Sir John Herschel was so impressed with the form, structure, and immensity of the Galactic Circle, as he sometimes terms it, that he says (in a footnote p. 575, 10th ed.), 'This circle is to sidereal what the invariable ecliptic is to planetary astronomy—a plane of ultimate reference, the ground-plane of the sidereal system.' We have now to consider what are the relations of the whole body of the stars to this Galactic Circle—this plane of ultimate reference for the whole stellar universe.
If we look at the heavens on a starry night, the whole vault appears to be thickly strewn with stars of various degrees of brightness, so that we could hardly say that any extensive region—the north, east, south, or west, or the portion vertically above us—is very conspicuously deficient or superior in numbers. In every part there are to be found a fair proportion of stars of the first two or three magnitudes, while where these may seem deficient a crowd of smaller stars takes their place.
But an accurate survey of the visible stars shows that there is a large amount of irregularity in their distribution, and that all magnitudes are really more numerous in or near the Milky Way, than at a distance from it, though not in so large a degree as to be very conspicuous to the naked eye. The area of the whole of the Milky Way cannot be estimated at more than one-seventh of the whole sphere, while some astronomers reckon it at only one-tenth. If stars of any particular size were uniformly distributed, at most one-seventh of the whole number should be found within its limits. But Mr. Gore finds that of 32 stars brighter than the second magnitude 12 lie upon the Milky Way, or considerably more than twice as many as there should be if they were uniformly distributed. And in the case of the 99 stars which are brighter than the third magnitude 33 lie upon the Milky Way, or one-third instead of one-seventh. Mr. Gore also counted all the stars in Heis's Atlas which lie upon the Milky Way, and finds there are 1186 out of a total of 5356, a proportion of between a fourth and a fifth instead of a seventh.
The late Mr. Proctor in 1871 laid down on a chart two feet diameter all the stars down to magnitude 91/2 given in Agrelander's forty large charts of the stars visible in the northern hemisphere. They were 324,198 in number, and they distinctly showed by their greater density not only the whole course of the Milky Way but also its more luminous portions and many of the curious dark rifts and vacuities, which latter are almost wholly avoided by these stars.
Later on Professor Seeliger of Munich made an investigation of the relation of more than 135,000 stars down to the ninth magnitude to the Milky Way, by dividing the whole of the heavens into nine regions, one and nine being circles of 20° wide (equal to 40° diameter) at the two poles of the Galaxy; the middle region, five, is a zone 20° wide including the Milky Way itself, and the other six intermediate zones are each 20° wide. The following table shows the results as given by Professor Newcomb, who has made some alterations in the last column of 'Density of Stars' in order to correct differences in the estimate of magnitudes by the different authorities.
| Regions. | Area in Degree. | Number of Stars. | Density. |
| I. | 1,398.7 | 4,277 | 2.78 |
| II. | 3,146.9 | 10,185 | 3.03 |
| III. | 5,126.6 | 19,488 | 3.54 |
| IV. | 4,589.8 | 24,492 | 5.32 |
| V. | 4,519.5 | 33,267 | 8.17 |
| VI. | 3,971.5 | 23,580 | 6.07 |
| VII. | 2,954.4 | 11,790 | 3.71 |
| VIII. | 1,796.6 | 6,375 | 3.21 |
| IX. | 468.2 | 1,644 | 3.14 |
N.B.—The inequality of the N. and S. areas is because the enumeration of the stars only went as far as 24° S. Decl., and therefore included only a part of Regions VII., VIII., and IX.
DIAGRAM OF STAR-DENSITY
From Herschel's Gauges
(as given by Professor Newcomb, p. 251).