part. Still it was formerly only too clear that,
so long as we were unable to ascertain directly those
components of the stars’ motions which lay in
the line of sight, the speed and 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.
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 obscure companion was inclined to the
line of sight, the companion would pass above or below
the bright star and produce no variation of its light.
Such systems might be numerous in the heavens.
In Vogel’s photographs, Spica, which was not
variable, by a small shifting of its lines revealed
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