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where. The sun-spots are therefore the effect of operations going on all the time, all over the sun, but giving rise to a spot only in the exceptional cases when they are very intense.

It was formerly supposed that the spots were openings or depressions in the photosphere, showing a darker region within. This view was based on the belief that, when a spot was near the edge of the sun's disk, the shaded border next the edge looked broader than the other. But this view is now abandoned. We cannot certainly say that a spot is either above or below the photosphere. We shall hereafter see that the latter is not a mere surface as it seems to us, but a shell or covering many miles, perhaps a hundred or more, in thickness. The spots doubtless belong to this shell, being cooler portions of it, but lying neither above nor below it.

The next remarkable feature of the sun to be described consists in the prominences. Our knowledge of these objects has an interesting history—which will be mentioned in describing eclipses of the sun. The spectroscope shows us that large masses of incandescent vapour burst forth from every part of the sun. They are of such extent that the earth, if immersed in them, would be as a grain of sand in the flame of a candle. They are thrown up with enormous velocity, sometimes hundreds of miles a second. Like the faculæ, they are more numerous in the sun-spot zones, but are not confined to those zones. The glare around the sun caused by the reflection of light