direction of the solar motion in space, and many of the
great problems of the constitution of the heavens must have remained
more or less imperfectly known. Now the spectroscope had placed in our
hands this power, which, though so essential, had previously appeared
almost in the nature of things to lie forever beyond our grasp; it
enabled us to measure directly, and, under favorable circumstances, to
within a mile per second, or even less, the speed of approach or of
recession of a heavenly body. This method of observation had the great
advantage for the astronomer of being independent of the distance of
the moving body, and was, therefore, as applicable and as certain in
the case of a body on the extreme confines of the visible universe, so
long as it was bright enough, as in the case of a neighboring planet.

ALGOL AND SPICA.

By observations with the Potsdam spectograph, Professor Vogel found
that the bright star of Algol pulsated backward and forward in the
visual direction in a period corresponding to the known variation of
its light. The explanation which had been suggested for the star's
variability, that it was partially eclipsed at regular intervals of
68.8 hours by a dark companion large enough to cut off nearly
five-sixths of its light, was, therefore, the true one. The dark
companion, no longer able to hide itself by its obscureness, was
brought out into the light of direct observation by means of its
gravitational effects. Seventeen hours before minimum Algol was
receding at the rate of about 241/2 miles a second, while seventeen
hours after minimum it was found to be approaching with a speed of
about 281/2 miles. From these data, together with those of the variation
of its light, Vogel found, on the assumption that both stars have the
same density, that the companion, nearly as large as the sun, but with
about one-fourth his mass, revolved with a velocity of about
fifty-five miles a second. The bright star of about twice the size and
mass moved about the common center of gravity with the speed of about
26 miles a second. The system of the two stars, which were about 31/4
millions of miles apart, considered as a whole, was approaching us
with a velocity of 2.4 miles a second. The great difference in
luminosity of the two stars, not less than fifty times, suggested
rather that they were in different stages of condensation, and
dissimilar in density. It was obvious that if the orbit of a star with
an obs