Page:A short history of astronomy(1898).djvu/487

probability turn out to be genuine binaries (chapter xii., § 264).

In addition to a number of minor papers Struve published three separate books on the subject in 1827, 1837, and 1852.^[1] 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

Fig. 101.—The orbit of ξ Ursae, shewing the relative positions of the two components at various times between 1781 and 1897. (The observations of 1781 and 1802 were only enough to determine the direction of the line joining the two components, not its length.)

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

William Herschel's observations of binary systems (chapter xii., § 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

1. Catalogus novus stellarum duplicium, Stellarum duplicium et multiplicium mensurae micrometricae, and Stellarum fixarum imprimis duplicium et multiplicium positiones mediae pro epocha 1830.