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Turning now to the other solar features, let us once more consider the sun as a whole. Fig.27 is a photograph taken from a part of the sun near its edge. We notice on it, what we see on every careful delineation of the sun, that its general surface is not uniformly bright, but that it grows darker as we approach the edge, where it is marked by whiter mottlings called faculæ, “something in the sun brighter than the sun itself,” and looking in the enlarged view which we present of one of them (Fig.28), as if the surface of partly cooled metal in a caldron had been broken into fissures showing the brighter glow beneath. These “faculæ,” however, are really above the solar surface, not below it, and what we wish to direct particular attention to is that darkening toward the edge which makes them visible.

FIG. 28.—FACULA. (FROM A DRAWING BY CHACORNAC.)

This is very significant, but its full meaning may not at first be clear. It is owing to an atmosphere which surrounds the sun, as the air does the earth. When we look horizontally through our own air, as at sunrise and sunset, we gaze through greater thicknesses of it than when we turn our eyes to the zenith. So when we look at the edge of the sun, the line of sight passes through greater depths of this solar atmosphere, and it dims the light shining behind it more than at the centre, where it is thin.