i8 4 POPULAR SCIENCE MONTHLY 



showing displacement, apparently due to the motions of the bright 

 components of a close binary. These displacements were explained 

 in 1891 by Pickering as the result of the revolution of the unlike com- 

 ponents of a binary system, having a relative velocity of 300 miles per 

 second, and a radius of 50,000,000 miles. Belopolsky has also in- 

 vestigated this object, obtaining results which differ somewhat from 

 those given above. Professor G. W. Myers has made a mathematical 

 discussion of the problem, reaching the conclusion that the phenomena 

 can be explained on the theory of a binary system, composed of two 

 gaseous, scarcely separated, components of different masses, mutually 

 eclipsing each other during their revolutions. Indeed the two com- 

 ponents may not have separated, but exist still as a single body of 

 unusual form, such as Poincare's, or Darwin's figures of equilibrium. 

 The problem is extremely complicated, and well illustrates the almost 

 infinite diversity which is met with in the various problems about 

 variable stars. The binary character of this type of variables seems sure 

 in many cases, while in others even three bodies appear to be present; 

 but the details involved are still in doubt. 



The variable stars found in clusters have periods ranging for the 

 most part from ten to fourteen hours. The elements of about 300 of 

 them have been determined by the writer. The uniformity of the 

 periods found in the same cluster is remarkable, pointing unmistakably 

 to a common cause. What that cause is has not yet been found. The 

 form of light-curve is shown in Pigs. 3 and 4. Owing to the faintness 

 of these stars, which generally vary between the twelfth and fifteenth 

 magnitudes, it has not yet been possible to determine either the nature 

 of the spectrum or the radial motion. The light-curve shows no in- 

 dication of eclipsing phenomena. The uniformity in the period, traced 

 in many cases through more than 5,000 returns of maximum, points 

 to axial rotation or orbital revolution. Variability might result, un- 

 doubtedly, from the rotation of an elliptical, or unevenly luminous 

 body; but the light-curves of cluster variables are difficult of explana- 

 tion on this theory. They may be binaries with small, elliptical orbits, 

 but even this is hardly consistent with the form of light-curve. The 

 rejection of these hypotheses, nevertheless, seems to leave the phe- 

 nomena without plausible explanation. A few cluster variables have 

 been found where the maxima succeed each other at intervals of about 

 six hours, one half the usual period. This indicates pretty clearly a 

 double variable with alternating maxima, both components having the 

 same period. These apparently accidental cases of duplicity may throw 

 some light on the physical condition of all these stars. 



V. Of the Algol variables Algol itself is a good example. Its 

 light usually remains at a uniform brightness of the second magnitude, 

 but once in a little less than three days it falls to the third magnitude, 

 where it remains for some twenty minutes before beginning to regain 



