other systems like our own?"

"Yes," replied the doctor, "our recent investigations of the periods of
some of the variable stars show irregularities in brightness, period, and
proper motion. A close study of these irregularities has convinced some of
our astronomers that there are invisible bodies near them, evidently
planets circling around a central sun. The theory is that the dark bodies
cause slight perturbations in the star, which account for the
irregularities in period, motion, etc. So Neptune was discovered by the
effect it had upon the observed movements of Uranus. This is the first
evidence we have had tending to prove that there are other groups of
worlds like ours, and it is considered quite significant."

"I can readily believe it," said Thorwald, "and I know how helpful every
bit of evidence is, in your search for knowledge. But if I mistake not you
have the aid of another instrument, which is destined to play an important
part in your future studies. You get much nearer those distant orbs when a
spectroscope is placed at the end of the telescope, and the ray of light
coming from sun and star is widened out into a band of color, which tells
a marvelous story. That light, that has been for years, and perhaps for
centuries, on its way to you, now discloses the very nature of the
substances which compose those fiery globes. And what are those
substances? It must have been a startling truth to the man who first read
from the spectrum of the star he was studying, that it contained matter
with which he was familiar, materials of which the earth itself is made.
By this science you have learned beyond doubt that many of the commonest
elements of the earth's crust exist also in other worlds, and, what is of
great significance, that the materials most closely connected with living
organisms on the earth, such as hydrogen, sodium, magnesium, and iron, are
the very ones which are found most widely diffused among the stars. I