No direct proof that the nebulæ are gaseous could be obtained, however, before the spectroscope was known. The attempt to analyze the light of the nebulæ with this instrument was made in 1864, by Dr. Huggins, who directed his spectroscope to the planetary nebula, No. 4,373, in Draco. Its spectrum was found to consist of three bright, distinct lines, the brightest of which corresponded with the strongest nitrogen line, and the feeblest with the hydrogen C line. Besides these lines, it gave also a very faint, continuous spectrum, apparently due to a central point of condensation. By this observation, the gaseous nature of a nebula was for the first time demonstrated. Dr. Huggins thus analyzed 70 nebulæ, of which one-third gave a gaseous spectrum, consisting of several bright lines, the brightest of which invariably corresponded with the lines of nitrogen. The others gave a continuous spectrum, with the red end usually deficient. These results indicate that if some of the so-called nebulæ are due to an aggregation of stars, either too minute or too remote in space to be individually resolved, others are in a gaseous state. Yet the faint, continuous spectrum, given by some nebulæ, in addition to their gaseous spectra, seems to show that these nebulæ have some stars or matter in a different state, either involved in them or projected on their surface.
The idea of diffused matter distributed here and there in space, and gradually condensing into stars, is by no means new. As early as 1572, Tycho Brahé proposed such an hypothesis, to explain the sudden apparition of a new star in Cassiopeia, which he considered as formed by the recent agglomeration of the "celestial matter" diffused in space. Kepler adopted the same idea to explain the new star which appeared in Ophiuchus, in 1604. Halley, Lacaille, Mairan and others, entertained the same opinion. The hypothesis of a self-luminous, nebulous matter diffused in space, and forming here and there immense masses, has been proposed from the origin of the telescope, and was adopted by Sir William Herschel, who in his grand speculations on the universe considered the nebulæ as immense masses of phosphorescent vapors, gradually condensing around one or several centres into stars or clusters of stars. The evidence afforded by the spectroscope seems to be in favor of such an hypothesis, and shows us that gaseous agglomerations exist in space.
According to our modern conception, the visible universe is but an infinitely small portion of the infinite universe perceived by our mind. The great blazing centre around which our little, non-luminous globe pursues its endless journey, is only an humble member of a cluster comprising four hundred equally powerful suns, as they are believed to be, although they appear to us as little twinkling stars. The nearest of these stars is 221,000 times as far from the Sun as the Sun is from the Earth, and yet this entire cluster is only one among the several hundred Star-clusters composing the great galactic nebula in which we are involved, comprising thirty or fifty millions of such suns. Among the 4,000 irresolvable nebulæ in the sky, perhaps over one-half are supposed to be galaxies, like our own galaxy, composed of star-clusters, and millions of stars. Besides these remote galaxies, vast agglomerations of yet uncondensed, nebulous matter exist in space, and form the nebulæ proper, in which the genesis of suns is slowly elaborated. Although the visible universe is limited by the penetrating power of our instruments, yet we see in imagination the infinite universe stretching farther and farther; but we know not whether this invisible universe is totally devoid of matter, or whether it also is filled with millions and millions of suns and galaxies.
[APPENDIX]
KEY TO THE PLATES
PLATE I.—GROUP OF SUN-SPOTS AND VEILED SPOTS.
Observed June 17, 1875, at 7h. 30m. A. M.
The background shows the sun's visible surface, or photosphere, as seen with a telescope of high power at the most favorable moments, composed of innumerable light markings, or granules, separated by a network of darker gray. The granules, each some hundreds of miles in width, are thought to be the flame-like summits of the radial filaments or columns of gas and vapor which compose the photospheric shell. The two principal sun-spots of the group here represented show the characteristic dark umbra in the centre, overhung by the thatch-like penumbra, composed of whitish gray filaments. The penumbral filaments are not supposed to differ in their nature from those constituting the ordinary photosphere, save that they are seen here elongated and violently disturbed by the force of gaseous currents. Both spots are traversed partly or wholly by bright overlying faculæ, or so-called luminous bridges, depressed portions of which, in the left-hand spot, form the gray and rosy veils commonly attendant upon this class of spots. In each of these spots, also, the inner ends of projecting penumbral filaments have fallen so far within the umbra as to appear much darker than the rest. At the right of the upper portion of the left-hand spot, is a mass of white facular clouds, honey-combed by dark spaces, through which are seen traces of the undeveloped third spot of the triple group first observed. If seen upon the sun's limb, this would have presented the appearance of a lateral spot. Above the right-hand spot is a small black "dot," or incipient spot, without distinct penumbra. The irregular dark rift below the two large spots and connecting them is a spot of the crevasse type, with very slight umbra, a still better example of which is seen in a westward prolongation of the penumbra of the left-hand spot. In the upper left-hand corner of the Plate are seen several small faculæ, appearing as irregular whitish streaks amongst the granules. In the pear-shaped darkening of the solar surface below and at their left, is seen a veiled spot, two of which attended this group.
Approximate scale, 2500 miles—1 inch.
PLATE II.—SOLAR PROTUBERANCES.
Observed May 5, 1873, at 9h. 40m. A. M.
A view of an upheaval of the chromosphere, or third outlying envelope of the sun, as observed with the tele-spectroscope, or telescope with spectroscope attached.