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confusion. In the larger, he found 582 single stars, 46 clusters, and 291 nebulae.
The Milky "way —Via Lactea or Galaxy, as it is variously termed—is that luminous, cloud-like band that stretches across the heavens in a great circle. It is inclined to the celestial equator about 63°, and intersects it in the constellations Cetus and Virgo. This stream of suns is divided into two branches from a Centauri to Cygnus. To the naked eye it presents merely a diffused light; but with a powerful telescope it is found to consist of myriads of stars densely crowded together. These stars are not uniformly distributed through its entire extent. In some regions, within the space of a single square degree we can discern as many as can be seen with the naked eye in the entire heavens. In other parts there are broad open spaces. A remarkable instance of this occurs near the Southern Cross. There is a dark pear-shaped vacancy with a single bright star at the centre, glittering on the blue background of the sky. In viewing it, one is said to be impressed with the idea that he is looking through an opening into the starless depths beyond the Milky Way.
The number of stars in the galaxy which may be seen by Herschel's great reflector is estimated at twenty-one and a half millions. With the more powerful instruments now being made it is probable the number will be largely increased. The northern galactic pole is situated near Coma Berenices, and the southern in Cetus. Advancing from either pole toward the Milky Way, the number of stars increases, at first slowly and then more rapidly, until the proportion at the galaxy itself is thirty-fold.
HerscJid's theory.—Sir W. Herschel has conjectured that the stars are not indifferently scattered through space, but are collected in a stratum something like that shown in the cut, and that our sun
occupies a place at S, near where the stream branches. A and E are the galactic poles. It is evident that, to an eye viewing the stratum of stars in the direction SB, SC, or SD, they would seem much denser than in the direction SA or SE. Thus are we to think of our own sun as a star of the second or third magnitude, and our little solar system as plunged far into the midst of this vortex of worlds, a mere atom along that
"Broad and ample road Whose dust is gold and parement stars."
Nebular Hypothesis.—This is a theory which was advanced by Laplace, to show how the solar system was formed. In the "beginning," all the matter which now composes the sun and the various planets, with their moons, was in a gaseous and highly heated state. It filled all the space now occupied by the system, and extended far beyond the orbit of Neptune. In other words, the solar system was simply an immense nebula. The heat, which is the repellant force, overcame the attraction of gravitation. Gradually the mass cooled by radiation. As centuries passed, the repellent force becoming weaker, the attractive force drew the matter and condensed it toward one or more centres. The nebula then presented the appearance of a nebulous star—a nucleus enveloped to a great distance by a gaseous atmosphere. According to a well-known law in philosophy, seen in every-day life, in a whirlpool, a whirlwind, or even in water poured into a funnel, wherever matter seeks a centre, a rotary motion is established. As this rotary motion increased, the centrifugal force finally overcame at the exterior the attraction of gravitation, and so threw off a ring of condensed vapor. Centuries elapsed, and again, under the same conditions, a second ring was detached. Thus, one by one, concentric rings were separated from the parent nebula, all revolving in the same plane and in the same direction. These different rings, becoming gradually consolidated, formed the planets,—generally however, in this process, while still in the vaporous state and slowly condensing, themselves throwing off rings which were in turn consolidated into satellites. In the case of Saturn, several of these secondary rings did not break up, and so condense into globes, but still remain as rings which revolve about the planet.* Mitchell naively remarks, "Saturn's rings were left unfinished to show us how the world was made." The ring which formed the minor planets broke up into small fragments, none large enough to attract the rest and thus form a single globe. The central mass of vapor finally condensed itself into the sun, which remains the largest member of the system. According to this theory, the sun may yet give off a few more planets, whose orbits will not exceed its present diameter. After a time its heat will have all been radiated into space, its fire will become extinct, and life on the planets will cease. "We know not when this remote event may occur. We cannot fathom the purpose of God in creating and maintaining this system of worlds, nor foretell how soon it may complete its mission. We are assured, however,
"That nothing walks with aimless feet,
* It is possible that these rings may yet break up and form new satellites for that planet. Indeed, some hold that one at least of the rings has thus been resolved into small meteorites. These may be attracted, and so picked up, one by one, by the larger in succession, until they form another moon, which will continue to revolve about the planet as the ring does now.
Spectrum Analysis.—The rainbow—that child of the sun and shower—is familiar to all. The brilliant band of colors, seen when the sunbeam is passed through a prism, is scarcely less beautiful. The ray of light containing the primary colors is spread out fan-like, and each tint reveals itself. This variously colored band is called in philosophy a spectrum (plural, spectra). There are three different kinds of spectra—
1st. When the light of a solid or liquid body, as