a certain corona, resembling the Iris, surrounded the orb of the Sun, and obscured his light.' Taken alone, this passage would certainly not seem intended to describe the phenomena of a total eclipse; but as Philostratus afterwards remarks that the darkness was so great as to resemble night, we may assume with some confidence that a total solar eclipse had occurred.

Plutarch more distinctly describes the appearance actually presented by the corona when he endeavours to explain why the darkness during a total eclipse is not so great as that of night. Even though the Moon,' he says, should hide at any time the whole of the Sun, still the eclipse is deficient in duration as well as amplitude, for a peculiar effulgence is seen around the circumference which does not allow a deep and very intense shadow.'

I do not propose to record in full the observations which have been made upon the corona. To do so would occupy, indeed, much more space than can here be spared. Referring the reader who wishes for a more complete account of the earlier observations to Professor Grant's admirable History of the Physical Sciences,' I shall consider here those observations alone which tend to throw light on the nature of the

corona.

Some of the earlier observers of total solar eclipses would seem to have been misled by the great brightness of the corona close by the Sun, and to have supposed that a ring of direct sunlight had remained uncovered.

We shall see presently that modern observers have also been struck with the brightness of the light close by the Sun; and it seems obvious that this bright light is to be regarded as wholly distinct from the light of the chromosphere, the redness of which is too marked to escape recognition. Remembering that before the invention of the telescope the corona was the only marked phenomenon to which observers were able to direct their attention, a certain weight attaches to their comments on the brightness near the Moon's disc.

Clavius having expressed his belief that the eclipse of 1567 was annular, Kepler was led to investigate the subject, and he proved that that eclipse must needs have been total. In 1605 he witnessed a total eclipse at Naples, and found, in the features it presented, the explanation of the remarks of Clavius. The whole body of the Sun,' he says, 'was completely covered for a short time, but around it there shone a brilliant light.' We might suppose that he referred to the chromosphere, because he says that the light was of a reddish hue;' but as he adds that it was ' of uniform breadth, and occupied a considerable part of the heavens,' there can be no doubt that he is speaking of the corona.

Dr. Wyberd gives a remarkable account of the appearance of the corona during the total eclipse of March 29, 1652. When the Sun was reduced to a narrow crescent of light,' he says, 'the Moon all at once threw herself within the margin of the solar

disc with such agility that she seemed to revolve like an upper millstone, affording a pleasant spectacle of rotary motion. In reality, however, the Sun was totally eclipsed, and the appearance was due to a corona of light round the Moon, arising from some unknown cause. It had a uniform breadth of half a digit, or a third of a digit at least ;* it emitted a bright and radiating light, and appeared concentric with the Sun and Moon' when the middle of totality was reached.

It need scarcely be remarked that the agility with which the Moon seemed to throw herself within the margin of the Sun's disc was merely apparent. It has been noticed by many observers that the total obscuration of the Sun seems to occur suddenly, the Moon covering the last sickle of sunlight, apparently at a leap. Irradiation is no doubt in question. The sickle of light, even when in reality it is indefinitely fine, appears, through the effects of irradiation, to have a definite breadth; so that the Moon seems to traverse a definite distance in obliterating what is in truth the finest possible curve of light. But beyond this there is a circumstance which cannot but give an appearance of somewhat agile motion to the eclipsing disc of the Moon. Up to the very moment when totality is about to begin, the air between the observer and the Moon is illuminated by direct sunlight. This is easily seen by a consideration of fig. 79, in

* A digit signifies the twelfth part of the solar diameter. is nearly obsolete.

The term

which ABCD represents the portion of the Moon's shadow in the atmosphere, E the place of an observer towards whom this shadow is swiftly advancing. Then, until the shadow actually reaches E (at which moment totality begins) a line drawn from E to D will pass to the left of the line A D m-as in the direction E Df; so that, since the line A D m is necessarily directed towards the Moon's limb, the directly illuminated air (bounded, of course, by A D) extends between the observer and the Moon. Hence, the Moon's disc seems lighted up by this atmospheric glare until the

Illustrating the condition of the Earth's atmosphere a few minutes before totality in a solar eclipse.

very moment when totality begins; and as the curtain of glare is drawn suddenly away towards the edge where the last sickle of the Sun's direct light is vanishing, all the circumstances tend to give an appearance of agile motion in that direction.

As respects the rotatory motion which seemed to accompany this leap forward on the Moon's part, we can very well understand it as referring to the completion of the corona, which must needs take place by a sweeping round of the bounding rays to close in upon each other opposite the point where the last part of the

Sun's disc disappears.

It will be well to notice,

however, as we proceed, whether we have convincing evidence of an apparent motion of the coronal beams after totality has begun. The evidence on this point cannot but have an important bearing on the views we are to form respecting the corona.

Dr. Wyberd saw a corona of very limited extentindeed only half a digit wide-corresponding to a height not half so great as that of many prominences which have been observed during recent eclipses.

Our next observation refers, however, to a much more favourable view of the corona.

In May 1706 Plantade and Capiés saw a bright ring of white light surrounding the eclipsed Sun, and extending to a distance equal to about one-tenth part of the Moon's apparent diameter. Outside this bright ring a fainter light could be recognised, which extended no less than four degrees from the eclipsed Sun, fading off insensibly, until its light was lost in the obscure background of the sky.

In 1724 Maraldi noted a circumstance of some importance. At the beginning of the total eclipse which was observed in France in that year he perceived that the corona was wider on the side towards which the Moon was advancing than on the opposite side. At the close of totality the widest part of the corona was on the opposite side. As this would exactly correspond to what would be observed if the corona lies beyond the Moon, and so is traversed by the Moon precisely as the Sun himself is, it will be well for us to