Plate XXIV. The Great Nebula in the Constellation of Orion
From a photograph taken at the Yerkes Observatory.
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[32] The name Al gûl, meaning the Demon, was what the old Arabian astronomers called it, which looks very much as if they had already noticed its rapid fluctuations in brightness.
[33] Mr. Gore thinks that the companion of Algol may be a star of the sixth magnitude.
[34] Presidential Address to the British Association for the Advancement of Science (Leicester, 1907), by Sir David Gill, K.C.B., LL.D., F.R.S., &c. &c.
CHAPTER XXV
THE STELLAR UNIVERSE
The stars appear fairly evenly distributed all around us, except in one portion of the sky where they seem very crowded, and so give one an impression of being very distant. This portion, known as the Milky Way, stretches, as we have already said, in the form of a broad band right round the entire heavens. In those regions of the sky most distant from the Milky Way the stars appear to be thinly sown, but become more and more closely massed together as the Milky Way is approached.
This apparent distribution of the stars in space has given rise to a theory which was much favoured by Sir William Herschel, and which is usually credited to him, although it was really suggested by one Thomas Wright of Durham in 1750; that is to say, some thirty years or more before Herschel propounded it. According to this, which is known as the "Disc" or "Grindstone" Theory, the stars are considered as arranged in space somewhat in the form of a thick disc, or grindstone, close to the central parts of which our solar system is situated.[35] Thus we should see a greater number of stars when we looked out through the length of such a disc in any direction, than when we looked out through its breadth. This theory was, for a time, supposed to account quite reasonably for the Milky Way, and for the gradual increase in the number of stars in its vicinity.