[1184] Bull. Astr., t. ii., p. 223.
[1185] Montpellier Méms., t. viii., p. 242.
[1186] Amer. Jour., vol. xxxviii. (1864), p. 1.
[1187] Wolf, Bull. Astr., t. ii., p. 76.
CHAPTER X
RECENT COMETS
On the 2nd of June, 1858, Giambattista Donati discovered at Florence a feeble round nebulosity in the constellation Leo, about one-tenth the diameter of the full moon. It proved to be a comet approaching the sun. But it changed little in apparent place or brightness for some weeks. The gradual development of a central condensation of light was the first symptom of coming splendour. At Harvard, in the middle of July, a strong stellar nucleus was seen; on August 14 a tail began to be thrown out. As the comet wanted still over six weeks of the time of its perihelion-passage, it was obvious that great things might be expected of it. They did not fail of realisation.
Not before the early days of September was it generally recognised with the naked eye, though it had been detected without a glass at Pulkowa, August 19. But its growth was thenceforward surprisingly rapid, as it swept with accelerated motion under the hindmost foot of the Great Bear, and past the starry locks of Berenice. A sudden leap upward in lustre was noticed on September 12, when the nucleus shone with about the brightness of the pole-star, and the tail, notwithstanding large foreshortening, could be traced with the lowest telescopic power over six degrees of the sphere. The appendage, however, attained its full development only after perihelion, September 30, by which time, too, it lay nearly square to the line of sight from the earth. On October 10 it stretched in a magnificent scimitar-like curve over a third and upwards of the visible hemisphere, representing a real extension in space of fifty-four million miles. But the most striking view was presented on October 5, when the brilliant star Arcturus became involved in the brightest part of the tail, and during many hours contributed, its lustre undiminished by the interposed nebulous screen, to heighten the grandeur of the most majestic celestial object of which living memories retain the impress. Donati's comet was, according to Admiral Smyth's testimony,[1188] outdone "as a mere sight-object" by the great comet of 1811; but what it lacked in splendour, it surely made up in grace, and variety of what we may call "scenic" effects.
Some of these were no less interesting to the student than impressive to the spectator. At Pulkowa, on the 16th September, Winnecke,[1189] the first director of the Strasburg Observatory, observed a faint outer envelope resembling a veil of almost evanescent texture flung somewhat widely over the head. Next evening, the first of the "secondary" tails appeared, possibly as part of the same phenomenon. This was a narrow straight ray, forming a tangent to the strong curve of the primary tail, and reaching to a still greater distance from the nucleus. It continued faintly visible for about three weeks, during part of which time it was seen in duplicate. For from the chief train itself, at a point where its curvature abruptly changed, issued, as if through the rejection of some of its materials, a second beam nearly parallel to the first, the rigid line of which contrasted singularly with the softly diffused and waving aspect of the plume of light from which it sprang. Olbers's theory of unequal repulsive forces was never more beautifully illustrated. The triple tail seemed a visible solar analysis of cometary matter.
The processes of luminous emanation going on in this body forcibly recalled the observations made on the comets of 1744 and 1835. From the middle of September, the nucleus, estimated by Bond to be under five hundred miles in diameter, was the centre of action of the most energetic kind. Seven distinct "envelopes" were detached in succession from the nebulosity surrounding the head, and after rising towards the sun during periods of from four to seven days, finally cast their material backward to form the right and left branches of the great train. The separation of these by an obscure axis—apparently as black, quite close up to the nucleus, as the sky—indicated for the tail a hollow, cone-like structure;[1190] while the repetition of certain spots and rays in the same corresponding situation on one envelope after another served to show that the nucleus—to some local peculiarity of which they were doubtless due—had no proper rotation, but merely shifted sufficiently on an axis to preserve the same aspect towards the sun as it moved round it.[1191] This observation of Bond's was strongly confirmatory of Bessel's hypothesis of opposite polarities in such bodies' opposite sides.