image. In a third of Fr. Secchi's photographs (fig. 87) the corona is yet more distinctly shown.

The extension of the corona in fig 87 is regarded by Fr. Secchi as corresponding to the solar equator, whose position is very nearly indicated by the cross-wire

The Eclipse of 1860. From a Photograph by Secchi.

shown in the figure. La couronne,' he says, 'est très irrégulière, mais on peut remarquer qu'elle présente une étendue plus considérable à droite et à gauche que dans les autres directions, c'est à-dire qu'elle est plus développée dans le plan de l'équateur (solaire) que suivant la ligne des pôles.' But, as Fr. Secchi himself points out further on, the figure indicates

rather an extension opposite four points lying between the equator and poles, than an extension at the equator. In fact, fig. 87 presents a very striking resemblance to Mr. Whipple's picture further on (fig. 90). It is worthy of notice, too, that not only does the outline of the corona present this quadrilateral aspect, but in the bright parts close to the Moon's limb there are four corresponding regions of greatest brilliancy. In the second of Mr. De La Rue's photographs the brightest portions seem similarly disposed.*

Some of the direct observations made in 1860 serve also to throw important light on the nature of the corona. To the Astronomer Royal the corona presented much the same aspect as in 1851. Bruhns of Leipsic states that when the last rays of the Sun disappeared, the corona shone out with a white light of such brilliancy that the protuberances were almost

* One cannot wholly agree with Father Secchi's remark, that Mr. De La Rue's photographs afford no evidence of the peculiar quadrilateral expansion of the corona corresponding to the position of the zone of spots. It is true, however, that when Fr. Secchi ' published his results astronomers did not conceal their doubts.' If the expansion of the corona in four directions be regarded as an ordinary phenomenon (and we have seen how often it has been noticed), some very perplexing questions would be presented as to the cause of the peculiarity. A rectangular figure, like that shown in fig. 87, would correspond to a cylindrical real figure; but it would also correspond to other figures of three dimensions. It is indeed possible that there may be no corona at all opposite the solar poles, the light we see there being merely a foreshortened view (on this supposition) of the great extension over the spot belts. In this case the true figure of the corona would resemble that due to the rotation of two hyperbolas, having the same axes, around one of these axes.

z 2

obliterated. He adds, the black-looking Moon was surrounded by a clear light of unequal breadth. It was considerably wider below than above, varying from nearly one-half to a quarter of a degree, and its general appearance gave me the idea that the Moon was eccentrically placed within it. Its general outline was circular, but on the eastern side a long ray shone out to a distance of about a degree (that is, twice the Moon's apparent diameter); it was of a tapering figure. During the ten seconds that my attention was directed to it, neither the direction nor the length of the ray varied; its light was considerably feebler than that of the corona, which was of a glowing white, and seemed to coruscate or twinkle. My assistant, M. Auerbach, noticed, in the south-western part of the corona, a curved ray about a tenth of a degree in length.' Father Secchi found that the corona could be seen with the naked eye for about forty seconds after the reappearance of the Sun, the solar light shining like an electric lamp projecting tremulous shadows.' Mr. De La Rue states that several minutes before totality the whole contour of the Moon could be distinctly seen: when totality had commenced, the Moon's disc appeared of a deep brown in the centre of the corona, which was extremely bright near the Moon's limb, and appeared of a silvery white, softening off with a very irregular outline, and sending forth some long streams. It extended generally to about from seven to eight tenths of the Moon's diameter.' This description corresponds

6

very

satisfactorily with the appearance represented in the accompanying picture (fig. 88) by Feilitsch.

During the eclipse of April 1865 it was noticed that the corona continued visible for thirty-six seconds after the appearance of the first rays of direct sunlight.

The eclipse of March 1867 was only annular, yet it presented a feature well worth careful consideration,

FIG. 88

?

and that at a station considerably removed from the line of central eclipse. O. Struve and Wagner, using the great equatorial of the Pulkowa observatory, noticed that when the eclipse reached its greatest phase, the outline of that part of the Moon's disc which was outside the Sun's, could be distinctly seen. Schwabe, who observed the same phenomenon at Dessau, noticed that this part of the Moon's disc was

rendered visible by its superior blackness; a circumstance which proves that the light of the corona must be considerably stronger than the illuminated atmosphere near the Moon's place, even when a portion of the Sun's direct light is falling upon the air there.

*

The total eclipse of August 1867 attracted very much less attention than those which occurred during the same month of the two following years. Yet the observations made by Herr Grosch, of the Santiago observatory, Chili, in conjunction with Vice-Director Vergaza and Lieut. Vidal, are full of significance in connection with the main object of this chapter, which is the determination of the true theory of the corona. At the moment when totality began, there appeared,' says Herr Grosch,† around the Moon a reddish glimmering light, similar to that of the aurora, and almost simultaneously with this (I mean very shortly after it) the corona. This reddish glimmer,

which surrounded the Moon with a border of the breadth of at most five minutes, was not sharply bounded in any part, but was extremely diffused, and less distinct in the neighbourhood of the poles. I can decidedly confirm this at least as regards the Sun's north point, but not so much so of the south point, as that part was less observed by me, but was more particularly attended to by Lieutenant Vidal,

By the atmosphere near the Moon's place, I mean that part of the atmosphere which lies nearly in the same direction as the Moon. Of course it is, in reality, very far removed from the Moon's true place.

+ I quote (from the Student for March 1869) Mr. Lynn's translation of Herr Grosch's narrative.