Page:Popular Science Monthly Volume 15.djvu/526

510 an infinite universe of stars to show everywhere such a uniformity of plan.

How the fixed stars are actually distributed through space, an inquiry into which we are led by study of their number and brightness, it has been but recently found worth while to consider. So long as they were believed to be simply lights set in a hollow revolving structure that divided the waters beneath from the waters above; so long as the idea of the "firmament" retained the association with solidity that now only remains in the word; so long as the "spangled heavens, a shining frame" was a reality of opinion and not an unmeaning archaism of poetry—this question was never heard. No significance could attach to it, and it excited no curiosity. But when this solid celestial framework was broken up by the discovery of the earth's rotation, and its lights scattered afar on the deep ocean of unbounded space, when contemplation of the beautiful adjustments and proportions of our solar system had suggested the hope of discovering the same harmonies throughout the universe, it began to be asked if some of these far-distant orbs, or perhaps the mighty whole, our sun and its attendant planets included, were not connected in a system of similar character. Kant was one of the first to advance this idea. The elder Herschel, contenting himself with a working hypothesis to give form to his observations, supposed our firmament to be a mighty cluster of stars equally distributed within finite limits, so that the number visible in the field of his great reflector at any pointing would show the extent of occupied space in that direction; and he undertook to gauge the depths and discover the shape of this cluster by counting telescopic fields in different parts of the sky. The elder Struve considered the density of the stars as varying with the distance from the Milky Way, as does that of the atmosphere with its distance from the earth's surface; being equal in parallel plans. Argelander, of Bonn, relieved his laborious task of cataloguing over 300,000 northern stars, by investigation into the subject; Mr. Proctor has devoted to it numerous memoirs and popular lectures, and speaks of it as his chief incentive to the labor of constructing his set of twelve star maps; Mr. Peirce gives it considerable space in his "Photometric Researches." From these sources we have a few conjectures and a few facts.

The richest parts of the sky, in bright and faint stars alike, are almost all about the Milky Way. This stream of suffused light follows, with some irregularities, the course of a great circle; and toward the plane of this circle, passing not very far, perhaps, from the sun, stars at all distances appear to become more densely packed. The Milky Way itself is evidence of this for the faintest magnitudes; and Herschel's star-gauges, from which he inferred for our cluster the shape of a disk or lens, give the comparison in a numerical form.