_Variable Stars._--It has often happened in the history of different
branches of physical science that observation and experiment were so
far ahead of theory that hopeless confusion appeared to reign; and
then one chance result has given a clue, and from that time all
differences and difficulties in the previous researches have stood
forth as natural consequences, explaining one another in a rational
sequence. So we find parallax, proper motion, double stars, binary
systems, variable stars, and new stars all bound together.

The logical and necessary explanation given of the cause of ordinary
spectroscopic binaries, and of irregular proper motions of Sirius and
Procyon, leads to the inference that if ever the plane of such a
binary orbit were edge-on to us there ought to be an eclipse of the
luminous partner whenever the non-luminous one is interposed between
us. This should give rise either to intermittence in the star's light
or else to variability. It was by supposing the existence of a dark
companion to Algol that its discoverer, Goodricke of York,[16] in
1783, explained variable stars of this type. Algol (beta Persei)
completes the period of variable brightness in 68.8 hours. It loses
three-fifths of its light, and regains it in twelve hours. In 1889
Vogel,[17] with the Potsdam spectrograph, actually found that the
luminous star is receding before each eclipse, and approaching us
after each eclipse; thus entirely supporting Goodricke's opinion.
There are many variables of the Algol type, and information is
steadily accumulating. But all variable stars do not suffer the sudden
variations of Algol. There are many types, and the explanations of
others have not proved so easy.

The Harvard College photographs have disclosed the very great
prevalence of variability, and this is certainly one of the lines in