5 io THE POPULAR SCIENCE MONTHLY. 



an infinite universe of stars to show everywhere such a uniformity of 

 plan. 



How the fixed stars are actually distributed through space, an in- 

 quiry 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 struc- 

 ture 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 propor- 

 tions 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 ex- 

 tent of occupied space in that direction ; and he undertook to gauge 

 the depths and discover the shape of this cluster by counting tele- 

 scopic fields in different parts of the sky. The elder Struve consid- 

 ered 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 nu- 

 merous 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 fol- 

 lows, with some irregularities, the course of a great circle ; and to- 

 ward 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. 



* Professor Newcomb's account of these researches and speculations, in hia " Popular 

 Astronomy," pages 462-476, is full and interesting. 



