is much too soon to form any theories of small stars revolving round large ones". It was, of course, obvious that double stars might be optical only—caused by two stars happening to lie in the same line of vision. Nevertheless, despite his caution, he kept a careful watch on the relative positions of the components of double star systems for twenty years. In the paper accompanying his catalogue of 500 new nebulæ, read 1st July, 1802, Herschel expressed the view that "casual situations will not account for the multiplied phenomena of double stars," and announced that he was about to communicate a series of observations, from which it would be evident that "many of them have actually changed their situation with regard to each other, in a progressive course, denoting a periodical revolution round each other; and that the motion of some of them is direct, while that of others is retrograde". These observations were tabulated in two papers on "the changes that have happened in the relative situation of double stars," read in 1803 and 1804 respectively. In the first paper, Herschel brought forward evidence in regard to the orbital motions of Alpha Geminorum (Castor), Gamma Leonis, Epsilon Bootis, Zeta Herculis, Delta Serpentis, and Gamma Virginis. The second paper gave details concerning other fifty stars. Herschel also assigned periods to several of the more prominent binaries. These investigations, Herschel claimed, went to prove that many double stars are "not merely double in appearance, but must be allowed to be real binary combinations of two stars intimately held together by the bond of mutual attraction".
The importance of the discovery of binary stars may be realised when we recollect that previously there was no scientific proof of the prevalence of the law of gravitation outside of the Solar System. There were, of course, strong reasons for believing the law to be universal; and John Michell, a man of remarkable sagacity, had argued
