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been found, leading to a belief in motion
round an invisible companion. Vogel, in
1889, discovered this in the case of Spica
ˆ
(I l Virginis), whose period is 4d. 0h. 19m.,
and the diameter of whose orbit is six mil-
lion miles. Great numbers of binaries of this
type have since then been discovered, all of
short period.
Also, in 1889, Pickering found that at
611
