The so-called "globular clusters" usually include stars of different brightness; comparatively bright telescopic stars of the 10th to 13th magnitude with faint stars of the 15th to 17th magnitude. Prof. Perrine of the Lick Observatory finds that (_a_) "the division of the stars in globular clusters into groups, differing widely in brightness, is characteristic of these objects"; (_b_) "the globular clusters are devoid of true nebulosity"; and (_c_) "stars fainter than 15th magnitude predominate in the Milky Way and globular clusters, but elsewhere are relatively scarce." He found that "exposures of one hour or thereabouts showed as many stars as exposures four to six times as long; the only effect of the longer exposures being in the matter of density." This last result confirms the late Dr. Roberts' conclusions. Perrine finds that for clusters in the Milky Way, the faint stars (15th to 17th magnitude) "are about as numerous in proportion to the bright stars (10th to 13th magnitude) as in the globular clusters themselves." This is, however, not the case with globular clusters at a distance from the Milky Way. In these latter clusters he found that "in the regions outside the limits of the cluster there are usually very few faint stars, hardly more than one-fourth or one-tenth as many as there are bright stars"; and he thinks that "this paucity of faint stars" in the vicinity of these clusters "gives rise to the suspicion that all regions at a distance from the Galaxy may be almost devoid of these very faint stars." The late Prof. Keeler's series of nebular photographs "in or near the Milky Way" tend to confirm the above conclusions. Perrine finds the northernmost region of the Milky Way "to be almost, if not entirely, devoid of globular clusters."[380] According to Sir John Herschel, "the sublimity of the spectacle afforded" by Lord Rosse's great telescope of 6 feet in diameter of some of the "larger globular and other clusters" "is declared by all who have witnessed it, to be such that no words can express."[381] In his address to the British Association at Leicester in 1907, Sir David Gill said-- "Evidence upon evidence has accumulated to show that nebulæ consist of the matter out of which stars have been and are being evolved.... The fact of such an evolution with the evidence before us, can hardly be doubted. I most fully believe that, when the modifications of terrestrial spectra under sufficiently varied conditions of temperature, pressure, and environment, have been further studied, this connection will be greatly strengthened." CHAPTER XVIII Historical