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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. It must be added, however, that although Angstrom has announced that the zodiacal light gives the same spectrum as the aurora, recent observations by Liais appear to show that its spectrum is continuous.

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 argu

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 arc separating its base from the Sun's place is least at the time when the light is first visible after sunset or before sunrise.

ments by which all other theories of the zodiacal light may be negatived. In the Monthly Notices for November 1870 of the present year there is 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 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.

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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 consideration 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.*

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 1870-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."'

Writing in November 1870, and summing up the evidence obtained up to that time, I made, in the first edition of this work, the following remarks:- 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 evidence against all other theories but one, and that one theory is confirmed by line after line of positive reasoning. To doubt what general view we should form of the corona and zodiacal light under these circumstances seems to me to savour-not of that wise caution which prevents the true philosopher from overlooking difficulties, but rather—of an inaptitude to estimate the value of evidence. As to details we may be doubtful. Other matter than meteoric or cometic matter may well be in question; other modes of producing light, save heat, electricity, or direct illumination, may be in operation in this case; and lastly there may be other forces at work than the attractive influence of solar gravity, or the form of repulsive force evidenced by the phenomena of comets. As regards, also, the true shape and position of the coronal and zodiacal appendage-and yet more as regards its variations in shape-we may still have much to learn. But of the general fact that the corona and zodiacal light form a solar appendage of amazing extent and importance, that they are not merely terrestrial phenomena, but worthy of all the attention astronomers

and physicists can direct to them, it seems to me that no reasonable doubts can any longer be entertained.'

I have purposely suffered the reasoning with which this chapter was brought to a close in the first edition to remain almost untouched, in order that the results of later observations, as well as the theoretical considerations based upon such observations, might hold their due place in the history of this portion of my subject. The reasoning in the last twenty-four pages was based on observations made before the eclipse of December 1870; but that foundation was quite wide enough for its support. I do not find that any of the conclusions enforced above require to be abandoned— or even modified-in consequence of later observations. On the contrary, they have all been very strikingly enforced. But fortunately these observations have not availed only to make plain what had before been established. Had they done this only, they would have been wholly wasted; since scientific observations should have a nobler end than to supply simple proofs of facts already in reality demonstrated. All who heard of the laborious and costly preparations for the eclipse of December 1870 hoped that some new discoveries would reward the observers, and this hope was not disappointed. On the contrary, few eclipses have revealed more striking facts than were discovered on that little favourable occasion-the socalled Mediterranean eclipse.

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