Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/855

] marked with spots, is shown in fig. 28, which presents the aspect of the sun, seen with a small telescope on September 25, 1870, as drawn by Mr Proctor. Three views of spots, as seen with large telescopes, are presented in figs. 29, 30, and 31, which show the same spot in different stages of its history, as observed at the Harvard Observatory, Cambridge, U.S., in March and April 1870. These views, though showing the spots as they have only been seen with the improved instruments of our day, will serve to illustrate the following account of the history of research into these objects better than pictures resembling the imperfect drawings made by the first observers.

—Sun Spot seen in 1870. —Another Phase of Spot, fig. 29.

{{ti|1em|The phenomena of the solar spots, as observed by Scheiner and Hevelius, may be summed up in the follow ing particulars : 1. Every spot which has a nucleus, or comparatively dark part, has also an umbra, or fainter shade surrounding it. 2. The boundary between the nucleus and umbra is always distinct and well defined. .}. The increase of a spot is gradual, the breadth of the nucleus and umbra dilating at the same time. 4. In like manner the decrease of a spot is gradual, the breadth of the nucleus and umbra contracting at the same time. 5. The exterior boundary of the umbra never consists of sharp angles, but is always curvilinear, how irregular so ever the outline of the nucleus may be. 6. The nucleus of a spot, whilst on the decrease, often changes its figure by the umbra encroaching irregularly upon it, insomuch that in a small space of time new encroachments are dis cernible, whereby the boundary between the nucleus and umbra is perpetually varying. 7. It often happens, by these encroachments, that the nucleus of a spot is divided into two or more nuclei. 8. The nuclei of the spots vanish sooner than the umbra. 9. Small umbrae are often seen without nuclei. 10. An umbra of any considerable size is seldom seen without a nucleus in the middle of it. 1 1 . When a spot which consisted of a nucleus and umbra is about to disappear, if it is not succeeded by a facula or spot brighter than the rest of the disk, the place where it was is soon after not distinguishable from the rest.}} — Phase of Spot, figs. 29, 30. In the Philosophical Transactions, vol. Ixiv. (1774), Dr Alexander Wilson, professor of astronomy at Glasgow, gave a dissertation on the nature of the solar spots, in which he mentioned the following appearances: 1. When the spot is about to disappear on the western edge of the sun s limb, the eastern part of the umbra first contracts, then vanishes, the nucleus and western part of the umbra remaining ; then the nucleus gradually contracts and vanishes, while the western part only of the umbra remains. At last this disappears also ; and if the spot remains long enough to become again visible, the eastern part of the umbra first becomes visible, then the nucleus ; and when the spot approaches the middle of the disk, the nucleus appears environed by the umbra on all sides, as already mentioned. 2. When two spots lie very near to one an other, the umbra is deficient on that side which lies next to the other spot; and this will be the case, though a large spot should be contiguous to one much smaller ; the umbra &quot; of the large spot will be totally wanting on that side next the small one. If there are little spots on each side of the large one, the umbra does not totally vanish, but appears flattened or pressed in towards the nucleus on each side. When the little spots disappear, the umbra of the large one extends itself as usual. This circumstance, he observes, may sometimes prevent the disappearance of the umbra in the manner above mentioned ; so that the western umbra may disappear before the nucleus if a small spot happens to break out on that side. These observations led Dr Wilson to adopt the opinion,