SUN-SPOTS (the great Sun-Spot of 1865).

The spot entering the Sun's disc, Oct. 7th (foreshortened view). 2. Oct 10th. 3. Oct. 14th: central view, showing the formation of a bridge, and the nucleus. 4. Oct. 16th.

LESSON VIII.-TELESCOPIC APPEARANCE OF THE SUNSPOTS. PENUMBRA, UMBRA, NUCLEUS. FACULE. GRANULES. RED FLAMES.

109. We have already said that the first things which strike us on the Sun's surface, when we look at it with a powerful telescope, are the spots. In Plate IV. we give drawings of a very fine one, visible on the Sun in 1865. We shall often refer to them in the following description. The spots are not scattered all over the Sun's disc, but are generally limited to those parts of it a little above and below the Sun's equator, which is represented by the middle lines in Fig. 3. The arrows show the direction in which the spots, carried round by the Sun's rotation, appear to travel across the disc.

110. The spots float, as it were, in what, as we have already seen in the case of the stars, is called the photosphere; the half-shade shown in the spot is called the penumbra (that is, half shade); inside the penumbra is a still darker shade, called the umbra, and inside this again is the nucleus. Diagrams 3 and 4 of Plate IV. will render this perfectly clear. The white surface represents the photosphere; the half tones the penumbra; the dark, irregular central portions the umbra; and the blackest parts in the centre of these dark portions, the nucleus.

111. Sun-spots are cavities, or hollows, eaten into the photosphere, and these different shades represent different depths.

112. Diligent observation of the umbra and penumbra, with powerful instruments, reveals to us the fact that change is going on incessantly in the region of the spots.

Sometimes changes are noticed, after the lapse of an hour even here a portion of the penumbra is seen setting sail across the umbra; here a portion of the umbra is melting from sight; here, again, an evident change of position and direction in masses which retain their form. The enormous changes, extending over tens of thousands of square miles of the Sun's surface, which took place in the great sun-spot of 1865, are shown in Plate IV.

113. Near the edge of the solar disc, and especially about spots approaching the edge, it is quite easy, even with a small telescope, to discern certain very bright streaks of diversified form, quite distinct in outline, and either entirely separate or uniting in various ways into ridges and network. These appearances, which have been termed faculæ, are the most brilliant parts of the Sun. Where, near the edge, the spots become invisible, undulated shining ridges still indicate their place-being more remarkable thereabout than elsewhere, though everywhere traceable in good observing weather. Faculæ may be of all magnitudes, from hardly visible, softly-gleaming, narrow tracts 1,000 miles long, to continuous complicated and heapy ridges 40,000 miles and more in length, and 1,000 to 4,000 miles broad. Ridges of this kind often surround a spot, and hence appear the more conspicuous; such a ridge is shown in Fig. 1, Plate IV.; but sometimes there appears a very broad white platform round the spot, and from this the white crumpled ridges pass in various directions.

114. So much for the more salient phenomena of the Sun's surface, which we can study with our telescopes. There is much more, however, to be inquired into; and here. we may remark that the Sun himself has bestowed a great boon upon observational Astronomy; and, whether brightly shining or hid in dim eclipse, now tells his own story, and prints his image on a retina which never forgets, and withal

so docilely, that each day he is visible at the Kew Observatory a young lady takes observations which surpass immeasurably in value those made by the hardest-headed astronomers of bygone times.

115. We may begin by saying, that the whole surface of the Sun, except those portions occupied by the spots, is coarsely mottled; and, indeed, the mottled appearance requires no very large amount of optical power to render it visible : in a large instrument, it is seen that the surface is principally made up of luminous masses-described by Sir William Herschel as corrugations. The luminous masses present to different observers almost every variety of irregular form: they have been stated to resemble "rice grains," "granules or granulations," and

so on.

F

116. The word "willow-leaf" very well paints the appearance of the minute details sometimes observed in the penumbra of spots, which occasionally are made up apparently of elongated masses of unequal brightness, so arranged that for the most part they point like so many arrows to the centre of the nucleus, giving to the penumbra a radiated appearance. At other times, and occasionally in the same spot, the jagged edge of the penumbra projecting over the nucleus has caused the interior edge of the penumbra to be likened to coarse thatching with straw.

117. There are darker or shaded portions between the granules, often pretty thickly covered with dark dots, like stippling with a soft lead-pencil; these are what have been called "pores" by Sir John Herschel, and