this kind which rendered Algol a star of questionable import to the ancient students of the heavens. It was accordingly known as the Demon Star, for this is the equivalent of the name by which we now know it. As to the peculiarities of Algol which have given it notoriety, these are very simply described. For two days and ten hours the star remains of uniform lustre, being ranked about the second magnitude; then a decline of brightness sets in, and the star in a few hours parts with three-fifths of its brightness. At the lowest point it remains for about twenty minutes, and then the brilliancy commences to increase, so that in a few hours more Algol has resumed its original character. The entire period required for the decline and the rise is about ten hours, and the whole cycle of the changes has been determined with much accuracy, and is at present 2 days, 20 hours, 48 minutes, 52 seconds. The length of the period seems to undergo some trifling fluctuations of a few seconds, but on the whole the uniformity of the movement is a striking part of the phenomenon. Considering that these changes can be observed without any telescope, it is not surprising that they have been known for centuries. Indeed, it fortunately happens that there is a smaller star near Algol which serves as a convenient standard of comparison. Under ordinary circumstances Algol is much brighter than its neighbour, but when it sinks to its lowest point the two stars have almost equal lustre. It is only within the last year or two that the mystery of the variability of Algol has been at last revealed and the phenomenon of the Demon Star has received its true interpretation.
It had been suggested long ago that the loss of light
