A Popular History of Astronomy During the Nineteenth Century / Fourth Edition

by Agnes M. Clerke

The spectrum of the tail was, in this comet, found to be not essentially different from that of the head. Professor Wright of Yale College ascertained a large percentage of its light to be polarized in a plane passing through the sun, and hence to be reflected sunlight.[1303] A faint continuous spectrum corresponded to this portion of its radiance; but gaseous emissions were also present. At Potsdam, on June 30, the hydro-carbon bands were indeed traced by Vogel to the very end of the tail;[1304] and they were kept in sight by Young at a greater distance from the nucleus than the more equably dispersed light. There seems little doubt that, as in the solar corona, the relative strength of the two orders of spectra is subject to fluctuations.

The comet of 1881 iii. was thus of signal service to science. It afforded, when compared with the comet of 1807, the first undeniable example of two such bodies travelling so nearly in the same orbit as to leave absolutely no doubt of the existence of a genetic tie between them. Cometary photography came to its earliest fruition with it; and cometary spectroscopy made a notable advance by means of it. Before it was yet out of sight, it was provided with a successor.

At Ann Arbor Observatory, Michigan, on July 14, a comet was discovered by Dr. Schaeberle, which, as his claim to priority is undisputed, is often allowed to bear his name, although designated, in strict scientific parlance, comet 1881 iv. It was observed in Europe after three days, became just discernible by the naked eye at the end of July, and brightened consistently up to its perihelion passage, August 22, when it was still about fifty million miles from the sun. During many days of that month, the uncommon spectacle was presented of two bright comets circling together, though at widely different distances, round the North pole of the heavens. The newcomer, however, never approached the pristine lustre of its predecessor. Its nucleus, when brightest, was comparable to the star Cor Caroli, a narrow, perfectly straight ray proceeding from it to a distance of 10°. This was easily shown by Brédikhine to belong to the hydrogen type of tails;[1305] while a "strange, faint second tail, or bifurcation of the first one," observed by Captain Noble, August 24,[1306] fell into the hydro-carbon class of emanations. It was seen, August 22 and 24, by Dr. F. Terby of Louvain,[1307] as a short nebulous brush, like the abortive beginning of a congeries of curving trains; but appeared no more. Its well-attested presence was significant of the complex constitution of such bodies, and the manifold kinds of action progressing in them.

The only peculiarity in the spectrum of Schaeberle's comet consisted in the almost total absence of continuous light. The carbon-bands were nearly isolated and very bright. Barely from the nucleus proceeded a rainbow-tinted streak, indicative of solid or liquid matter, which, in this comet, must have been of very scanty amount. Its visit to the sun in 1881 was, so far as is known, the first. The elements of its orbit showed no resemblance to those of any previous comet, nor any marked signs of periodicity. So that, although it may be considered probable, we do not know that it is moving in a closed curve, or will ever again penetrate the precincts of the solar system. It was last seen from the southern hemisphere, October 19, 1881.

The third of a quartette of lucid comets visible within sixteen months, was discovered by Mr. C. S. Wells at the Dudley Observatory, Albany, March 17, 1882. Two days later it was described by Mr. Lewis Boss as "a great comet in miniature," so well defined and regularly developed were its various parts and appendages. Discernible with optical aid early in May, it was on June 5 observed on the meridian at Albany just before noon—an astronomical event of extreme rarity. Comet Wells, however, never became an object so conspicuous as to attract general attention, owing to its immersion in the evening twilight of our northern June.

But the study of its spectrum revealed new facts of the utmost interest. All the comets till then examined had been found (with the two transiently observed exceptions already mentioned) to conform to one invariable type of luminous emission. Individual distinctions there had been, but no specific differences. Now all these bodies had kept at a respectful distance from the sun; for of the great comet of 1880 no spectroscopic inquiries had been made. Comet Wells, on the other hand, approached its surface within little more than five million miles on June 10, 1882; and the vicinity had the effect of developing a novel feature in its incandescence.

During the first half of April its spectrum was of the normal type, though the carbon bands were unusually weak; but with approach to the sun they died out, and the entire light seemed to become concentrated into a narrow, unbroken, brilliant streak, hardly to be distinguished from the spectrum of a star. This unusual behaviour excited attention, and a strict watch was kept. It was rewarded at the Dunecht Observatory, May 27, by the discernment of what had never before been seen in a comet—the yellow ray of sodium.[1308] By June 1, this had kindled into a blaze overpowering all other emissions. The light of the comet was practically monochromatic; and the image of the entire head, with the root of the tail, could be observed, like a solar prominence, depicted, in its new saffron vesture of vivid illumination, within the jaws of an open slit.

At Potsdam, the bright yellow line was perceived with astonishment by Vogel on May 31, and was next evening identified with Fraunhofer's "D." Its character led him to infer a very considerable density in the glowing vapour emitting it.[1309] Hasselberg founded an additional argument in favour of the electrical origin of cometary light on the changes in the spectrum of comet Wells.[1310] For they were closely paralleled by some earlier experiments of Wiedemann, in which the gaseous spectra of vacuum tubes were at once effaced on the introduction of metallic vapours. It seemed as if the metal had no sooner been rendered volatile by heat, than it usurped the entire office of carrying the discharge, the resulting light being thus exclusively of its production. Had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. Similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent. For the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons.

Sir William Huggins succeeded in photographing the spectrum of comet Wells by an exposure of one hour and a quarter.[1311] The result was to confirm the novelty of its character. None of the ultra-violet carbon groups were apparent; but certain bright rays, as yet unidentified, had imprinted themselves. Otherwise the spectrum was strongly continuous, uninterrupted even by the Fraunhofer lines detected in the spectrum of Tebbutt's comet. Hence it was concluded that a smaller proportion of reflected light was mingled with the native emissions of the later arrival.

All that is certainly known about the extent of the orbit traversed by the first comet of 1882 is that it came from, and is now retreating towards, vastly remote depths of space. An American computer[1312] found a period indicated for it of no less than 400,000 years; A. Thraen of Dingelstädt arrived at one of 3617.[1313] Both are perhaps equally insecure.