The
nebulae 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 might seem to detract from the unity of the whole; but
when we seek to learn something definite by taking their average,
we find this average to be, so far as can yet be determined, much
the same in opposite regions of the universe. Quite recently it
has become probable that a certain class of very bright stars
known as Orion stars--because there are many of them in the most
brilliant of our constellations--which are scattered along the
whole course of the Milky Way, have one and all, in the general
average, slower motions than other stars. Here again we have a
definable characteristic extending through the universe.
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