EVOLUTION OF THE STABS 223 



influence in the same way that the comets have transported very distant 

 materials into the terrestrial region, is wholly unknown. 



Stars Clusters 



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 multi- 

 tudes 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 dis- 

 tant 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 Nebulae 



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 indis- 

 tinguishable 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 nebula?. Lord Ross's reflecting 

 telescope showed that a few of the very bright and large nebulas, per- 

 haps 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 nebulae 

 on the surface of the sky is most remarkable. Proctor's chart, pub- 

 lished 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 out- 

 lines. Each dot on the chart represents a nebula. He who runs may 



