d be satisfactorily accounted for by supposing an obscure body 0.764
the bright star's diameter to revolve round it in a period identical
with that of its observed variation. This theoretical forecast was
verified with singular exactitude at Potsdam in 1889.[1456] A series of
spectral photographs taken there showed each of Algol's minima to be
preceded by a rapid recession from the earth, and succeeded by a rapid
movement of approach towards it. They take place, accordingly, when the
star is at the furthest point from ourselves of an orbit described round
an invisible companion, the transits of which across its disc betray
themselves to notice by the luminous vicissitudes they occasion. The
diameter of this orbit, traversed at the rate of twenty-six miles a
second, is just 2,000,000 miles; and it is an easy further inference
from the duration and extent of the phases exhibited that Algol itself
must be (in round numbers) one million, its attendant 830,000 miles in
diameter. Assuming both to be of the same density, Vogel found their
respective masses to be four-ninths and two-ninths that of the sun, and
their distance asunder to be 3,230,000 miles.

This singularly assorted pair of stars possibly form part of a larger
system. Their period of revolution is shorter now by six seconds than it
was in Goodricke's time; and Dr. Chandler has shown, by an exhaustive
discussion, that its inequalities are comprised in a cycle of about 130
years.[1457] They arise, in his view, from a common revolution, in that
period, of the close couple about a third distant body, emitting little
or no light, in an orbit inclined 20 deg. to our line of vision, and of
approximately the size of that described by Uranus round the sun. The
time spent by light in crossing this orbit causes an apparent delay in
the phases of the variable, when Algol and its eclipsing satellite are
on its further side from ourselves, balanced by acceleration while they
traverse its hither side. Dr. Chandler derives confirmation for his
plausible and ingenious theory from a supposed undulation in the line
traced out by Algol's small proper motion; but the reality of this
disturbance has yet to be established.[1458] Meanwhile, M.
Tisserand,[1459] late Director of the Paris Observatory, preferred to
account for Algol's inequalities on the principle later applied by
Belopolsky to those of Castor. That is to say, he assumed a revolving
line of apsides in an elliptical orbit tr