The spots when more closely examined will be found to present endless irregularities in outline and size, as will be seen from the accompanying plates and figures. On the whole, there is comparative fidelity to two main features--a dark central nucleus, known as the umbra, and a lighter border, the penumbra; but sometimes there are umbræ which have no penumbra, and sometimes there are spots which can scarcely be called more than penumbral shadings. The shape of the spot is sometimes fairly symmetrical; at other times the most fantastic forms appear. The umbra appears dark upon the bright disc, but is in reality of dazzling lustre, sending to us, according to Langley, 54 per cent. of the amount of heat received from a corresponding area of the brilliant unspotted surface. Within the umbra a yet darker deep, if it be a deep, has been detected by various observers, but is scarcely likely to be seen with the small optical means which we are contemplating. The penumbra is very much lighter in colour than the umbra, and invariably presents a streaked appearance, the lines all running in towards the umbra, and resembling very much the edge of a thatched roof. It will be seen to be very much lighter in colour on the edge next the umbra, while it shades to a much darker tone on that side which is next to the bright undisturbed part of the surface (Figs. 14 and 15). Frequently a spot will be seen interrupted by a bright projection from the luminous surface surrounding it which may even extend from side to side of the spot, forming a bridge across it (Plate VI., and Figs. 16, 17, and 18). These are the outstanding features of the solar spots, and almost any telescope is competent to reveal them. But these appearances have to be interpreted, so far as that is possible, and to have some scale applied to them before their significance can in the least be recognised. The observer will do well to make some attempt at realizing the enormous actual size of the seemingly trifling details which his instrument shows. For example, the spot in Figs. 14 and 15 is identical with that measured by Mr. Denning on the day between the dates of my rough sketches; and its greatest diameter was then 27,143 miles. Spots such as those of 1858, of February, 1892, and February, 1904, have approached or exceeded 140,000 miles in diameter, while others have been frequently recorded, which, though not to be compared to these leviathans, have yet measured from 40,000 to 50,000 miles in diameter, with umbræ of 25,000 to 30,000 miles. Of course, the accurate measurement of the spots demands appliances which are not likely to be in a beginner's hands; but there are various ways of arriving at an approximation which is quite sufficient for the purpose in view--namely, a realization of the scale of any spot as compared with that of the sun or of our own earth. [Illustration: FIG. 14.--SUN-SPOT, JUNE 18, 1889.] [Illustration: FIG. 15.--SUN-SPOT, JUNE 20, 1889.]