Ragnarok : the age of fire and gravel

WHAT IS A COMET? 67

Tf the comet shines by reflected light, it is pretty good evidence 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 doublerefracting spar. Polarization of this character is only possible when the light that is operated upon has already been reflected from an imperfectly transparent medium.” *

There is considerable difference of opinion as to whether the head 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. N ewton, from the first, maintained that the comet is made partly of solid substance, and partly of an investment of thin, elastic vapors. Tf 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 eighthmagnitude star was temporarily quenched as the nucleus of Encke’s comet passed over at.” +

Others, again, have held that stars have been seen through the comet’s nucleus.

* “ Edinburgh Review,” October, 1874, p. 207. + Ibid., p. 206,