Now, if the atmospheric glare theory is true, all the cone O c c' in our atmosphere is illuminated at the time of central eclipse except only the core belonging to the cone o m. This is certain, because we see the Moon dark and the corona bright; so that we require om to be dark and the remainder of o cc' to be bright. Now, so far as undeflected solar rays are concerned, the whole region a b' is in shadow. The light from the prrominences can get into this region, and so perhaps can solar light deflected by some possible action at the Moon's surface. But the problem which the supporters of the atmospheric glare' theory have to solve is to get the light into the cone occ', growing brighter and brighter up to the very boundary of the dark cone mo (to correspond with the increase of the corona's light up to the Moon's limb), and there suddenly ceasing. This done, they must show further that if another observer is stationed somewhere else within bb' as at o', the cones formed by lines from his eye to the Moon's limb and the corona's boundary, are

means of forming an opinion. From such observations made in 1819, Dalton estimated the extreme height of the auroral arch at 102 miles. Sir John Herschel estimated the height of the auroral arch seen on March 9, 1861, at 83 miles. Observations of meteors afford another means of solving the problem. A height greater than any of those above-mentioned has been deduced from observations of this sort. Lastly, polariscopic observations have led Liais and others to the conclusion that our atmosphere extends to a height of more than 200 miles from the Earth. The rarity of the atmosphere at such an elevation, assuming the law of diminution of density which prevails lower down to continue unchanged, would be altogether inconceivable. A quantity of air which a healthy person of average height could draw into his lungs at a single inspiration would suffice, when so reduced in density, to fill a sphere exceeding in diameter the orbit of Jupiter.

BB

respectively dark and illuminated in exactly the same way—that is, they must show that the same regions of the air are at once illuminated and in darkness. This may fairly be regarded as impossible.

Yet even if this could be demonstrated, much more would still remain to be done before the atmospheric glare' theory could be regarded-I will not say as established-but as worthy of consideration. Until something of this sort has been done-and nothing of the sort has yet been attempted-we need not inquire how far those spectroscopic observations can be explained away which Professors Young and Harkness justly regard as of themselves demonstrating the nonterrestrial character of the coronal light.*

It appears, then, that whatever view we are to form of the actual constitution of the corona, we can at least have no doubt that it is a true solar appendage. There may not be any closer bond of union between the material substance (of whatever sort) which emits the light forming the corona than exists between the nucleus of a comet and the comet's tail. But certainly

It will be observed that the above considerations dispose of a modified form of the atmospheric glare theory more recently put forward, according to which some action on the Sun's rays as they pass near the Moon's surface may account for coronal phenomena. Such action does not render the atmospheric glare theory at all more satisfactory. We have, if the theory is true, a certain region of our atmosphere illuminated and a certain other region dark, and the theory does not in any way explain how this comes about. Moreover, supposing it did explain the matter for one observer at one moment, the explanation would not avail to show how in the case of another observer that same illuminated region would be dark and that same dark region illuminated at the very same moment.

the evidence seems to force on us the conclusion that a relation as unquestionable associates the corona and the Sun as that which compels us to regard the tail and coma as real appendages of a comet.

The corona thus viewed becomes one of the most important and interesting of all the phenomena of the solar system. We no longer have to deal with sunbeams shining through our atmosphere, or with mirages in some lunar envelope, but with luminous spaces of inconceivably vast extent. Let us consider what evidence we already have bearing on the nature of this wonderful solar appendage. We shall then more justly appreciate the interest attaching to those efforts which are being made to gain fresh information.

The general aspect of the corona, as described in the preceding pages, does not suggest the idea that we have to deal solely with a solar atmosphere. Those radial projections are not the appearances we should expect to find in an atmospheric envelope. Nor again is it easy to understand how the irregular masses of light, the spots resembling hanks of thread in disorder, and other peculiarities of like nature, can be accounted for on the theory that the corona is a solar atmosphere.

But this view of the corona may be regarded as disposed of completely by the nature of the lines seen in the spectra of the coloured prominences. We have seen (p. 295) that the gases forming the prominences probably exist at a comparatively low pressure--that almost certainly the pressure near the summits of the

loftier prominences falls very far below the atmospheric pressure at the summit of Dhawala Giri and Mount Everest. Now, if we supposed the extension of the corona limited to that shown in Whipple's photograph, we should yet, on the supposition that the corona is an atmosphere, deduce a pressure far greater than this, let our estimate of the tenuity of the upper parts of such an atmosphere be what it (reasonably) may. But when we remember that under favourable circumstances the corona has been seen to extend to a distance very far exceeding the diameter of the eclipsed Sun, so that its depth (still regarding it as an atmosphere) would be more than a million of miles, or exceed sixfold the height of the loftiest solar prominences, we find ourselves compelled to reject the idea that we have indeed to deal in the main with a solar atmosphere properly so called.*

We conclude, then, that the matter (of whatever sort) existing where we know that the coronal beams extend, does not constitute a solar envelope. This being the case, we have to account for its subsistence or continuance in these regions, by some other conception than that of the combined forces of attraction and molecular or atomic repulsion which keep an atmosphere in equilibrium (statical or dynamical, as the case

*These remarks relate to the theory that the whole extent of the corona is atmospheric. It is by no means impossible that an atmosphere of enormous extent surrounds the Sun, reaching far above the loftiest prominences. It will be seen, indeed, further on, that the phenomena of the corona can hardly be explained without assuming that such an atmosphere exists.

may be). Now, there may be, and very probably there is, in partial question here the action of repulsive forces exerted by the Sun. Most unquestionably, as Sir John Herschel has pointed out, the Sun does exert repulsive forces, and those of a magnitude inconceivably enormous. But, in the absence of any exact knowledge of the condition of the coronal matter, or of the nature and mode of action of solar repulsive forces, we must for the present limit our consideration to those forms of solar action which we can measure and estimate. We must inquire how matter swayed wholly or principally by gravitation might remain in the Sun's neighbourhood without being brought to his surface. We know that this can only happen when that matter is in motion with suitable velocity. Place any particle at rest at the distance of Mercury, and that particle would move off towards the Sun, and in the course of about fifteen days and a half it would fall upon that luminary. But endow the particle with Mercury's velocity, no matter in what direction (except directly towards the Sun's globe), and it will revolve around the Sun in an orbit having the same mean distance as Mercury's. It has become at once an attendant on the Sun, or we may say it is now a solar appendage. Take, then, a million, or a million millions, of such bodies and give them adequate velocities, even though in a million different directions, and the Sun forthwith has as an appendage a cloud of cosmical bodies, which will continue for ever, or for an indefinitely long period, as a cloud appendage. It will not be fixed-