the first time, we are able to apply the celestial callipers to gauge the diameter of a star. So far as surveying and measuring goes, this is the most significant piece of work in sidereal astronomy since the epoch, half a century ago, when the determination of a stellar distance first emerged from the mistiness of mere guess-work and took a respectable position among the solved problems of astronomy. Nor is our gratification at the result of Vogel's striking work lessened by the fact of its unexpectedness. Who would have predicted some few years ago that the spectroscope was to be the instrument to which we should be indebted for the means of putting a measuring tape round the girth of a star? The process and the results are alike full of interest and are of happy augury for the future.
To explain exactly how it is possible to deduce a presumable value for the diameter of Algol would lead into some technicalities that need not be here mentioned. But the principle of the method is so plain that it would be unfitting to leave it without some attempt at exposition. We are first to notice that Algol, at the moment of its greatest eclipse, has lost about three-fifths of its light: it therefore follows that the dark satellite must have covered three-fifths of the bright surface. It is also to be noticed that the period of maximum obscuration is about twenty minutes, and that we know the velocity of the bright star, which along with the period of revolution gives the magnitude of its orbit. These facts, added to our knowledge that ten hours is required for the brilliancy to sink from and regain its original lustre, enable the sizes of the two globes to be found. There is only one element of uncertainty in the matter. We have assumed that the densities of the two bodies are the same. Of
