yet known to the astronomer to learn anything about the universe as a whole? We may commence by answering this question in a somewhat comprehensive way. It is possible only because the universe, vast though it is, shows certain characteristics of a unified and bounded whole. It is not a chaos, it is not even a collection of things, each of which came into existence in its own separate way. If it were, there would be nothing in common between two widely separate regions of the universe. But, as a matter of fact, science shows unity in the whole structure, and diversity only in details. The Milky Way itself will be seen by the most ordinary observer to form a single structure. This structure is, in some sort, the foundation on which the universe is built. It is a girdle which seems to span the whole of creation, so far as our telescopes have yet enabled us to determine what creation is; and yet it has elements of similarity in all its parts. What has yet more significance, it is in some respects unlike those parts of the universe which lie without it, and even unlike those which lie in that central region within it where our system is now situated. The minute stars, individually far beyond the limit of visibility to the naked eye, which form its cloud-like agglomerations, are found to be mostly bluer in color, from one extreme to the other, than the general average of the stars which make up the rest of the universe.
There are two points in the sky called poles of the Milky Way, which bear the same relation to it that the north and south poles of the earth bear to the equator: they lie in opposite directions and are each 90° from the central line of the milky arch. The careful counts of stars made since the time of the Herschels show that, as a general rule, there are fewest stars on a given surface of the sky round the galactic poles (as those of the Milky Way are called) and that their thickness gradually increases as we approach the great girdle itself. This feature of the sky will, with a little care, be evident even to the observer without a telescope, who will see that the stars are somewhat more numerous along the outskirts of the Milky Way than around its poles. The regions of the heavens occupied by the poles of the Milky Way are as far apart as two points in the sky can be. And yet, the most careful counts of the stars show that, although they are fewer in number around these poles than elsewhere, they are about equally thick in these two opposite directions. For every point in the sky there is an opposite point, one of the two being below the horizon when the other is above it. The relation is that of our antipodes to us. It is a noteworthy feature of the universe that a certain resemblance is found in any two opposite regions of the sky, no matter where we choose them. If we take them in the Milky Way, the stars are more numerous than elsewhere; if we take opposite regions in or near the Milky Way, we shall find more stars in both of them than elsewhere; if we take them in the region anywhere around the poles of the Milky Way, we shall find fewer stars, but they will be equally numerous in each of the two regions. We infer from this that whatever cause determined the number of the stars in space was of the same nature in every two antipodal regions of the heavens.
Another unity marked with yet more precision is seen in the chemical elements of which stars are composed. We know that the sun is composed of the same chemical elements which we find on the earth and which we resolve in our laboratories. These same elements are found in the most distant stars. It is true that some of these bodies seem to contain elements which we do not find on earth. But as these unknown elements are scattered from one extreme of the universe to the other, they only serve still farther to enforce the unity which runs through the whole. The nebulas are composed, in part at least, of forms of matter dissimilar to any with which we are acquainted. But, different though they may be, they are alike in their general character throughout the whole field we are considering. Even in such a feature as the proper motions of the stars, the same unity is seen. The reader doubtless knows that each of these objects is flying through space on its own course with a speed comparable with that of the earth around the sun. These speeds range from the smallest limit up to more than one hundred miles a second. Such diversity
