The cluster which, according to Pickering, may he called the finest in the sky, is ω Centauri. It lies just within the border of the Milky Way, in right ascension, 13h. 20.8m., and declination—46° -47'. There are no bright stars near. To the naked eye it appears as a hazy star of the fourth magnitude. Its actual extreme diameter is about 40'. The brightest individual stars within this region are between the eighth and ninth magnitudes. Over six thousand have been counted on one of the photographs and the whole number is much greater.
The most remarkable and suggestive feature of the principal clusters is the number of variable stars which they contain. This feature has been brought out by the photographs taken at the Harvard Observatory and at its branch station in Arequipa. The count of stars and the detection of the variables was very largely made by Professor Bailey, who, for several years past, has been in charge of the Arequipa station. The proportion of variables is very different in different clusters. In the double cluster, 869-884, only one has been found among a thousand stars. The richest in variables is Messier, 3, in which one variable has been detected among every seven stars. It might be suspected that the closer and more condensed the cluster the greater the proportion of variables. This, however, does not hold universally true. In the great cluster of Hercules only two variables are found among a thousand stars.
Very remarkable, at least in the case of ω Centauri, is the shortness of the period of the variables. Out of one hundred and twenty-five found, ninety-eight have periods less than twenty-four hours. On the subject of the law of variation in these cases, Pickering says:
"The light curves of the ninety-eight stars whose periods are less than twenty-four hours may be divided into four classes. The first is well represented by No. 74. The period of this star is 12h. 4m. 3s. and the range in brightness two magnitudes. Probably the change in brightness is continuous. The increase of light is very rapid, occupying not more than one-fifth of the whole period. In some cases, possibly in this star, the light remains constant for a short time at minimum. In most cases, however, the change in brightness seems to be continuous. The simple type shown by No. 74 is more prevalent in this cluster than any other. There are, nevertheless, several stars, as No. 7, where there is a more or less well marked secondary maximum. The period of this star is 2d. llh. 51m. and the range in brightness one and a half magnitudes. The light curve is similar to that of well-known short-period variables, as δ Cephei and η Aquilæ. Another class may be represented by No. 126, in which the range is less than a magnitude and the times of increase and decrease are about equal. The period is 8h. 12m. 3s. No. 24 may perhaps be referred to as a fourth type. The range is about seven-tenths of a magnitude and the
