Now it appears clear that this flight of cosmical bodies may be looked upon as constituting an extension of the comet - an invisible train as it were. But for the accident that the comet's track intersects the earth's path in space, we should have remained for ever ignorant of the fact that the comet has any other extent than that which is indicated by its telescopic figure. Now, however, that we know otherwise, we recognise the probability that other comets which have been looked upon as tailless may have invisible extensions reaching far behind them into space, or even completely around their orbit.
But the members of the November shooting-star system have been subjected to spectroscopic analysis. We know that they contain several terrestrial elements; and we recognise the probability that if we could examine one of them before its destruction (in traversing our own atmosphere) we should find a close resemblance in its constitution to that of those aerolites or meteorites which have reached the surface of the earth.
But here we encounter a new difficulty. One theory respecting the tails of comets has accounted for them by the supposition of a propulsive effect exerted by the solar rays; and another theory has ascribed them to the action of vapours ascending in the solar atmosphere. But if the tails of comets really consist of solid matter very widely dispersed, it must be quite evident that neither of these causes could suffice to account for the great extension of these appendages. Then the rapid manner in which the tails seem to be formed remains wholly mysterious. And we are also left without any explanation of the rapid change of position exhibited by the tail while the comet is sweeping around the sun at the time of nearest approach to that luminary. Sir John Herschel compared this motion to that of a stick whirled round by the handle - the whole extent of the tail partaking in the movement as if comet and tail formed a rigid mass.
The difficulties here discussed seem in the present state of our knowledge wholly insoluble. In fact, it seems impossible even to conceive of a solution to the last mentioned phenomenon, so long as we look upon the comet's tail as a distinct unvarying entity. For instance, if the tail, a hundred millions of miles long, which extended backwards from Halley's comet before, perihelion passage, consisted of the same matter as the tail which projected forwards to the same extent a few days later, then certainly there is nothing in our present experience of matter and its relations which can enable us to deal with so astounding a phenomenon. It will be understood, of course, Sir John Herschel does not say in so many words, that the tail of Halley's comet was brandished round in the manner described above, but that, although it appeared to move in this manner, it is impossible so to conceive of its motion.
We refrain, however, from speaking further on a point respecting which we have no means of reasoning satisfactorily. Mere guess-work is an altogether unprofitable resource in the discussion of scientific matters.
Now that we have so powerful an instrument of research as the spectroscope, there really seems hope that even the hitherto inscrutable mysteries presented by comets' tails may one day be interpreted. Each comet which has been subjected to spectroscopic analysis has revealed something new. Observations, such as those which have been made on Brorsen's comet, and on the two telescopic comets previously examined by Dr. Huggins, are not merely valuable in themselves, but as affording promise of what may be achieved when some brilliant comet shall be subjected to spectroscopic analysis. When we consider that all the comets yet examined have been absolutely invisible to the naked eye on the darkest night, whereas several of the great comets have blazed forth as the most conspicuous objects in the heavens, and have even been visible in the full splendour of the midday sun, we see good reason for the hope that far fuller information will be gained respecting the structure of comets so soon as one of the more important members of the family shall have paid us a visit.
Whenever such an event may happen it is not likely to find our spectroscopists unprepared. It is probable that, before long, every important observatory will be supplied with spectroscopes. Already some of the most powerful telescopes in use have been fitted with them. We hear also, that the giant reflector of the Parsons-town Observatory - commonly known as the Rosse telescope - has been armed with a spectroscope especially constructed for the purpose by Mr. Browning, F.R.A.S., the optician. Not only in England, but at the principal Continental observatories, spectroscopic work is in progress, and observers are daily becoming more and more familiar with the powers of the new analysis. Stars which are far too small to be viewed by the naked eye have already been examined with the spectroscope. The Padre Secchi at Rome has just published a list of minute red stars thus examined. It is such delicate work as this which will fit observers to deal with the difficulties involved in the spectroscopic analysis of comets.
We shall see when we come to deal with the second comet of the year 1868, that we have yet better reason than the analysis of Brorsen's comet has afforded, for hoping that before long we may have interesting and exact information respecting the structure of these mysterious wanderers. We may even hope to gain some knowledge respecting the purposes which comets subserve in the economy of the solar and sidereal systems.