comet, I remember
to have read the statement of an astronomer that, after passing round
the sun, a new tail was formed opposite the original one. Now, it seems
to me that that is just what would happen, for in moving round the sun
the comet would travel say 3,000,000 miles; the greater portion of the
tail then, would extend millions of miles upon one side of the sun,
while from the nucleus upon the opposite side of the sun a new tail
would appear to be formed.
Upon this hypothesis, the extraordinary length of their tails and the
fact that stars are visible through the densest portion of them is
explained; as also the fact that they so rapidly disappear from view
when moving from the sun, the light received by them from the sun
being in proportion to their distance from it, and but little of that
reflected.