Those singular meteors, the comets, which flash through heaven with long trails of light, and of old astonished the nations as if they were harbingers of some overwhelming calamity, were also the frequent subjects of our astronomer's investigations. He brought some of his fine and powerful instruments to bear on a comet discovered by Mr. Pigott in 1807, and closely and carefully investigated its physical constitution.
The nucleus, or head, was circular and well determined, and evidently shone by its own light. Very small stars seemed to grow pale, "to hide their diminished heads," when seen through its _coma_ or tail. It is true, however, that this faintness may have been only apparent, and due to the circumstance of the stars being projected on a luminous background. Such was Herschel's explanation. A gaseous medium, capable of absorbing sufficient solar radiance to efface the light of some "lesser stars," appeared to him to possess in each stratum a sensible quantity of matter. Hence it would cause a real diminution of the light transmitted, though nothing would indicate the existence of such a cause.[1]
[Footnote 1: This conclusion is disputed by many astronomers.]
Herschel examined the beautiful comet of 1811 with equal accuracy. "Large telescopes showed him, in the midst of the gaseous head, a rather reddish body of planetary appearance, which bore strong magnifying powers, and showed no sign of _phase_ (that is, of change of aspect, as in the case of the Moon). Hence Herschel concluded that it was self-luminous. Yet, if we reflect that the planetary body under consideration was not a second in diameter, the absence of a phase," says Arago, "does not appear a demonstrative argument."
"The light of the head had a bluish-green tint." Was this a real tint, or did the central reddish body, only through contrast, make the surrounding vapour appear to be coloured? Herschel did not examine the question from this point of view.
"The head of the comet appeared to be enveloped at a certain distance, on the side towards the Sun, by a brilliant narrow zone, embracing about a semicircle, and of a yellowish colour. From the two extremities of the semicircle arose, towards the region away from the Sun, two long luminous streaks which limited the tail. Between the brilliant circular semi-ring and the head, the cometary substance appeared to be dark, of great rarity, and very diaphanous.