Page:Sir William Herschel, his life and works (1881).djvu/157

Rh 1782 he published his "Catalogue of Double Stars," and his last published memoir (1822) was on the same subject.

The question of determining the parallax of stars first brought to the discovery of double stars. If two stars, A and B, appear very close together, and if, in reality, the star B is very many times more distant from the earth than A, although seen along the same line of sight, then the revolution of the earth in its orbit will produce changes in the relative situation of A and B, and, in fact, B will describe a small orbit about A, due to this revolution. This idea had been proposed by, and measures on this plan had been made by , with negative results. But, in reviewing their work, declares that the stars chosen by were not suitable to the purpose. It is necessary, among other things, to the success of this method, that it should be certain that the star B is really very much more distant than the star A. The only general test of the distance of stars is their brilliancy, and decided to use only