Not less interesting are those stars whose variations cover only short periods, extending from less than thirty days down to a few hours. Of these, perhaps the most easily observed, as it is also one of the most remarkable, is Beta Lyræ. This star is one of the two bright stars of nearly equal magnitude which form an obtuse-angled triangle with the brilliant first-magnitude star Vega. The other star of the pair is Gamma Lyræ, and between them lies the famous Ring Nebula, to be referred to later. Ordinarily Beta Lyræ is of magnitude 3·4, but from this it passes, in a period of rather less than thirteen days, through two minima, in one of which it descends to magnitude 3·9 and in the other to 4·5. This fluctuation seems trifling. It really means, however, that at maximum the star is two and three-quarter times brighter than when it sinks to magnitude 4·5; and the variation can be easily recognised by the naked eye, owing to the fact of the nearness of so convenient a comparison star as Gamma Lyræ. Beta Lyræ is a member of the class of spectroscopic binaries, and belongs to that type of the class in which the mutually eclipsing bodies are both bright. In such cases the variation in brilliancy is caused by the fact that when the two bodies are, so to speak, side by side, light is received from both of them, and a maximum is observed; while, when they are end on, both in line with ourselves, one cuts off more or less of the other's light from us, thus causing a minimum. A third class, distinct from either of the preceding, is that of the Algol Variables, so-called from the bright star Beta Persei, which has already been mentioned as a spectroscopic binary. Than this star there is no more notable variable in the heavens, and its situation fortunately renders it peculiarly easy of observation to northern students. Algol shines for about fifty-nine hours as a star of small second magnitude, then suddenly begins to lose light, and in four and a half hours has fallen to magnitude three and a half, losing in so short a space two-thirds of its normal brilliancy. It remains in this degraded condition for only fifteen minutes, and then begins to recover, reaching its normal lustre in about five hours more. These remarkable changes, due, as before mentioned, to the presence of an invisible eclipsing companion, are gone through with the utmost regularity, so much so that, as Gore says, the minima of Algol 'can be predicted with as much certainty as an eclipse of the sun.' The features of the type-star are more or less closely reproduced in the other Algol Variables--a comparatively long period of steady light emission, followed by a rapid fall to one or more minima, and a rapid recovery of light. The class as yet is a small one, but new members are gradually being added to it, the majority of them white, like the type-star.