SECTION III.

COMETS DEVOID OF NUCLEUS AND TAIL.

Gradual condensation of nebulous matter at the centre-Imperceptible transition from comets without apparent tails to the immense luminous trains of great historic comets.

LET us before proceeding further make a few general remarks on the heads and tails of comets. The remaining sections of the chapter we will devote to a more complete examination of their structure.

Fig. 21. Cometary nebulosities; central condensation; absence of tail and nucleus. Since a systematic search has been made for comets, and powerful instruments have been employed, the number of those discovered has, as might be expected, considerably increased; but the majority are telescopic comets, and amongst them are

many nebulosities devoid of nucleus. This fact had been already ascertained by Sir William Herschel in 1807. Out of sixteen telescopic comets that I have examined, fourteen,' he observes, 'exhibited nothing remarkable at their centres.'

The following are some examples of comets which were simple nebulosities, and apparently without tail or nucleus. Encke's comet, observed by Mr. J. Tebbutt, June 24, 1865: 'The comet,' he observes, was about two minutes in diameter,

faint, and without the slightest condensation of light in the centre.' In October 1871 the same comet presented, according to Mr. Hind, when first observed, the aspect of a faint and nearly round nebulosity, without any condensation of its parts. But on the 9th of November the same comet exhibited an appearance anything but globular. According to Mr. Carpenter the nebulosity had expanded like a fan, the apex of which was the most brilliant part; but there was no nucleus. The comet discovered on July 12, 1870, by M.

Winnecke was similar in appearance, and is described as a round nebulosity, of moderate brilliancy, and of 2 minutes in diameter.

The following is another instance in which the trace of a brilliant nucleus is just discernible. We refer to Brorsen's comet, observed at Marseilles, on September 1, 1873, by M. Stéphan, who thus describes it: 'Nebulosity ovoid, diffuse, and exceedingly faint, with a trace of condensation towards its centre.' And likewise Winnecke's comet, seen in April and May 1869: It is a faint nebulous patch of some little size,' says Mr.

Hale Wortham, 'appearing occasionally to brighten somewhat to a centre.' According to Father Perry, 'there seems to be a slight condensation towards the centre, but no decided nucleus.' However, we must not forget that the absence of a nucleus may proceed either from the distance of the comet rendering a very slight condensation invisible, or from the position of the comet relatively to the sun. If the nucleus shines by a light which is not its own, its light would increase as the comet draws near to its perihelion. And we see, in fact, that in fig.

17, Encke's comet exhibits a visible condensation, while in fig. 23 it has a brilliant and defined nucleus. In like manner Brorsen's comet, observed in October 1873, showed considerable condensation about the centre. On its apparition in 1868 the brightest portion was very eccentric, and there were three or four centres of condensation or brilliant nuclei. (See fig. 18, p. 120.)

coma.

The comet of 1867, II., telescopically observed by Mr. Huggins, appeared to consist of a slightly oval coma, surrounding a minute and not very bright nucleus.' This bright point was not central, but near to the following (eastern) edge of the The double comet of Biela, as we shall presently see, possesses a well-defined luminous nucleus in the centre of each of the nebulosities which compose its two parts. The same fact is observable in respect to other telescopic comets. In May 1873 Tempel's comet exhibited a head of oval form, with a central nucleus about as bright as a star of the 12th or 13th magnitude. Faye's comet, seen at Marseilles, in September of the same year, although extremely faint, had a small sharplydefined nucleus, which enabled it to be easily observed. Lastly, the comet of 1873, IV., discovered by M. P. Henry at the Observatory of Paris, was round, very brilliant, nearly visible to the naked eye, and had a central condensation. It is shown under this aspect in the left hand drawing of fig. 32.

In some comets, as we have seen in the preceding section, the nuclei have been equal in brilliancy to Jupiter himself; others that we have yet to mention have even exceeded him in the brilliancy of their light. Between simple nebulosities, therefore, devoid of nucleus or luminous condensation, and those comets which have surpassed in lustre the most brilliant of the planets, there is no distinct line of demarcation. The transition from the one extreme to the other is imperceptible. We shall find a similar gradation in respect to cometary tails, from

the comets destitute of tail, that we have just described, from hardly visible traces of these appendages in telescopic comets, to the immense luminous trains of the great comets of 1680, 1769, 1811, 1843, 1858, &c., which during their apparition swept the heavens. These differences of aspect the reader will be enabled to follow by the aid of our engravings.