It has been found, with the aid of the spectroscope, that the most slowly moving of all stars are the extremely hot bluish Orion stars with an average velocity of eight miles per second, while the most rapidly moving stars are the deep-red stars with an average velocity of twenty-one miles per second, and there is in all cases a relationship existing between the color, or spectrum, of a star and its velocity. The reason for this connection between the two still remains undiscovered. The spectroscope has also told us some astonishing facts in recent years about the velocities of the spiral nebulæ. It is now known that these mysterious objects are moving with the tremendous average velocity of _four hundred and eighty miles per second_, which exceeds the average velocity of the stars fully twenty-five fold. They possess, moreover, internal motions of rotation that are almost as high as their velocities through space. It is now generally believed that spiral nebulæ are far distant objects of enormous size and mass, exterior to our own system of stars and similar to it in form. In place of the universe of the "fixed stars" and the immovable sun or earth of a few centuries ago we find that modern astronomical discovery is substituting a universe of inconceivable grandeur and immensity in a state of ceaseless flux and change. Our earth--an atom spinning about on its axis and revolving rapidly around a huge sun that is equal in volume to more than a million earths--is carried onward with this sun through a vast universe of suns. Only an average-sized star among several hundred million other stars is this huge sun of ours, moving with its planet family through the regions of the Milky Way, where are to be found not only moving clusters and groups of stars, speeding along their way in obedience to the laws of motion of the system to which they belong, but also strangely formed nebulæ covering vast stretches of space, whirling and seething internally and shining with mysterious light, and still other stretches of dark obscuring matter shutting off the rays of suns beyond. The extent and form of this enormous system of stars and nebulæ and the laws that govern the motions of its individual members are among the problems that the astronomers of today are attempting to solve. On both sides of these regions of the Milky Way, wherein lies our own solar system, lie other vast systems, such as the globular star clusters, composed of thousands, possibly hundreds of thousands, of suns; the Magellanic clouds, which resemble detached portions of the Milky Way, and, probably, the much discussed spiral nebulæ, possible "island universes" similar to our own. We have come far in the past three hundred years from the conception of an immovable earth at the center of the universe to this awe-inspiring conception of the universe that we have today, which is based upon modern astronomical discoveries.