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

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."'

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. 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.

As

CHAPTER VII.

PHYSICAL CONDITION OF THE SUN.

IN the course of the last four chapters a number of facts bearing on the physical condition of the Sun have been dealt with at greater or less length. Considered separately these facts are full of interest and seem to afford somewhat satisfactory information on the points they severally relate to. We feel little difficulty, for instance, in giving a general interpretation to the dark lines of the solar spectrum, regarding them as undoubtedly due to the existence of the vapours of certain metallic and other elements in the solar atmosphere. We regard with a certain confidence, again, the conception that the spots are depressions of greater or less depth, and further that the light received from the umbra of a spot shines through absorbing vapours, some of which exist at a greater pressure and at a lower temperature than over the rest of the photosphere. We are able, also, to form certain sufficiently definite opinions respecting the prominences, more particularly as regards the pressure at which their substance exists and the motions to which they are subjected. While, lastly, the

corona has been studied with results which cannot but be regarded as trustworthy.

But when we attempt to combine these several results, and further to determine what the general condition of that orb may be which presents these several features, we recognise at once that a problem of enormous difficulty lies before us. The more we have learned respecting the Sun, nay the more we have learned respecting those physical laws by which we are to interpret solar phenomena, the more insuperable have our difficulties become. It was easy to theorise when as yet but little was known. It was easy to suppose that the few physical laws we imagined we understood sufficed to account for all the phenomena presented by the solar orb. But as one fact after another has been discovered, the true complexity of the problem has been revealed to us; and as the physical laws which it is in our power to discuss and experiment on have been more carefully studied, we have begun to recognise how very limited our experience has hitherto been. It is not too much to say that theories respecting the Sun's physical condition which would have been regarded twenty years since as deserving of careful study, have now no worthier standing in science than the idea of Anaximander that the Sun is a great vessel filled with fire, at the top of which is an opening through which the fire escapes.'

It is easier to consider those facts which have revealed to us the enormous difficulty of the problem we are upon, than to present any considerations tending to render our conceptions clearer.