In order that our knowledge of the position of a star may be
complete, we must know its distance. This can be measured only
through the star's parallax--that is to say, the slight change in
its direction produced by the swing of our earth around its orbit.
But so vast is the distance in question that this change is
immeasurably small, except for, perhaps, a few hundred stars, and
even for these few its measurement almost baffles the skill of the
most expert astronomer. Progress in this direction is therefore
very slow, and there are probably not yet a hundred stars of which
the parallax has been ascertained with any approach to certainty.
Dr. Chase is now completing an important work of this kind at the
Yale Observatory.

To the most refined telescopic observations, as well as to the
naked eye, the stars seem all alike, except that they differ
greatly in brightness, and somewhat in color. But when their light
is analyzed by the spectroscope, it is found that scarcely any two
are exactly alike. An important part of the work of the astro-
physical observatories, especially that of Harvard, consists in
photographing the spectra of thousands of stars, and studying the
peculiarities thus brought out. At Harvard a large portion of this
work is done as part of the work of the Henry Draper Memorial,
established by his widow in memory of the eminent investigator of
New York, who died twenty years ago.

By a comparison of the spectra of stars Sir William Huggins has
developed the idea that these bodies, like human beings, have a
life history. They are nebulae in infancy, while the progress to
old age is marked by a constant increase in the density of their
substance. Their temperature also changes in a way analogous to