our seeing them, but we have already shown how this obstacle may be easily done away with. It was, doubtless, to these phenomena that we must ascribe the pretended passage of Mercury across the solar disc in the year 807; the black spot, which was supposed to be the obscure disc of that planet, was visible for eight days. In the year 840, it was Venus that was supposed to be seen passing over the solar disc during a space of 91 days. Again, in 1096, it was asserted that signs were seen on the Sun,' signa in Sole. But in those days nothing was known of solar spots; after they were discovered these appearances gave rise to no more mistakes. Several observers have seen sun-spots with the naked eye; for instance, in August 1612, Galileo and some of his friends saw a spot of at least l' in diameter upon the solar disc at sunrise. was seen for three successive days. The sight of a solar spot visible to the naked eye in 1779 determined William Herschel to turn his attention to the physical nature of the Sun. Herr Schwabe, who has devoted so many years to the investigation of sun-spots, has seen many that were large enough to be distinguished without the aid of a telescope. ‹ The principal spots appeared,' he says, in the years 1828, 1829, 1831, 1836, 1837, 1838, 1839, 1847, and 1848. I consider as large spots those which cover 50" at least; it is at about this size that they become visible to persons with good eyes, without the aid of a telescope.' It On the 28th June, 1868, a very large spot on the solar disc was noticed by Mr. W. S. Gilman, of Palisades, Rockville Co., New York, who wrote of it as 'a spot visible to the naked eye;' and doubtless such would often be seen if people paid more attention to the subject. Fig. 31.-Faculæ in the neighbourhood of a Spot, from a Drawing by The faculæ, or brilliant spots, were observed for the first time, as we have stated, by Galileo. Most frequently they accompany the spots, and are seen upon the external borders of the penumbra, so that it might easily be imagined that they resulted from an effect of contrast between the black surface of the spot and the brilliancy of the surrounding portions of the disc; but this is not at all the case, for, besides the fact that faculæ do not always surround the penumbra, that certain spots are devoid of faculæ, the latter are sometimes seen to appear isolated on the disc, and their apparition usually denotes the approaching formation of a spot at that point. Faculæ sometimes take the form of converging currents, terminating on various points of the periphery of a spot, like so many streams of brilliant matter. Fig. 31, for which I am indebted to M. Chaornac, furnishes a remarkable example of this state of things, very different from those represented in fig. 29. Structure of the Photosphere; Pores or Granulations on the luminous Portions of the Disc.—' Willow-leaves,' their Disposition near a Spot and inside the Penumbra. When the disc of the Sun is examined with a moderately powerful glass, all those regions which are not covered with spots-often the entire disc-appear of uniform whiteness, and gives us the idea of a perfectly smooth and flat surface. It is not so, however, when we observe the Sun by means of a very powerful telescope. In this case the brilliant surface appears covered with a multitude of luminous lines alternating with darker lines, and crossing each other in every direction, so as to resemble, as several persons have remarked, the grain of an engraving. The figures 28, 32, and 33, will give some notion of this peculiar aspect presented by the solar disc. The more obscure portions to which we allude have been called pores or lucules; they are seen in every region of the disc, whilst the spots or faculæ only appear, as we shall see presently, in a certain limited zone on each side of the solar equator. We must except, however, the faculæ themselves, and the nuclei of the spots whose tints are nearly uniform; but those portions of the spots which, though of very variable tints, constitute the penumbra, when examined with a glass of sufficient power show a structure which is very similar to that of the granulated surface of the disc. The only difference appears to be that the pores in the penumbra are much larger, so that the less obscure portions of the penumbra appear to stand out separate from one another on the darker background. Their elongated forms caused them to receive from Mr. Nasmyth, a contemporary English astronomer, the name of willow-leaves.' Other observers have also noticed the existence of these luminous fragments, which have been compared by the late distinguished Mr. Dawes to slashed blades of straw, and by Mr. Stone to rice-grains; Mr. Huggins calls them simply granulations. The following is the manner in which the director of the Roman Observatory, Professor Secchi, describes the aspect presented by the surface of the solar disc, away from the spots, on their borders, and in the interior of the penumbra (he observed with a great refracting telescope, by Merz, having a diagonal ocular glass) :— Fig. 32.-Pores or Granulations on the Surface of the Solar Disc, according to Mr. Huggins. 'The luminous background of the Sun is seen as a kind of tissue over which are dispersed a multitude of |