stars for this research which had two components very greatly different in brightness. A must be very bright (and presumably near to us), and B must be very close to A, and very faint (and thus, presumably, very distant).
It was in the search for such pairs of stars that the Catalogue of Double Stars (1782) was formed. Herschel's first idea of a double star made such pairs as he found, to consist of two stars accidentally near to each other. A was near to us, and appeared projected in a certain place on the celestial sphere. B was many times more distant, but, by chance, was seen along the same line, and made with A an optical double. If the two stars were at the same distance from the earth, if they made part of the same physical system, if one revolved around the other, then this method of gaining a knowledge of their distance failed. Even in his first memoir on the subject, a surmise that this latter state might occur in some cases, was expressed by Herschel. The notes on some of the pairs declare that a motion of one of them was suspected. But this motion might be truly orbital—of
