From the consideration of spectroscopic binaries we naturally turn to that of variable stars, the two classes being, to some extent at least, coincident, as is evidenced by the case of Algol. While the discovery of spectroscopic binaries is one of the latest results of research, that of variability among stars dates from comparatively far back in the history of astronomy. As early as the year 1596 David Fabricius noted the star now known as Omicron Ceti, or Mira, 'the Wonderful,' as being of the third magnitude, while in the following year he found that it had vanished. A succession of appearances and disappearances was witnessed in the middle of the next century by Holwarda, and from that time the star has been kept under careful observation, and its variations have been determined with some exactness, though there are anomalies as yet unexplained. 'Once in eleven months,' writes Miss Clerke, 'the star mounts up in about 125 days from below the ninth to near the third, or even to the second magnitude; then, after a pause of two or three weeks, drops again to its former low level in once and a half times, on an average, the duration of its rise.' This most extraordinary fluctuation means that at a bright maximum Mira emits 1,500 times as much light as at a low minimum. The star thus subject to such remarkable outbursts is, like most variables, of a reddish colour, and at maximum its spectrum shows the presence of glowing hydrogen. Its average period is about 331 days; but this period is subject to various irregularities, and the maximum has sometimes been as much as two months away from the predicted time. Mira Ceti may be taken as the type of the numerous class of stars known as 'long-period variables.'