instantly correct or compensate for such movements. There are several methods in use of accomplishing this hand-guiding or correcting; the most refined method is by the use of an attachment called the double-slide plate-carrier. The observer sits with his eye at the guiding-microscope of the attachment, and with his fingers on the milled heads of two fine adjusting-screws, and he is able to introduce with extreme accuracy and quickness any corrections which he sees are necessary.
It was in this way that the accompanying photographs of the Pleiades and of the great cluster in the constellation of Hercules were obtained. The Pleiades photograph has already been described. Fig. 2 is from a negative of the Hercules cluster which was exposed in the two-foot reflector for seven hours, on two successive nights. This telescope is particularly well adapted for photographing extremely faint stars, and the chief value of this photograph lies in the fact that it records the very faint outlying stars of the cluster,—stars which are much too faint to be detected visually with any telescope; thus the enormous extent of the cluster is shown. When examined visually with the great telescope of this observatory—the 40-inch refractor—this cluster appears to be less than fifteen minutes of arc in diameter; the reflector photograph shows that it is at least thirty minutes of arc in extent. What this corresponds to in miles it is impossible to say, because we do not know the distance from us to any star-cluster. If we assume the distance of the great Hercules cluster to be such that its annual parallax is one one-hundredth of a second of arc—a distance which is probably many times too small—then the extent of this cluster is such that light would require more than two years to flash across its diameter.
If the reader will now turn to Fig. 3, he will see the central part of this same cluster in Hercules as photographed with the 40-inch refractor, the largest instrument thus far successfully used in celestial photography. The focal length of this telescope is 63 feet, more than eight times greater than that of the two-foot reflector; the scale of this photograph is greater in the same proportion, and the stars near the dense centre of the cluster are now shown well separated. It is needless to say that such large-scale photographs as this are admirably suited for very accurate measurement of star-positions. I have obtained with the 40-inch refractor two or more very sharp negatives of each of the finest globular clusters which are visible from our latitude. It is believed that these photographs will prove extremely valuable for comparison with similar ones obtained several years later, in the search for change and rotation in these clusters.
Several of the globular clusters are so remarkable that they should be briefly mentioned. Messier 5, in the constellation of Libra, is nearly as large as the great one in Hercules. Messier 2, in the constellation of Aquarius, and Messier 3, in the constellation of Canes Venatici, are slightly smaller, but are both magnificent telescopic objects. Messier 15, in the constellation of Pegasus, is an exceedingly rich cluster; its centre is so compressed that even the 40-inch refractor fails to show the component stars separated. All of these clusters are in the northern celestial hemisphere.
The southern constellations Sagittarius and Ophiuchus are remarkably rich in globular clusters. Messier 14 Ophiuchi is a globular assemblage of countless numbers of excessively faint stars of very uniform magnitude. Messier 19 Ophiuchi is also composed of myriads of extremely faint stars, which in the 40-inch refractor appear like glittering dust; this vast assemblage is nearly twice as large in angular diameter as the great Hercules cluster. Messier 75 Sagittarii is so extremely compressed that when seen in the 40-inch refractor it appears very much as the great Hercules cluster appears when viewed with a six-inch telescope; it is apparently much farther from us than the star-clusters in general.
The most magnificent of the globular clusters, Omega Centauri, is nearly 47° south of the celestial equator, and is therefore not visible from our northern latitude. Through the kindness of Professor E. C. Pickering, of the Harvard College Observatory, I am able to present a beautiful photograph of this incomparable cluster (Fig. 4), taken at the Harvard Observatory station at Arequipa, Peru.
