Page:Hector Macpherson - Herschel (1919).djvu/54

48 increases and decreases in proportion to its apparent brightness to the naked eye." In the most crowded part of the Galaxy, Herschel occasionally counted as many as 588 stars in a field of view, and in quarter of an hour's time no fewer than 116,000 stars were thus enumerated, while other fields were almost destitute of stars. Herschel made two important assumptions—(1) that the stars were, roughly speaking, of the same size, and (2) that they were scattered throughout space with some approach to uniformity. As a result of his star-gauges, he was enabled on these two assumptions to estimate the possible extent and shape of the sidereal system. He sketched it as a cloven disc of irregular outline, extending much further in the direction of the Milky Way than in that of the galactic poles, the cleft representing the famous division in the Milky Way. The Milky Way was regarded as more or less an optical phenomenon, as a vastly extended portion of the stellar system.

Herschel's gauges led him to the view that the galactic system was strictly limited in extent. "It is true," he said, "that it would not be consistent confidently to affirm that we were on an island unless we had actually found ourselves everywhere bounded by the ocean, and therefore I will go no further than the gauges will authorise; but considering the little depth of the stratum in all those places which have been actually gauged, to which must be added all the intermediate parts that have been viewed and found to be much like the rest, there is but little room to expect a connection between our nebula and any of the neighbouring ones." The stellar system which he designated as "our nebula" was in his view an island universe—"a very extensive branching, compound congeries of many millions of stars". The majority of nebulae and clusters he believed to be independent stellar units. He divided these nebulæ, or milky ways—for at this time the two terms were interchangeable in his