Page:Popular Science Monthly Volume 40.djvu/563

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IT is only, curiously enough, within the last decade or two that the science of astronomy has answered to its name. Until the methods of spectrum analysis and of photography were applied to the stars, astronomers were scarcely justified in their title, for they knew little about the stars, and, hardly hoping to know more, almost confined their attention to the solar system. Now, although sidereal astronomy, the science of astronomy par excellence, is still in its infancy, we may discern pretty clearly what will be the nature of its achievements. Surpassing the wildest dreams of the older astronomers in range and penetration, modern astronomy yet brings the whole cosmos within the grasp of human intelligence. Not only are the stars in process of being numbered, their motions, proper and relative, in course of measurement, their physical constitution subjected to analysis, and their distances brought within computation; but the entire sidereal system is recognized as limited in extent, and the form and magnitude of the vast group in space will at no distant date become susceptible of approximate delineation and calculation.

Of the methods referred to, photography has had, perhaps, the largest share in the recent advancement of sidereal science. The chemistry of the stars, it is true, is founded wholly on spectrum analysis, that profound and searching means of testing the composition of bodies by the action of elementary substances, under proper conditions, upon the infinitesimal undulations which give rise to the phenomena of light; but without the aid of photography, the mapping of star-spectra must have remained a slow and inaccurate process. The camera, on the other hand, has revealed almost all that is known concerning the number, distances, masses, and motions of the stars; the lens has no "personal equation," and never gets tired; sensitized gelatin responds with infinite celerity to the undulations which make no impression whatever upon the eye; and star-pictures of the heavens are not only permanent records, but, with the proper instruments and skill, can be so readily taken that before very long it is probable that some seven hundred thousand out of the whole sixty millions of stars will be accurately charted and indexed.

For such is the least number of the heavenly host—which a French astronomer somewhat extravagantly estimates to contain nearly seventy thousand millions of suns; for each star we see is a sun shining with its own light, and governing probably, like our own, the motions of a system of planets. Nor is the light

  1. The System of the Stars. By Agues M. Clerke. London: Longmans. 1890.