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]
Others, again, have held that stars have been seen through the comet's nucleus.
[1. "Edinburgh Review," October, 1874, p. 207.
2. Ibid., p. 206.]
{p. 68}
Amédée Guillemin says:
"Comets have been observed whose heads, instead of being nebulous, have presented the appearance of stars, with which, indeed, they have been confounded."[1]
When Sir William Herschel discovered the planet Urania, he thought it was a comet.
Mr. Richard A. Proctor says: