Page:Popular Science Monthly Volume 87.djvu/227

Rh influence in the same way that the comets have transported very distant materials into the terrestrial region, is wholly unknown.

The star clusters offer a wide range of character, as to their density of stellar contents and as to the symmetry of distribution. There are the large irregular clusters visible to the eye, such as the Pleiades, Praesepe, the Perseus clusters, in which the stars are widely separated and irregularly distributed. There are the globular clusters, invisible to the naked eye, except in three or four cases, which contain multitudes of faint stars densely crowded together and quite symmetrically arranged. The great cluster in Hercules is the most striking example in the northern skies. The accompanying photograph, secured with the 60-inch reflector of the Mount Wilson Observatory, records stars to the order of 30,000, each star a sun as truly as is our star. There are two still more extensive clusters in the Southern Hemisphere, but they have not yet been photographed on the same scale as the northern clusters. The globular clusters, of various degrees of stellar richness, exist to the number of several scores.

There are two great agglomerations of stars—two dense clouds of stars—occupying isolated positions in the far southern sky, quite distant from the Milky Way, which seem to have many of the Milky Way's attributes. They appear to be great irregular clusters of stars differing only in size from the vastly greater Milky Way cluster. These objects are known as the Greater and Lesser Magellanic Clouds.

The objects which probably concern our subject most directly are the nebulae. The word nebula means a "little cloud"; and like little clouds superimposed upon the dark background of the sky the first 10,000 nebulae looked to their discoverers. They were of various sizes, from that of the Orion nebula, and even larger, down to those indistinguishable in small telescopes from stars, and to those so faint as to be on the limit of telescopic vision. The Herschels, father and son, were the first great discoverers of the nebulæ. Lord Ross's reflecting telescope showed that a few of the very bright and large nebulæ, perhaps two dozen in all, are not formless masses, but spirals—indicating plainly that they have motions of rotation. It was noticed by Sir William Herschel, a century ago, that the distribution of the nebulæ on the surface of the sky is most remarkable. Proctor's chart, published in 1869, illustrates this fact. On this chart the cloud-like forms of the Milky Way are outlined across both hemispheres, as seen by the naked eye, but it should be said that telescopic vision of the Milky Way would present very different and vastly more uniform outlines. Each dot on the chart represents a nebula. He who runs may