176. _Why the Faculæ are most Conspicuous near the Limb of the Sun._--The reason why the faculæ are most conspicuous near the limb of the sun is this: The luminous surface of the sun is covered with an atmosphere, which, though not very thick compared with the diameter of the sun, is still sufficient to absorb a good deal of light. Light coming from the centre of the sun's disk penetrates this atmosphere under the most favorable conditions, and is but slightly reduced in amount. The edges of the disk, on the other hand, are seen through a much greater thickness of atmosphere; and the light is reduced by absorption some seventy-five per cent. Suppose, now, a facula were sufficiently elevated to penetrate quite through this atmosphere. Its light would be undimmed by absorption on any part of the sun's disk; but at the centre of the disk it would be seen against a background nearly as bright as itself, while at the margin it would be seen against one only a quarter as bright. It is evident that the light of any facula, owing to the elevation, would be reduced less rapidly as we approach the edge of the disk than that of the general surface of the sun, which lies at a lower level. Sun-Spots. 177. _General Appearance of Sun-Spots._--The general appearance of a well-formed sun-spot is shown in Fig. 193. The spot consists of a very dark central portion of irregular shape, called the _umbra_, which is surrounded by a less dark fringe, called the _penumbra_. The penumbra is made up, for the most part, of filaments directed radially inward. [Illustration: Fig. 193.] There is great variety in the details of form in different sun-spots; but they are generally nearly circular during the middle period of their existence. During the period of their development and of their disappearance they are much more irregular in form. There is nothing like a gradual shading-off of the penumbra, either towards the umbra on the one side, or towards the photosphere on the other. The penumbra is separated from both the umbra and the photosphere by a sharp line of demarcation. The umbra is much brighter on the inner than on the outer edge, and frequently the photosphere is excessively bright at the margin of the penumbra. The brightness of the inner penumbra seems to be due to the crowding together of the penumbral filaments where they overhang the edge of the umbra. There is a general antithesis between the irregularities of the outer and inner edges of the penumbra. Where an angle of the penumbral matter crowds in upon the umbra, it is generally matched by a corresponding outward extension into the photosphere, and _vice versa_.