Page images
PDF
EPUB

among the bright lines seen in the auroral spectrum is one agreeing in position with a line of iron, and it has been thought probable by Stewart, Angström, and others, that the light of the aurora is due in part to electrical discharges taking place in the upper regions of our atmosphere. But how can iron reach those upper regions save from meteoric visitants? and what can be more likely than that iron does actually reach the upper regions of our air in this way, when we consider how largely iron enters into the composition of nearly all the meteoric masses which have been so far subjected to analysis?

But it may be reasoned that if this is indeed the case, if solar action in the upper regions of the Earth's atmosphere (or terrestrial action excited in some way by the Sun) can cause these electrical discharges, then solar action exerted directly on similar material in the other parts of the Sun's domain ought to excite a similar luminosity, and that therefore we ought at night to see some traces-faint, it may be, but still recognisable of this particular form of phospho

rescence.

This amounts, in fact, to the consideration, that the limits of the corona as seen during total eclipse ought not to mark the real limits of the Sun's light-exciting action. And even supposing that but a small proportion of the coronal light is really due to this form of action-that is, to electrical discharges-it would still be likely that some signs of those meteoric systems

whose illumination by the Sun has been here regarded as the cause of the corona, should be seen beyond the observed limits of that aureola of light.

Here again it happens (and I know no surer test of the justice of a theory) that we have been led to see that a certain phenomenon should be manifested, which actually is a familiar phenomenon of the heavens, and which would most assuredly have required explanation if it had not thus been led up to.

For precisely in that region where we should expect to find a faint gleam of light-precisely where the known relations of the planetary scheme would lead us to look for an abundance of meteoric material, there appears that mysterious luminosity known as the Zodiacal Light. And just as our reasoning has led us to regard the meteoric appendage of the Sun—an appendage really extending far beyond the orbits of the most distant planets—as variable in configuration, however constant when regarded as a whole, so we find the zodiacal light varying from year to year in brightness, and extent, and position. Its light, again, presents that faint tinge of pink which has been recognised in the corona and forms so marked a phenomenon of the aurora. It has even been observed to fluctuate in brightness and to be traversed by flickerings and coruscations-to thrill, as it were, responsive to mysterious influences, precisely as we should expect on the supposition that it is analogous to the aurora. But lastly, as if to remove all doubt, comes the fact

that the light of the zodiacal gleam gives the very same spectrum as the aurora, a spectrum which, as we have already seen, resembles closely that of the corona.

I have said that if we were not led by our consideration of the corona to anticipate as it were the existence of the zodiacal light, we should have to explain this latter phenomenon. Let us view the zodiacal light apart for a moment.

We have a glow or radiance which is commonly seen along the zodiac, that is, in the region of the sky where planets are to be looked for. This glow obeys all the usual laws observed in the motion of celestial bodies. It rises and sets precisely as the fixed stars and planets are observed to do. If we travel towards or from the equator, it is seen higher or lower, precisely as the part of a planet's path near the Sun's place would shift. It presents all those peculiarities, in fine, which force on the astronomer the conclusion that he has to do with an extra-terrestrial phenomenon, and a further peculiarity showing that it is a phenomenon specially associated with the planetary scheme.*

*Space forbids my entering here into a consideration of the arguments by which all other theories of the zodiacal light may be negatived. In the Monthly Notices for November of the present year there will be (I write this in October) a paper of mine, showing by mathematical considerations of a very plain kind that the only admissible theory of the zodiacal light is that same theory which I have here urged in explanation of the corona,-the theory, namely, that there exists around the Sun a region of meteoric matter continually changing in configuration and constitution, owing to the continual arrival and departure of individual meteors. Every peculiarity of the zodiacal light is in accordance with this view, and many of its features, as also many features of the

It has therefore been regarded by every astronomer who has studied the subject with due attention, as indicating the existence of a lens-shaped region around the Sun within which cosmical matter is strewn with considerable profusion.

Now, regarding the zodiacal light in this way, and considering its general aspect when seen under favourable conditions, the conclusion is forced upon us that the density of aggregation of this cosmical material increases with proximity to the solar globe. For we see that the borders of the zodiacal light are very much fainter than the central part or core of the gleam. We see, again, that the light grows brighter and brighter towards the horizon, that is, with proximity to the place of the Sun. And these relations are observed even in those countries where at certain seasons the zodiacal light is vertical, and where therefore the actual are separating its base from the Sun's place is least at the time when the light is first visible after sunset or before sunrise.

The obvious conclusion is, that if the zodiacal light could be traced yet farther towards the Sun's place this increase of lustre would continue, and that therefore all round the Sun there would be seen a luminosity corresponding precisely with the observed aspect of the corona. So that again we are led by the con

corona, seem individually explicable on no other hypothesis; while assuredly no other theory can account for all the observed peculiarities of these remarkable phenomena of our system.

sideration of a well-recognised feature of the solar system to the conclusion that the corona is a phenomenon to be expected when the Sun is totally eclipsed, rather than one whose appearance should be regarded as surprising and perplexing.*

In conclusion I would remark that while the exact nature of the corona remains-and perhaps may long remain a mystery, I know of few instances in which the general nature of a phenomenon has seemed more satisfactorily exhibited than in the case of the corona and zodiacal light. We have the strongest negative

[ocr errors]

To the considerations above adduced, I may add some which are touched upon in a paper of mine which appeared in Fraser's Magazine for February last :- There is one feature of comets' tails,' I there point out, which has long since attracted attention, and will remind the reader of the peculiarities common to the zodiacal light and the aurora. I refer to the sudden changes of brilliancy, the flickerings or coruscations, and the instantaneous lengthening and shortening of these mysterious appendages. Olbers spoke of "explosions and pulsations, which in a few seconds went trembling through the whole length of a comet's tail with the effect now of lengthening now of abridging it by several degrees.” And the eminent mathematician Euler was led by the observation of

similar appearances to put forward the theory "that there is a great affinity between these tails, the zodiacal light, and the aurora borealis." The late Admiral Smyth, commenting on this opinion of Euler's, remarks that most reasoners seem now to consider comets' tails as consisting of electric matter" (that is, I suppose, indicating the occurrence of electric discharges), adding that "this would account for the undulations and other appearances which have been noticed-as, for instance, that extraordinary one seen by Chladni in the comet of 1811, when certain undulatory ebullitions rushed from the nucleus to the end of the tail, a distance of more than ten millions of miles in two or three seconds of time." To this may be added the theory suggested by Sir John Herschel, that the matter forming the zodiacal light is “loaded, perhaps, with the actual materials of the tails of millions of comets, which have been stripped of these appendages in the course of successive passages round the immediate neighbourhood of the Sun."'

« PreviousContinue »