The discovery by Dawes that within the umbra, or what was formerly called the nucleus of every fullydeveloped spot, there exists a darker region, or (so far as telescopic research has yet gone) a true nucleus, not only opens a new field for speculation, but renders the interpretation of the phenomena before recognised altogether more difficult. By using a solar eye-piece of his own invention, in which the field of view was so contracted as to exclude even the light from the penumbræ of large spots, he detected in the stratum which had before been regarded as black, a mottled appearance-the degree of darkness being by no means uniform, and suggesting the idea that the surface is far from level, the lighter parts being probably the most elevated, and feebly reflecting the light received from the self-luminous strata above it. . . In all spots which are tolerably symmetrical, and large enough to admit of accurate scrutiny, this umbra will be found to be perforated near its centre by a perfectly black hole, which is to be regarded as the true nucleus.' From the researches of Fr. Secchi it would appear as though the umbral portion of large spots were formed of luminous matter which is undergoing a continual process of dissipation towards its interior edge. He compares the process to the gradual dissipation of cumulus clouds under the heat of a summer Sun, and regards the umbra as of the nature of a veil of clouds; the nucleus as a region where an intenser heat has caused these clouds to melt away. Within spots such as these he has recognised the presence of such leaves. Ther this matter.' On April 14, the these singular obje picted in fig. 53. says Secchi, full of like the great spot of *Memoria III. sugli spett the following dimensions to Total diameter in length coloured matter, which he compares to the coloured envelope whence the prominences spring; and it was partly on account of his having noted such appearances that he pronounced, so far back as 1860, his conviction that an envelope of this coloured matter-the chromosphere of the next chapter-surrounds the whole globe of the Sun. These appearances have been noticed also by other observers. Mr. Lockyer has recognised the gradual disappearance of portions of luminous matter (as if by subsidence in some semi-transparent medium) in the umbræ of large spots; while the presence of various tints of red within the spots has been noticed also by Schwabe, Capocci, Schmidt, and other ob servers. The processes of formation, enlargement, and disappearance of spots are well worthy of study; and although no regular law has been detected in their succession, we can yet recognise certain distinctive features ordinarily belonging to each stage of development. The formation of a spot is usually preceded by the appearance of faculæ. Then a dark point makes its appearance which increases in size, the penumbral fringe being presently recognised around it, and the distinction between the umbra and the penumbra being well defined. The same clearness of definition continues ordinarily until and after the spot has reached its greatest development. But when the spot is about to diminish, there is a change in this respect. The edges seem less sharp, and an appearance is presented as though there floated over them a luminous cloud veil, brighter in some places than in others, and not unfrequently attaining a brightness which seems to exceed even that of the facula. At certain parts of the spot's circumference, this bright matter projects, hiding the whole width of the penumbra and forming a sort of cape or promontory, with sharply serrated edges, singularly well defined against the dark background of the umbra. It is usually in this manner that the formation of a bridge begins, two promontories on opposite sides of a spot, or even on the same side, joining their extremities, so as to form either a bridge of light across the umbra, or a curved streak having both its extremities on one side of the spot. But indeed no strict law or sequence has yet been assigned to these processes of change. In a large spot the wildest and most fantastic variations will take place, and often when the spot seems approaching the stage of disappearance, it will seem to renew its existence, as though fresh forces were at work in disturbing the region it belongs to. Some of the processes of change which take place in large spots are very well exemplified in fig. 54, where the drawings 1 to 4 show the successive changes of appearance presented by the great spot of 1865, from October 7, when it was on the Sun's eastern limb, until October 16, when it had passed the central part of the disc. These drawings were made by the Rev. Mr. Howlett, one of our most enthusiastic solar observers, and specially skilled in the delineation of Sun spots. Were not his accuracy beyond all question, it would be |