dræ, R. Leonis, etc. Mira Ceti varies from about the second magnitude to a little below the ninth, with a mean period of about 331 days from maximum to maximum. Owing to its unusual brilliancy at maximum, and the great range of its light-fluctuations, this is perhaps the most interesting and remarkable of all the variable stars. The period of Chi Cygni is about 406 days, and its variation from about the fourth to nearly the thirteenth magnitude. R. Hydræ varies from the fourth to the eleventh mgnitude, with a period of about 437 days; and R. Leonis from about the fifth to the tenth magnitude, with a period of about 313 days. Most of the long-period variables are reddish in color, and show a banded spectrum, which seems to be a characteristic feature of this type of variable. Various theories have been proposed to account for the variation of light in long-period variables, but none of them are very satisfactory. The periodical outbreak of sun-spots on a large scale has been suggested, and also the clashing together of meteoric swarms revolving in an elongated orbit; but it must be confessed that the subject is still, to a great extent, a matter of mystery.
Class 3 includes the irregular variables—that is, stars which are undoubtedly variable, but have no regular periods. Sometimes these stars remain for long periods without any perceptible change, while at other times their fluctuations of light are very noticeable. Of these, perhaps the most remarkable are Mu Cephei (Sir William Herschel's "Garnet Star"), Alpha Herculis, Alpha Orionis (Betelgeuse), and Beta Pegasi. The variation is usually small, not exceeding one magnitude. Like the regular variables, these have also banded spectra.
In Class 4 are some very interesting objects—variable stars of short period. The greater number of these have periods of under eight days. The variation of light is generally small, but regular. In but few cases does it much exceed one magnitude, and in several it is less. In some, as in Beta Lyræ, Zeta Geminorum, and Eta Aquilæ, all the light-changes may be observed with the naked eye, while in others an opera-glass is necessary to follow the fluctuations.
In Class 5 are placed stars of the Algol type. These are the rarest of the regular variables, only ten having been hitherto detected. In these stars the light remains constant, or nearly so, for the greater portion of the period. A sudden diminution of brightness then commences, and all the light-changes are completed in the course of a few hours, after which the star returns to its normal brightness. The brightest of these remarkable stars are Algol (Beta Persei), Lambda Tauri, and Delta Libræ. The others are much fainter, only two being visible to the naked eye when at their normal brightness. A star of this class recently