Its tail was only equalled (if it were equalled) in length by that of the comet of 1843. It extended in space to the vast distance of 200 millions of miles from the head; but, so imperfectly were its proportions displayed to terrestrial observers, that it at no time covered an arc of the sky of more than 30°. This apparent extent was attained, during a few days previous to September 25, by a faint, thin, rigid streak, noticed only by a few observers—by Elkin at the Cape Observatory, Eddie at Grahamstown, and Cruls at Rio Janeiro. It diverged at a low angle from the denser curved train, and was produced, according to Brédikhine,[1334] by the action of a repulsive force twelve times as strong as the counter-pull of gravity. It belonged, that is, to type 1; while the great bifurcate appendage, obvious to all eyes, corresponded to the lower rate of emission characteristic of type 2. This was remarkable for the perfect definiteness of its termination, for its strongly-forked shape, and for its unusual permanence. Down to the end of January, 1883, its length, according to Schmidt's observations, was still 93 million miles; and a week later it remained visible to the naked eye, without notable abridgment.
Most singular of all was an anomalous extension of the appendage towards the sun. During the greater part of October and November, a luminous "tube" or "sheath," of prodigious dimensions, seemed to surround the head, and project in a direction nearly opposite to that of the usual outpourings of attentuated matter. ([See Plate III.]) Its diameter was computed by Schmidt to be, October 15, no less than four million miles, and it was described by Cruls as a "truncated cone of nebulosity," stretching 3° or 4° sunwards.[1335] This, and the entire anterior part of the comet, were again surrounded by a thin, but enormously voluminous paraboloidal envelope, observed by Schiaparelli for a full month from October 19.[1336] There can be little doubt that these abnormal effluxes were a consequence of the tremendous physical disturbance suffered at perihelion; and it is worth remembering that something analogous was observed in the comet of 1680 (Newton's), also noted for its excessively close approach to the sun, and possibly moving in a related orbit. The only plausible hypothesis as to the mode of their production is that of an opposite state of electrification in the particles composing the ordinary and extraordinary appendages.
The spectrum of the great comet of 1882 was, in part, a repetition of that of its immediate predecessor, thus confirming the inference that the previously unexampled sodium-blaze was in both a direct result of the intense solar action to which they were exposed. But the D line was, this time, not seen alone. At Dunecht, on the morning of September 18, Drs. Copeland and J. G. Lohse succeeded in identifying six brilliant rays in the green and yellow with as many prominent iron-lines;[1337] a very significant addition to our knowledge of cometary constitution, and one which lent countenance to Brédikhine's assumption of various kinds of matter issuing from the nucleus with velocities inversely as their atomic weights. All the lines equally showed a slight displacement, indicating a recession from the earth of the radiating body at the rate of 37 to 46 miles a second. A similar observation, made by M. Thollon at Nice on the same day, gave emphatic sanction to the spectroscopic method of estimating movement in the line of sight. Before anything was as yet known of the comet's path or velocity, he announced, from the position of the double sodium-line alone, that at 3 p.m. on September 18 it was increasing its distance from our planet by from 61 to 76 kilometres per second.[1338] M. Bigourdan's subsequent calculations showed that its actual swiftness of recession was at that moment 73 kilometres.
Changes in the inverse order to those seen in the spectrum of comet Wells soon became apparent. In the earlier body, carbon bands had died out with approach to perihelion, and had been replaced by sodium emissions; in its successor, sodium emissions became weakened and disappeared with retreat from perihelion, and found their substitute in carbon bands. Professor Riccò was, in fact, able to infer, from the sequence of prismatic phenomena, that the comet had already passed the sun; thus establishing a novel criterion for determining the position of a comet in its orbit by the varying quality of its radiations.
Recapitulating what was learnt from the five conspicuous comets of 1880-82, we find that the leading facts acquired to science were these three. First, that comets may be met with pursuing each other, after intervals of many years, in the same, or nearly the same, track; so that identity of orbit can no longer be regarded as a sure test of individual identity. Secondly, that at least the outer corona may be traversed by such bodies with perfect apparent impunity. Finally, that their chemical constitution is highly complex, and that they possess, in some cases at least, a metallic core resembling the meteoric masses which occasionally reach the earth from planetary space.
A group of five comets, including Halley's, own a sort of cliental dependence upon the planet Neptune. They travel out from the sun just to about his distance from it, as if to pay homage to a powerful protector, who gets the credit of their establishment as periodical visitors to the solar system. The second of these bodies to affect a looked-for return was a comet—the sixteenth within ten years—discovered by Pons, July 20, 1812, and found by Encke to revolve in an elliptic orbit, with a period of nearly 71 years. It was not, however, until September 1, 1883, that Mr. Brooks caught its reappearance; it passed perihelion January 25, and was last seen June 2, 1884. At its brightest, it had the appearance of a second magnitude star, furnished with a poorly developed double tail, and was fairly conspicuous to the naked eye in Southern Europe, from December to March. One exceptional feature distinguished it. Its fluctuations in form and luminosity were unprecedented in rapidity and extent. On September 21, Dr. Chandler[1339] observed it at Harvard as a very faint, diffused nebulosity, with slight central condensation. On the next night, there was found in its place a bright star of the eighth magnitude, scarcely marked out, by a bare trace of environing haze, from the genuine stars it counterfeited. The change was attended by an eight-fold augmentation of light, and was proved by Schiaparelli's confirmatory observations[1340] to have been accomplished within a few hours. The stellar disguise was quickly cast aside. The comet appeared on September 23 as a wide nebulous disc, and soon after faded down to its original dimness. Its distance from the sun was then no less than 200 million miles, and its spectrum showed nothing unusual. These strange variations recurred slightly on October 15, and with marked emphasis on January 1, when they were witnessed with amazement, and photometrically studied by Müller of Potsdam.[1341] The entire cycle this time was run through in less than four hours—the comet having, in that brief space, condensed, with a vivid outburst of light, into a seeming star, and the seeming star having expanded back again into a comet. Scarcely less transient, though not altogether similar, changes of aspect were noted by M. Perrotin,[1342] January 13 and 19, 1884. On the latter date, the continuous spectrum given by a reddish-yellow disc surrounding the true nucleus seemed intensified by bright knots corresponding to the rays of sodium.
A comet discovered by Mr. Sawerthal at the Royal Observatory, Cape of Good Hope, February 19, 1888, distinguished itself by blazing up, on May 19, to four or five times its normal brilliancy, at the same time throwing out from the head two lustrous lateral branches.[1343] These had, on June 1, spread backward so as to join the tail, with an effect like the playing of a fountain; ten or eleven days later, they had completely disappeared, leaving the comet in its former shape and insignificance. Its abrupt display of vitality occurred two full months after perihelion.
On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva, New York, eminent as a successful comet-hunter, secured one of his customary trophies. The faint object in question was moving through the constellation Cetus, and turned out to be a member of Jupiter's numerous family of comets, revolving round the sun in a period of seven years. Its past history came then, to a certain extent, within the scope of investigation, and proved to have been singularly eventful; nor had the body escaped scatheless from the vicissitudes to which it had been exposed. Observing from Mount Hamilton, August 2 and 5, Professor Barnard noticed this comet (1889, v.) to be attended in its progress through space by four outriders, "The two brighter companions" (the fainter pair survived a very short time) "were perfect miniatures," Professor Barnard tells us,[1344] "of the larger comet, each having a small, fairly defined head and nucleus, with a faint, hazy tail, the more distant one being the larger and less developed. The three comets were in a straight line, nearly east and west, their tails lying along this line. There was no connecting nebulosity between these objects, the tails of the two smaller not reaching each other, or the large comet. To all appearance they were absolutely independent comets." Nevertheless, Spitaler, at Vienna, in the early days of August, perceived, as it were, a thin cocoon of nebulosity woven round the entire trio.[1345] One of them faded from view September 5; the other actually outshone the original comet on August 31, but was plainly of inferior vitality. It was last seen by Barnard on November 25, with the thirty-six inch refractor, while its primary afforded an observation for position with the twelve-inch, March 20, 1890.[1346] A cause for the disruption it had presumably undergone had, before then, been plausibly assigned.
The adventures of Lexell's comet have long served to exemplify the effects of Jupiter's despotic sway over such bodies. Although bright enough in 1770 to be seen with the naked eye, and ascertained to be circulating in five and a half years, it had never previously been seen, and failed subsequently to present itself. The explanation of this anomaly, suggested by Lexell, and fully confirmed by the analytical inquiries both of Laplace and Leverrier,[1347] was that a very close approach to Jupiter in 1767 had completely changed the character of its orbit, and brought it within the range of terrestrial observation; while in 1779, after having only twice traversed its new path (at its second return it was so circumstanced as to be invisible from the earth), it was, by a fresh encounter, diverted into one entirely different. Yet the possibility was not lost sight of that the great planet, by inverting its mode of action, might undo its own work, and fling the comet once more into the inner part of the solar system. This possibility seemed to be realized by Chandler's identification of Brooks's and Lexell's comet.[1348] An exceedingly close approach to Jupiter in 1886 had, he found reason to believe, produced such extensive alterations in the elements of its motion as to bring the errant body back to our neighbourhood in 1889. But his inference, though ratified by Mr. Charles Lane Poor's preliminary calculations, proved dubious on closer inquiry, and was rendered wholly inadmissible by the circumstances attending the return of Brooks's comet in 1896.[1349] The companion-objects watched by Barnard in 1889 had by that time, perhaps, become dissipated in space, for they were not redetected. They represented, in all likelihood, wreckage from a collision with Jupiter, dating, perhaps, so far back as 1791, when Mr. Lane Poor found that one of the fateful meetings to which short-period comets are especially subject had taken place.
The Lexell-Brooks case was almost duplicated by the resemblance to De Vico's lost comet of 1844[1350] of one detected November 20, 1894, by Edward, son of Lewis Swift. Schulhof[1351] announced the identity, and Chandler,[1352] under reserve, vouched for it. Had the comet continued to pursue the track laboriously laid down for it at Boston, and shown itself at the due epoch in 1900, its individuality might have been considered assured; but the formidable vicegerent of the sun once more interposed, and, in 1897, swept it out of the terrestrial range of view. Hence the recognition remains ambiguous.