that there must be some material substance there to reflect the light.

"A considerable portion of the light of the comet is, nevertheless,
borrowed from the sun, for it has one property belonging to it that
only reflected light can manifest. It is capable of being polarized
by prisms of double-refracting spar. Polarization of this character
is _only possible_ when the light that is operated upon has already
been reflected _from an imperfectly transparent medium_."[1]

There is considerable difference of opinion as to whether the bead of
the comet is solid matter or inflammable gas.

"There is nearly always a point of superior brilliancy perceptible in
the comet's head, which is termed its nucleus, and it is necessarily
a matter of pressing interest to determine what this bright nucleus
is; whether it is really a kernel of hard, solid substance, or merely
a whiff of somewhat more condensed vapor. Newton, from the first,
maintained that the comet is _made partly of solid substance_, and
_partly of an investment of thin, elastic vapors_. If this is the
case, it is manifest that the central nodule of dense substance
should be capable of intercepting light when it passes in front of a
more distant luminary, such as a fixed star. Comets, on this account,
have been watched very narrowly whenever they have been making such a
passage. On August 18, 1774, the astronomer Messier believed that he
saw a second bright star _burst into sight from behind the nucleus of
a comet which had concealed it the instant before_. Another observer,
Wartmann, in the year 1828, noticed that the light of an
eighth-magnitude star was _temporarily quenched as the nucleus of
Encke's comet passed over it_."[2]