Nor again can the atmospheric glare' theory explain this phenomenon, unless we suppose the Earth's atmosphere to extend nearly to the Moon's distance,which is altogether incredible. Indeed, even if we admitted this, the bright rays between which the dark rays are seen ought to grow brighter and brighter with increase of apparent distance from the Moon, which is the reverse of what is actually seen.

It remains, then, that we should account for the phenomenon by the theory to which we have been led by other considerations. In fact, since the meteoric theory has been shown to accord so well with other phenomena, while it derives a negative strength from the obvious flaws in all the other theories, we are justified in accepting with a certain degree of confidence any explanation of these dark radial bars which the meteoric theory may point to. In other words, instead of feeling bound to explain these dark bars before admitting the meteoric theory, we may employ the meteoric theory to supply the explanation we require.

Now, there are two phenomena-one belonging to the solar system, the other to the Earth's economy— which seem likely to aid us in this matter. One is the appearance of comets' tails; the other is the aspect of auroral streamers. As respects the former phenomenon, it is to be remarked that the directive forces, whatever they may be, which cause the tails of comets

cone), they shall subtend an angle four or five times as great as that subtended by the Moon's diameter.

to project from the Sun, reside undoubtedly in the solar globe, and act undoubtedly with very great energy on certain forms of matter near him. Hence, as we have abundant reason for believing that the corona is not free from a certain association with cometary matter, we need not be altogether surprised if we find in the corona evidences of the same sort of action that we recognise in the formation and projection of comets' tails. Yet again, as respects the second phenomenon, we have the striking evidence afforded by the spectroscope to show that a resemblance of some sort exists between the coronal light and that of our auroras; so that we are justified in finding some resemblance, and even in conceiving that some association exists, between the long straight streamers which form so remarkable a feature of the aurora borealis, and those straight radial bars, with dark intervening spaces, seen in the solar corona.

*

There is a circumstance which seems to render this relation more striking in the fact that the only explanation one can readily conceive of the observed characteristics of the auroral spectrum seems to bring us again upon that subject of meteoric astronomy which has thus far stood us in such useful stead. For

*It is worthy of notice that a difficulty exists in the relatively small section of the radial bars, both bright and dark; for this phenomenon would imply that the centre of that action to which these bars are due cannot have the dimensions of the solar globe which we are able to It seems far from unlikely (since indeed we have other evidence corroborating such a view) that the central and more condensed portions of the Sun's mass may be the real seat of this intense repulsive action.

measure.

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