which took place when, for the first time, we became able to measure the size of a star.
It is well known that the most difficult test-objects on which a telescope can be directed are some of those double stars of which the components have a suitable distance. If the two stars be so close together that they subtend at our system an angle not more than a few tenths of a second, then the telescopic separation of the two components is a feat to tax the powers of the most perfect instrument, and the eye of the most accomplished observer. It may, however, happen that there are double stars of which the components are much closer than this. In such a case there is not the slightest possibility of our being able to effect a visual decomposition of the pair into its components. The spectroscopic process has, however, placed at our disposal a striking method for detecting the existence of double stars, the components of which are so close that even if they were hundreds of times farther apart than they actually are they would still fall short of the necessary distance at which they must be situated before they can be separated telescopically. Indeed, we have here obtained an accession to our power so remarkable that we have not yet been able even to feel the limits within which its application must be confined.
As an illustration of this process I shall take a star which is probably as famous as Algol itself. It is Mizar, the middle star of the three which form the tail of the Great Bear (Fig. 27). Mizar has in its vicinity the small star Alcor, which is so easily seen as to make it hard for us to realise the significance of the proverb, "He can see Alcor." It is, however, possible that the lustre of Alcor may have waxed greater since ancient times. The rela-
