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: 

“The spectroscopic observations made by Mr. Huggins on the light of three comets show that a certain portion, at least, of the light of these objects is inherent. . . .  The nucleus gave in each case three bands of light, indicating that the substances of the nuclei consisted of glowing vapor."[2]

In one case, the comet-head seemed, as in the case of the, comet examined by Padre Secchi, to consist of pure carbon.

In the great work of Dr. H. Schellen, of Cologne, annotated by Professor Huggins, we read: 

“That the nucleus of a comet can not be in itself a dark and solid body, such as the planets are, is proved by its great transparency; but this does not preclude the possibility of its consisting of innumerable solid particles separated from one another, which, when illuminated by the sun, give, by the reflection of the solar light, the impression of a homogeneous mass.  It has, therefore, been concluded that comets are either composed of a substance which, like gas in a state of extreme rarefaction, is perfectly transparent, or of small solid particles individually separated by intervening spaces through which the light of a star can pass without obstruction, and which, held together by mutual attraction, as well as by gravitation toward a denser central conglomeration, moves through space like a cloud of dust.  In any case the connection lately noticed by Schiaparelli, between comets and meteoric