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§§ 308,309] probability turn out to be genuine binaries (chapter, § 264).

In addition to a number of minor papers Struve published three separate books on the subject in 1827, 1837, and 1852. A comparison of his own earlier and later observations, and of both with Herschel's earlier ones, shewed about 100 cases of change of relative positions of two members of a pair, which indicated more or less clearly a motion of revolution, and further results of a like character have been obtained

from a comparison of Struve's observations with those of later observers.

William Herschel's observations of binary systems (chapter, § 264) only sufficed to shew that a motion of revolution of some kind appeared to be taking place; it was an obvious conjecture that the two members of a pair