is that in Perseus. This constellation is situated in the Milky Way, not far from its region of nearest ap

It

proach to the pole. In the figure of the constellation the cluster forms the handle of the hero's sword. may be seen in the evening during almost any season

except summer. To the naked eye it seems more diffused and star-like than Præsepe; in fact, it has two distinct centres of condensation, so that it may be considered as a double cluster.

The two clusters last described may be resolved into stars with the smallest telescopes. But in the case of most of these objects the individual stars are so faint that the most powerful instruments scarcely suffice to bring them out. One of the most remarkable clusters in the northern heavens is that of Hercules. To the naked eye it is but a faint and insignificant patch, which would be noticed only by a careful observer, but in a large telescope it is seen. to be one of the most interesting objects in the heavens. Near the border the individual stars can be readily distinguished, but they grow continually thicker toward the centre, where, even in a telescope of two feet aperture, the observer can see only a patch of light, which is, however, as he scans it, suggestive of the countless stars that must there be collected. By the aid of photography, Professor Pickering nearly succeeded in the complete resolution of this cluster, and Keeler was even more successful with the Crossley reflector of the Lick Observatory.

In many cases the central portions of these objects are so condensed that they cannot be visually resolved into their separate stars, even with the most powerful telescopes. A closer approach to complete resolution has been made by photography. We reproduce photographs of two noted clusters which show their appearance in a powerful telescope.

The cluster which, according to Pickering, may be called the finest in the sky, is Omega 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

Its actual ex

as a hazy star of the fourth magnitude. treme 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. (See Figure on page 175.)

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 results of his examination of the photographs are given in the table below.' In this table, the first number is that of the new general catalogue of Dreyer. The second column gives the usual designation of the cluster, generally its number in Messier's list. The next two columns give the position referred to the equinox of 1900. Next follows the approximate number of stars examined. The other columns are sufficiently explained by their headings.

1Harvard College Observatory Circular No. 33.

It will be seen from this table that 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 Omega Centauri, is the shortness of the period of the variables. Out of 125 found, 98 have periods less than twenty

On the subject

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. 3. and the range in brightness two magnitudes. Probably