Ten years ago, all that astronomers could hope to do with comets was to note their appearance and changes of appearance when viewed with high telescopic powers. There was one instrument, indeed, the polariscope, which afforded doubtful evidence respecting the quality of the light we receive from comets, and thus allowed astronomers to form vague guesses respecting the structure of these mysterious wanderers. But beyond the unsatisfactory indications of this instrument, astronomers had no means whatever of ascertaining the physical nature of comets.
At present, however, an instrument of incomparably higher powers is applicable to the inquiry. The spectroscope has the power of revealing, not only the general character of any substance which is a source of light, but even of exhibiting, in many instances, the elementary constitution of such a substance. The indications of this wonderful instrument of analysis are not affected by the distance or dimensions of the object under examination. So long as the object is luminous the spectroscope will tell us with the utmost certainty whether the light is inherent or reflected; and if the light is inherent-that is, if the object is self-luminous - the spectroscope will tell us with the utmost certainty what terrestrial elements (if any) exist in the constitution of the object. It is with the revelations of the spectroscope respecting Brorsen's comet that I now propose to deal. I must make a few preliminary remarks, however, respecting the various peculiarities of structure which have been presented by comets.
I assume that my readers are familiar with the general appearance presented by comets-at least by those which are visible to the naked eye. It may be necessary to note, however, of the three features commonly recognised in comets-viz. the nucleus, coma, and tail-the coma alone is invariably exhibited. A comet which has neither nucleus nor tail presents simply a round mass of vapour slightly condensed towards the centre. The nucleus, when seen, appears as a bright point within the condensed part of a comet. The tail, as every one knows, is a long train of light issuing from the head.
It was noted in very early times that comets are almost perfectly translucent. . This peculiarity has been confirmed by modern and more exact observations. Sir W. Herschel watched the central passage of a comet over the fainter component of a double star; and he could detect no diminution of the star's brilliancy. Similar observations were made by MM. Olbers and Struve. Sir John Herschel watched the passage of Biela's comet over a small cluster of very faint telescopic stars. The slightest haze would have obliterated the cluster, yet no appreciable effect was produced by the interposition of cometic matter having a thickness (according to Herschel's estimate) of 50,000 miles. And there is another remarkable evidence of tenuity. From recognised optical principles, a star seen through the globular head of a comet, should appear displaced from its true position just as any object seen (non-centrally) through a globular decanter full of water seems thrown out of its true place. The astronomer Bessel made an observation on a star which approached within about eight seconds of the nucleus of Halley's comet, and he found that the place of the star was not affected to an appreciable extent.
Whether the nucleus of a comet is solid or not had long been a disputed point among astronomers. With telescopes of moderate power the bright point within the coma presents an appearance of solidity which might readily deceive the observer. But with an increase of power the nucleus assumes a different appearance. Instead of having a well-defined outline it appears to merge into the coma by a somewhat rapid gradation - but not by an abrupt variation - of light. Good observers have reported the extinction of telescopic stars behind the nuclei of comets, but there are peculiar difficulties about an observation of this sort; and it is very difficult to determine whether a star is really concealed from view by the interposition of the nucleus or simply obliterated by the glare of light.
The tail of a comet appears nearly always as an extension from the coma, and a dark interval is usually seen between the head and the tail. But there is an immense variety in the configuration of comets' tails. The comet of 1744 had six tails spread out like a fan. The comet of 1807 had two tails - both turned from the sun. The comet of 1823 had also two tails, but one was turned almost directly towards the sun. Other comets have had lateral tails.
The processes which seem to be passed through by comets during their approach towards and recession from the sun have proved very perplexing to astronomers and physicists. When first seen a comet usually appears as a light roundish cloud with a point of brighter light near the centre. As it approaches the sun the comet appears to grow considerably brighter on the side turned towards him. An emanation of light seems to proceed towards the sun for a short distance and then to curl backwards and stream out in a contrary direction. Gradually the backward streaming rays extend to a greater distance the nucleus continuing to throw out matter towards the sun. Thus the tail is formed; and it is often thrown out to a distance of many millions of miles in a few hours.
One of the most singular facts connected with the approach and recession of a comet, is the peculiarity that the comet grows gradually smaller and smaller as it approaches perihelion, and swells out in a corresponding manner as it passes away from the sun. The comet of 1652 was observed by Hevelius to increase so rapidly in dimensions as it passed away from the sun, that between December 20 and January 12 its volume had increased in the proportion of about 13,800 to 1. When it was last visible this comet exceeded the sun in volume. This observation, on which much doubt had been thrown, has been confirmed by the researches of the best modern observers. M. Struve measured Encke's comet as it approached the sun towards the end of the year 1828. He found that between October 28 and December 24 the comet collapsed to about the sixteen-thousandth part of its original volume. Sir John Herschel found in like manner that Halley's comet when passing away from the sun increased in volume upwards of fortyfold in a single week.