northern side, terminating close on Beta Ophiuchi. The main stream, after exhibiting several very remarkable condensations, passes through Aquila, Sagitta, and Vulpecula to Cygnus. In Cygnus there is a “confused and patchy” region marked by a broad vacancy, not unlike the Coal-Sack. From this region there is thrown off the offset to Beta Ophiuchi, already mentioned; the main stream is continued to Cassiopeia. [Illustration: Fig. 16.—The Midnight Sky, with Milky Way] There only remains to be noticed “a considerable offset or protuberant appendage,” thrown from the head of Cepheus directly toward the pole. Galileo was the first to prove, though earlier astronomers had entertained the notion, that the Milky Way was composed of a vast number of stars crowded closely together. But no attempt was made to offer a theory of its structure until, in 1754 Thomas Wright, in his _Theory of the Universe_, propounded views closely according with those entertained later by Sir W. Herschel. Wright, having examined a portion of the Galaxy with a reflecting telescope, only one foot in focal length, came to the conclusion that our sun is in the midst of a vast stratum of stars; that it is when we look along the direction in which this stratum extends that we see the zone of light constituting the Milky Way; and that as the line of sight is inclined at a greater and greater angle to the mean plane of the stratum, the apparent density of the star-grouping gradually diminishes. But it is to Sir W. Herschel, and the supplementary labors of Sir J. Herschel, that we owe the more definite views now commonly entertained respecting the Via Lactea. The elder Herschel, whose nobly speculative views of nature were accompanied by practical common-sense, and a wonderful power of patient observation, applied to the heavens his celebrated method of gauging. He assumed as a first principle, to be modified by the results of observation, that there is a tolerable uniformity in the distribution of stars through space. Directing his twenty-foot reflector successively toward different parts of the heavens, he counted the number of stars which were visible at any single view. The field of view of this reflector was fifteen minutes in diameter, so that the portion of the sky included in any one view was less than one-fourth of that covered by the moon. He found the number of stars visible in different parts of the heavens in a field of view of this size to be very variable. Sometimes there were but two or three stars in the field;[20] indeed, on one occasion he counted only three stars in four fields. In other parts of the heavens the whole field was crowded with stars. In the richer parts of the Galaxy as many as four hundred or five hundred stars would be visible at once, and on one occasion he saw as many as five hundred and eighty-eight. He calculated that in one-quarter of an hour 116,000 stars traversed the field of his