Such was the comet of 1555, whose rays shone like gold; and that of 1533, whose tail was of a beautiful yellow. Of Halley's comet, at its apparition in 1456, it is said 'the colour of the comet resembled that of gold.' It is true that 'at other times and perhaps in other places it appeared pale and whitish; it sometimes resembled a glistening flame.'

This last remark suggests a very natural reflection, and leads us to consider how far the state of the atmosphere, its more or less purity, and the greater or less height of the comet above the horizon, may have contributed to invest these bodies and their tails with the tints above described. It appears

certain, however, that the light of comets is far from being always of the same colour. The great comet of 1106 was of a remarkable whiteness. Situated towards that quarter of the heavens where the sun sets in winter, it extended a whitish beam, resembling a linen cloth. From the commencement of its apparition both the comet and its beam, which was as white as snow, diminished day by day.' According to other chronicles its rays were whiter than milk.' This, as may be seen, forms a complete contrast to the red and yellow colours of the preceding comets; nor is the contrast less with the comets of which we are about to speak.

Under the date of 1217 Pingré has the following: 'Several prodigies were observed; blue comets were seen.' The comet of 1356, observed in China, was of a whitish colour bordering upon blue. The comet of 1457, the tail of which resembled an upright spear, was of 'a livid dusky colour, very like that of lead.' The second comet of 1468 also 'was blue, but somewhat pale.' The one which appeared at the end of 1476 was of pale blue bordering upon black. And we must not forget the two comets, 'terrible and of a blackish hue,' whose apparition in 1456, before that of Halley's comet, has been mentioned by some authors.

Modern observers appear to have paid but little attention to the study of cometary light. Nevertheless, we find in Arago the following comparison between the tail of the great comet of 1843 and the zodiacal light: On the 19th of March the tail of the comet, situated close to the zodiacal light, was evidently tinged with red, inclining to yellow.' He says nothing about the colour of the nucleus. Amongst the numerous observations of the comet of Donati (1858) we have only met with the following mention of its colour, made by an observer at Neufchâtel, M. Jacquet: On Sunday, the 3rd of October, after a cloudless day, splendid twilight. The irregular line of mountains near where the sun has disappeared is traced against a sky glowing with gold and fire. It is six o'clock. We endeavour to see if the comet, in consequence of the purity of the air, may not be already visible. After a few moments' search we discover it, extremely small and pale, and of the silvery brightness of a planet seen by daylight.' Two days after, on October 5, at the same hour, the comet was visible in the neighbourhood of Arcturus. The clouds,' says M. Jacquet, 'pass from the region of Arcturus far too slowly for my patience; they disperse at last; I see a yellow star, and a little underneath and to the right a small white plume. My attention is caught by these two colours; one would say the comet is of gold and the plume of silver' (Souvenirs de la Comète de 1858). This evidently refers to the colour of the tail and the envelope surrounding the nucleus, for the next day the same observer speaks of the nucleus as 'small, bright, and of a reddish yellow.'

Coggia's comet (1874, III.), observed this summer, was distinguished by very appreciable phenomena of colour. Father Secchi remarks: 'The comet, when observed with an ordinary eyepiece, was magnificent. On the 9th of July it

formed a fan, of a reddish tint (by contrast), of about 180 degrees of opening, composed of curvilinear rays, springing from a nucleus of yellowish green.' The Roman astronomer thus attributes the colour of the tail to the effect of contrast. M. Tacchini is of a different opinion. After having described the continuous spectrum upon which was projected the discontinuous spectrum of the nucleus, he proceeds to add: "This beautiful coloured band, which presented itself only at the passage of the nucleus, when seen through a simple eyepiece appeared of a greenish white, whilst the fan itself was sensibly reddish, even when occulting the nucleus.'*

The question of the colour of the light in the several portions of a comet, its nucleus, atmosphere, and tail, is an interesting one, for it is intimately connected with the physical nature of the light itself. In conjunction with results afforded by spectroscopes and polariscopes it will doubtless help to determine if comets shine by their own light, or only reflect

'Questo bel nastro colorato presentavasi al solo passagio del nucleo, il quale guardato coll' oculare semplice appariva bianco-verdastro, mentre il ventaglio era sensibilmente roseo anche occultando il nucleo.' (Memorie della Societa degli Spettroscopisti Italiani. Luglio, 1874.)

[Mr. Huggins, describing the appearance of the comet in the telescope, writes, "The nucleus [of Coggia's comet] appeared of an orange colour. This may be due in part to the effect of contrast with the greenish light of the coma. Sir John Herschel described the head of the comet of 1811 to be of a greenish or bluish-green colour, while the central point appeared to be of a pale ruddy tint. The elder Strube's representations of Halley's comet, at its appearance in 1835, are coloured green, and the nucleus is coloured reddish yellow. He describes the nucleus on October 9 thus, "Der Kern zeigte sich wie eine kleine, etwas in gelbliche spielende, glühende Kohle von länglicher Form." Dr. Winnecke describes similar colours in the great comet of 1862' (Proc. Roy. Soc., vol. xxiii., p. 157). According to Mr. Lockyer, the colour, both of the nucleus and of the head, as observed in Mr. Newall's telescope, was a distinct orange yellow. Mr. Newall says the colour of the comet was greenish yellow. Messrs. Wilson and Seabroke, observing the comet on July 14, at Rugby, considered that it was reddish in colour (R.A.S. Notices, vol. xxxv., p. 84).—ED.]

that which they receive from the sun. Perhaps both hypotheses are true; but if so, to what extent do the atmosphere and the nucleus participate in this double cause of visibility? This is a question we are not yet in a position to answer, although, as we shall see, several steps have already been taken in this direction.

SECTION IV.

SUDDEN CHANGES OF BRILLIANCY IN THE LIGHT OF COMETARY

TAILS.

Rapid undulations occasionally observed in the light of cometary tails; observations of Kepler, Hevelius, Cysatus, and Pingré; comets of 1607, 1618, 1652, 1661, and 1769- Undulations in the tails of the comets of 1843 and 1860; do these undulations arise from a cause peculiar to the comet itself, or do they depend upon the state of the atmosphere ?-Objection made by Olbers to the first of these hypotheses; refutation by M. Liais.

THE tails of certain comets have exhibited variations of brilliancy, sudden changes of intensity, analogous to the phenomena of the same kind which are observed in the aurora borealis, and which, it is believed, have been remarked in the zodiacal light. This fact was unknown to the ancients; and when Seneca speaks of the augmented or diminished brilliancy of comets, it is evident that he alludes to the changes produced, in the course of their apparition, by the variations of their distance from the earth. He compares them 'to other stars which throw out more light and appear larger and more luminous in proportion as they descend and come nearer to us, and are smaller and less luminous as they are returning and increasing their distance from us.' (Quæstiones Naturales,

vii. 17.)

Kepler is the first observer* who has made mention of

* There are, however, some earlier observations of the same fact. The tail of the comet of 582 appeared, according to Gregory of Tours, like the smoke