Until recently the light from the heavenly bodies, even when collected by the largest telescopes, conveyed to us but very meagre information. With regard to the moon, sun, and some of the planets, in addition to their form and size, we have been able, by this means, to obtain some slight knowledge of their physical structure. But, with reference to the myriads of stars, clusters, and nebulæ, which people the depths of space, the telescope reveals little more than variety in color, brightness, and shape. (In relation to the nebulæ, this was illustrated by diagrams contrasting the appearance presented by the same objects when viewed in the telescopes of Sir John Herschel and of Lord Rosse.)
The discovery of "Spectrum Analysis"—the optical Analysis of Light—enables us to interpret symbols and indications hidden within the light itself. Wherever the tiny waves of light—the swift messengers of the celestial realms—can penetrate, they bear with them intelligence of their origin! "Bodies, so remote that astronomers fail to give us an idea of their distance, are brought, as it were, into our grasp, and are analyzed with certainty! We recognize in them the same elements which compose the soil we tread—the water we drink—the air we breathe!"
Before proceeding to explain the manner in which this new method of investigation decides the question of the existence of true nebulous-masses in the regions of space, it is necessary to recall certain well-known and long-established principles in optical science. In 1675, the immortal Newton demonstrated the composite nature of solar light. When a ray of sunlight is made to pass through a glass prism, it is refracted and spread out into a fan-like band, so as to exhibit exquisite gradations of color, from red at one end to violet at the other. This constitutes the Prismatic or Solar Spectrum. In 1802, Wollaston discovered that this spectrum is not continuous, but is interrupted by a number of dark lines. In 1815, Fraunhofer, by great improvements in the optical arrangements employed, rediscovered these lines—ascertained that their relative distances from each other were fixed for sunlight—and succeeded in mapping no less than fifty of them as belonging to the solar spectrum. Since that time, the number of these lines has been increased to thousands. The sagacious Fraunhofer traced these same dark fixed lines in reflected as well as in direct solar light: he found them quite unaltered in position, in the spectrum of moonlight and Venus-light. He, likewise, discovered, that the spectra of the fixed stars contained dark lines differing from those seen in the solar spectrum. He thence drew the important conclusion that these lines have their origin in the luminary. Fraunhofer thus opened the inquiry; but the explanation and import of these lines were reserved for a subsequent epoch.
Modern investigations have established the existence of three orders of spectra depending upon the source of the light: 1. A continu-
