The accompanying plate conveys an idea of what the spots on the sun consist of. Figures 2 and 3 represent the general aspect of these appearances. In the centre is a black space perfectly marked. To this succeeds a space in grey tinting, whose outlines melt by degrees into the rest of the luminous mass. The first region is called the Umbra; the second, the Penumbra. These words must be distinctly understood. The part indicated by the term _Umbra_ is only dark relatively to the illumined portion. This Umbra is very luminous, its brilliancy is two thousand times that of the full moon. We are merely dealing with comparisons here. The solar spots are often of very considerable dimensions. They have been found 30,000 leagues in breadth, and could swallow up the earth, which is only one-tenth of that magnitude. They are not permanent, sometimes they remain for months, or even years, but the greater number increase and decrease rapidly, and disappear in a few weeks. They are incessantly changing in form and in extent, and they grow and diminish. It is evident that they are regulated by a violent interior movement, and that they are the seat of tumultuous motion. Something like whirlwinds are seen to sweep across the regions occupied by the spots, and to carry them away, like the waves of a furious sea, or the flames of a conflagration. Gigantic bridges of apparently burning matter have been observed, thrown from one edge to the other of adjacent spots, uniting them by a shining band, and then this same band has stretched itself out and caught hold of other spots. Of a sudden the whole edifice has been seen to be swept away by fresh whirlwinds. Signs of a prodigious commotion, of gigantic perturbation, are always evident. These hurricanes, these tempests of flame, are of a widely different grandeur from the hurricanes and the tempests of our atmosphere, because the atmosphere of the sun is several thousands of yards in height, and covers an extent of surface 1,300,000 times greater than ours. [Illustration: Fig. 2.--Group of Solar Spots observed in 1864 by Nasmith.] [Illustration: Fig. 3.--Another Solar Spot observed by Nasmyth.] We have just said that the sun has an atmosphere. Such is the conclusion to which the careful examination of the great star has led. From the earliest times at which the sun was observed, a theory of its constitution was formulated, which was perpetuated down to the present age, without receiving authoritative contradiction. In the eighteenth century the astronomers Herschel and Wilson developed this theory, which was popularized in our time by the writings of Humboldt and Arago. According to this theory, the sun is composed of a dark nucleus, and a burning atmosphere, which is the only source of the light proper to this star. Arago and Humboldt called the incandescent atmosphere of the sun, the _photosphere_. Heat and light would not, therefore, come to us from the nucleus, but only from the photosphere.