Now, certain objections have been made to the nebular theory, the most important of which is based upon the direction of the rotation of the two outermost planets and their satellites. The satellites of the inner series of planets, from Mercury to Saturn, inclusive, revolve as the planets do, from

west to east.

Those of Uranus, however, revolve nearly at

Fig. 6. Motion of planets and satellites if formed simultaneously, as by Faye's hypothesis.

right angles with the plane of rotation of the planet, while those of Neptune revolve in a reverse direction. This would at first appear to be a serious objection to the theory; but recently the French astronomer, Faye, has propounded an hypothesis which explains this apparent anomaly. (See Figures 5 and 6, above and on the opposite page.) Faye's

explanation rejects the theory of Laplace in part, and goes back to that of Descartes, which assumes that the planetary rings were produced by a vortical or whirlpool motion of the original nebulous matter, and nearly simultaneously, instead of by the successive separation of concentric rings. Supposing that the entire mass from which the planets are formed revolves in one general direction as a whole, like a grindstone or wheel, it is evident that the outer edge of the mass would rotate much faster than the inner portions. This would also be true of a vortical ring formed within the mass, and, when that ring broke up, as its outer edge would tend to move faster than its inner edge this tendency would be impressed upon the resulting spheroid, which consequently would rotate on its axis in the same direction in which it moved around the sun. In this way, M. Faye thinks, the inner planets received their impulse of rotation from west to east. But before the rings of Neptune and Uranus (being formed somewhat later than the others) condensed into planets, the sun had attracted to itself nearly all the matter not already formed into planets, and the rings, being thus left separate, began to revolve, not as if they all formed parts of one disk, but independently. Thereupon their velocity varied inversely as their distance from the sun, their outer edges tended to move more slowly than their inner edges, and consequently the planets formed from them rotated in the opposite direction to their revolution around the sun. Of course the satellites formed from these planets would revolve around their primaries in the same direction in which the primaries rotated on their axes.

Dr. Karl Braun, a German philosopher, has suggested another theory of planetary evolution. He assumes that throughout the original mass various centers of condensation were formed, which ultimately became planetary bodies, revolving around the largest centre of condensation of all, which was the sun. All these theories agree in assuming that the original condition of the universe was that of a nebular mass, and that suns and satellites were evolved from it by the action of laws precisely similar to those which we behold still active in this world in which we dwell.

If, now, on looking at the starry heavens through the telescope, we find nebulous masses in the same condition in which we have supposed the sun and the earth to have formerly existed, it will go far to confirm the nebular hypothesis

of the origin of the solar system. That is precisely what we may observe. (A representation of a nebula in the constellation of Lyra was thrown upon the screen.

See Fig

ure 7.) If we look at this nebula, we find great rifts in it, and brighter spots in some places, which show that there is greater condensation in certain parts than elsewhere. A nebula in the constellation of Leo (shown upon the screen) shows a marked center of attraction, with evidences of a

tendency to circular motion. A nebula in Draco (illustrated from a photograph taken by Prof. Holden by the aid of the new Lick telescope, the largest in the world) shows a

spiral formation of a new order, as if a star had revolved around another star, which was itself in motion, and had left a train of nebulous matter behind it like a comet. (See spiral nebula in Canes, Figure 13, page 69.) A photograph (thrown upon the screen) of a strange nebulous object attached to the star Maia in the Pleiades, is interest

appearance.

ing as being the picture of an object which has never been seen by the eye of man, even by the aid of the most powerful telescope. The photographic plate is more sensitive than the retina of the human eye. This is an apparent illustration of the hypothesis that all nebulæ may have been

formed originally from an invisible gas. The great nebula in the sword of Orion (see Figure 8, page 63) shows many points of condensation. From it will be developed a little universe within a greater universe, forming in time not a single sun or star, but a cluster of stars.

We have seen in the nebulæ evidencies of rotation and contraction. Now let us consider the condition of a body after it has passed out of the nebulous into the solid state. (A picture of a portion of the sun's surface is thrown upon the screen, showing a dark back-ground, mottled with shining spots. See illustration, Figure 9, on page 64,

Fig. 10. Section of the Sun's Surface, showing a belt of sun-spots, near the center of the disk.

opposite.) The surface of the sun, of which we shall first speak, is not uniformly clear and shining, but broken up into bright parts interspersed with parts that are less luminous. The shining portions are sometimes called "rice-grains," from their appearance, and seem like clouds of luminous matter. These are the parts of the sun which give light. Sir John Herschel suggested that they might be "living organisms," but they are merely the hotter portions of the solar surface. Among them sometimes appear great rifts or spots (a picture of solar spots is thrown upon the screen) which are never seen at the poles, but always within a cer