served them from the time of sun-rising, and perceived that they were nearer his centre. . . . The first was composed of two others almost round and conjoyn'd. The second represented the shape of a scorpion. The third was round' (this is the first intimation we have of the triplicity of the group), and they were all three environed with a coronet, which was composed, as said above, of abundance of little obscure pricks. This coronet appeared to be clearer than the rest of the Sun when looked upon with the short glass, and darker when seen with the long. Without it there were other points, but very black ones, viz. five near the round spot on the south side, and another near the scorpion's tail on the north side. At eight a clock and fortyeight minuts, the figure of the scorpion was seen divided into several pieces, as if his tail and arms had been cut off. The northern point remained no more, there remaining none but those seen on the south side; and the length of the enclosure of all the spots, comprehended between the extremities, was of one minut and fifteen seconds, and the breadth of thirty seconds.' On August 12 Cassini found no great change had taken place, but the black points outside the spot were now spread in a straight row. On the 13th 'the edge of the coronet was turned to a point on the south side.' The spot had indeed changed in that strange way which all observers of the Sun must be familiar with, the following extremity of the penumbra drawn out to a point which was so curled round as to be directed towards the preceding end of the spot.

Cassini afterwards traced the progress of this spot to the Sun's limb. He remarks, The apparent velocity nigh the centre was such that if it had continued the same, the spots would have arrived almost in four days to the limb of the disque; but in the hypothesis'-that they are either attached to the Sun's surface, or not far from it-this apparent velocity was to lessen according as the spots should remove from the centre; as hath come to pass in effect. The diminution in the length of the misty crown' (a strange name for the penumbra) 'was in a manner proportionat to the diminution of the apparent velocity; since that when this crown was in the midle, and in a scituation wherein its true figure could be best seen, it appeared oblong and of the form of an human ear, its greatest diameter respecting east and west; but being nigh the limb, this same diameter seemed to shorten; and having appeared greatest in its first scituation, it appeared least in this, because it was almost in a circle that passed through the centre of the Sun, whose equal arches are by so much the more oblique by how much they approach more to the limb of his disque, and consequently appear less according to the rules of opticks; meantime the diameter that was turned from north to south apparently kept the same bigness it had near the centre, because it was in a circle almost parallel to the horizon of the Sun, which formed the representation of its limb, and whose equal arches' (for the same optical reasons)' do not appear contracted.' It will be seen by this reasoning-which,

being interpreted, signifies that fore-shortening caused the spot to appear longest in a direction square to a line from the Sun's centre-that Cassini had on this occasion come very near to the discovery afterwards made by Dr. Wilson, that the nucleus of a spot is at a lower level than the solar photosphere. For had he but noticed an excess of breadth in the penumbra nearest to the Sun's limb, the same just reasoning which he applied to the figure of the spot would have enabled him to pronounce at once respecting the relative level of the nucleus, the penumbra, and the photosphere. He gives a picture of the spot close by the Sun's limb, in which the penumbra is of equal width on the side next the centre and on that next the limb. such peculiarity as Dr. Wilson the case of this particular spot. In fact, so far as my own experience of the aspect of Sun-spots is concerned, spots such as the one observed by Cassini seldom exhibit the peculiarity in question in a manner which would enable an observer to theorise safely respecting the level at which nucleus, penumbra, and photosphere actually lie.

Most probably no detected existed in

In November 1769, Dr. Wilson, of Glasgow, began the careful study of a large spot (visible to the naked eye). The results which he deduced from its changes of appearance are of great interest and importance. When he first examined the spot (November 22) it was situated not very far from the western edge of the Sun's disc. On the next day he found that the spot

had changed in appearance. The penumbra, which on the previous day was equally broad on all sides of the nucleus,* was now very much contracted on the side which lay towards the centre of the disc, while the other parts retained nearly their former dimensions. On the 24th he again observed the spot. The distance from the limb was now only twenty-four seconds, and the contracted side of the penumbra had entirely vanished. The breadth of the nucleus on the same side also appeared to be more suddenly impaired than it ought to have been by the motion of the Sun across the disc.'

Dr. Wilson showed that these results correspond with those which would follow if the spot was a vast cavernous opening, having the nucleus at the bottom, and the penumbra forming its sloping sides. It only remained to be seen whether a corresponding succession of changes occurred when the spot re-appeared on the eastern edge, and thence passed across the solar disc. This actually happened. On December 11, the spot appeared on the opposite side of the disc. It was then distant about a minute and a half from the edge. The side of the penumbra next the edge, which formerly vanished, was now wholly visible, while that turned towards the centre of the disc appeared to be wanting. On December 12 it came into view, and he saw it distinctly, though narrower than the other side. He did not see the spot again until December 17, when it

* From Professor Grant's abstract of the original narrative, in his excellent History of Physical Astronomy.

had passed the centre of the disc, and the penumbra now appeared to surround the nucleus equally on all sides.

In fig. 41 the upper row of spots represents the succession of changes actually presented by this spot, while the lower shows what would occur as a spot traversed the Sun's disc, if the spot were simply a surface stain with a penumbral fringe. It will be seen

FIG. 41.

that the difference between the appearances depicted in the two rows is of a sufficiently marked character. It will also be noticed that even without any definite explanation of the peculiarities shown in the upper row, the mind at once recognises the fact that we have here to do with a cavity or depression.

Let us examine, however, the line of reasoning by which Dr. Wilson demonstrated this.