being then about equal to the faint stars near it. It remains thus obscured for only a few minutes, and then begins to brighten again, and in about four and a half hours more resumes its former brilliancy. This phenomenon is very easily observed, for, as will be seen by consulting our little map, Algol can be readily found, and its changes are
Map 6.— The Attendants of Alpha Persei.
so rapid that under favorable circumstances it can be seen in the course of a single night to run through the whole gamut. Of course, no optical instrument whatever is needed to enable one to see these changes of Algol for it is plainly visible to the naked eye throughout, but it will be found interesting to watch the star with an opera-glass. Its periodic time from minimum to minimum is two days, twenty hours, and forty-nine minutes, lacking a few seconds. Any one can calculate future minima for himself by adding the periodic time above given to the time of any observed minimum. For instance, there will be a minimum on November 12th at about 11.15 p. m., then the next minimum will occur two days, twenty hours and forty-nine minutes later, or at 8.04 p. m., on November 10th.
While spots upon its surface may be the cause of the variations in