the laws of dynamics to carry us a step further. Such a movement is possible on one condition and only one, and that is that there is an attracting body in the neighbourhood around which Algol revolves. Of course the student of mechanics knows that each of the bodies revolves around the common centre of gravity. The essential point to be noticed is that the spectroscopic evidence admits of no other interpretation save that there must be another mighty body in the immediate vicinity of Algol. We had already seen reason to believe in the possibility of the presence of such a companion for the Demon Star, simply from the fact of its variability. There cannot be any longer a doubt that the mystery has been solved. Algol must be attended by a companion star, which, if not absolutely as devoid of intrinsic light as the earth or the moon, is nevertheless dark relatively to Algol. Once in each period of revolution this obscure body intrudes itself between the earth and Algol, cutting off a portion of the direct light from the star and thus producing the well-known effect. Here we have such a remarkable concurrence between the facts of observation and the laws of dynamics that it is impossible to doubt the explanation they provide of the variability of this famous star.
There is, however, a further point in which the facts can be made to yield information of even a more striking character, inasmuch as it is unique of its kind. It is, of course, well known that stars in general show no appreciable discs, even in our best telescopes. In fact the better the instrument the smaller does the stellar point appear. This is, of course, due to the distance at which the stars are situated. It would be easy to show that if the sun were to be viewed by an observer placed on the
