But the number of binary stars known is not confined to those which have been discovered and measured by means of the telescope and micrometer. One of the most wonderful results of modern astronomical research has been the discovery of the fact that many stars have revolving round them invisible companions, which are either dark bodies, or else are so close to their primaries as for ever to defy the separating powers of our telescopes. The discovery of these dark, or at least invisible, companions is one of the most remarkable triumphs of the spectroscope. It was in 1888 that Vogel first applied the spectroscopic method to the well-known variable star, Beta Persei--known as Algol, 'the Demon,' from its 'slowly-winking eye.' The variation in the light of Algol is very large, from second to fourth magnitude; Vogel therefore reasoned that if this variation were caused by a dark companion partially eclipsing the bright star, the companion must be sufficiently large to cause motion in Algol--that is, to cause both stars to revolve round a common centre of gravity. Should this be the case, then at one point of its orbit Algol must be approaching, and at the opposite point receding from the earth; and therefore the shift of the lines of its spectrum towards the violet in the one instance and towards the red in the other would settle the question of whether it had or had not an invisible companion. The spectroscopic evidence proved quite conclusive. It was found that before its eclipses, Algol was receding from the sun at the rate of 26-1/3 miles per second, while after eclipse there was a similar motion of approach; and therefore the hypothesis of an invisible companion was proved to be fact. Vogel carried his researches further, his inquiry into the questions of the size and distance apart of the two bodies leading him to the conclusion that the bright star is rather more, and its companion rather less than 1,000,000 miles in diameter; while the distance which divides them is somewhat more than 3,000,000 miles. Though larger, both bodies prove to be less massive than our sun, Algol being estimated at four-ninths and its companion at two-ninths of the solar mass.