of the curve of outline; one has three at the angles of an equilateral triangle; a long nebula has two stars at the ends of its longest diameter.

These points are supposed to be centers about which the nebulous matter is accumulating. They have also been thought to be suns surrounded by dense atmospheres, made visible by the transmitted light, as a fog becomes visible about a lamp.

576. Double Nebulæ.-As we find double and multiple stars, so we find double and multiple nebulæ. In these grouped lights, are seen the same varieties of form which appear in the single nebula. We find associated two globular masses; two elliptical masses; an elliptical with a globular; two globes surrounded by bright arcs, like fragments of a broken ring; and, finally, a large elliptical mass of light, on whose outer edge are scattered, not very regularly, seven smaller globose masses, as small bunches are seen growing on a larger potato.

THE MILKY WAY.

577. The names, Galaxy from the Greek, Via Lactea from the Latin, and our own Milky Way, all refer to the broad white band which traverses the entire circuit of the sky. The Chinese call it The Celestial River; the North American Indians, The Road of Souls.

578. Its course is in a great circle inclined about 63° to the equinoctial, which it crosses in the constellations Cetus and Virgo. Beginning near the Eagle, we trace it south-east through Cassiopeia, to the right of Capella and Procyon, and to the left of Orion and Sirius; thence it passes through the Ship, and so beyond our horizon. Beyond Argo it divides into several fan-like branches, which unite again near the Cross. Beyond the Centaur,

it divides into two streams, which flow side by side through the Scorpion, Sagittarius, and the Eagle, to the place of beginning.

579. Its breadth and brightness. Near the Cross, where it is narrowest, it is only three or four degrees wide. At the Ship, and also at the Scorpion, it spreads over about twenty degrees of the sky. The brightest part in the north is near the Eagle and the Swan; the part in the south between the Ship and the Altar is yet more brilliant. Near this southern portion is a series of the brightest stars in the sky, beginning with Sirius, and including the beautiful stars of the Ship, the Cross, the Centaur, and the Scorpion. When this part of the sky rises above the southern horizon it. brings a glow of light like that of the new moon.

580. The telescope resolves the galaxy into countless multitudes of stars, irregularly grouped. Star-clusters are very numerous, especially in the southern part. In some regions the stars are strewn very uniformly, in others a rapid succession of closely clustering, rich patches are separated by comparatively poor intervals, or, in some instances, by spaces quite dark and devoid of any star, even of the smallest telescopic magnitude.

A bright portion near the Cross surrounds a dark place of considerable breadth, and of pear-shaped form, called the coal-sack. Similar spaces are found in the Scorpion, and in Ophiuchus. They are like windows opened through the dense wall of stars, through which we look forth into vast regions of starless space.

In many places the galaxy is so completely resolved by the telescope that the stars seem to shine out against a black ground; in others a faint white glimmer remains unresolved, showing that in these directions it has not yet been fathomed.

NEBULÆ—1, Crab Nebula in Taurus. 2, Great Neb. in Andromeda. 3, Dumb. bell Neb. in Vulpecula. 4, in Leo. 5, Multiple Neb. in Nubecula Major. 6, Double

Nebula. 7, in Coma Berenices.

F.297.

581. The distribution of the stars. As the galaxy passes round the sky in nearly a great circle, the two points of the sky on either side, equidistant from that circle, may be called the galactic poles. Herschel I. made an elaborate investigation of the distribution of stars in the heavens by a system of "star-gauging." By counting the stars visible at once in the field of his telescope, and then comparing results from different parts of the sky, he found:

That the spaces near the galactic poles contained the smallest average number of stars;

That the average was generally the same at the same distance from the poles;

That the averages increased with the distance, at first slowly, afterward much more rapidly;

That in the galaxy, the stars were crowded so thickly as to defy counting.

THEORIES OF THE GALAXY.

582. Herschel's theory.-That the sun is a member of an immense system of stars which form a layer or bed of circular shape, having but little thickness in comparison with its breadth. That this bed is split near its southern edge, the two flat surfaces diverging as if a wedge had been driven between them.

The figure shows a section of this supposed star-system, the sun being at S, not far from the split.

Herschel supposed that the stars are distributed pretty uniformly throughout this space. If so, a telescope pointed toward b would include in a single field but few stars, because the line of sight would soon pass out of the layer; if turned toward e more would be seen at once, and if toward f, a still larger number. Hence, he thought that his labors of star-gauging would show,