The chances are over four hundred thousand to one that the association in position is not accidental. This argument becomes overwhelming when the same association is found in many other cases. There were two hundred and three doubles in the Catalogue of 1782 alone, and many thousands are now known.
By a process like this, HERSCHEL reached his grand discovery of true binary systems, where one sun revolves about another. For he saw that if the two stars are near together in space, they could not stand still in face of each other, but that they must revolve in true orbits. Here was the discovery which came to take the place of the detection of the parallaxes of the fixed stars.
He had failed in one research, but he was led to grand conclusions. Was the force that these distant pairs of suns obeyed, the force of gravitation? This he could not settle, but his successors have done so. It was not till about 1827 that SAVARY, of the Paris Observatory, showed that one of HERSCHEL'S doubles was subjected to the law of gravitation, and thus extended the power of this law from our system to the universe at large. HERSCHEL himself lived to see some of his double stars perform half a revolution.
Of HERSCHEL'S discoveries, ARAGO thinks this has "le plus d'avenir." It may well be so. The laws which govern our solar system have been extended, through his researches, to regions of unknown distance. The binary stars will afford the largest field for research into the laws which govern them, and together with the clusters and groups, they will give a firm basis by which to study the distribution of stars in general, since here we have the great advantage of knowing, if not the real distance of the two stars from the earth, at least that this distance is alike for both.
After HERSCHEL'S first publication on the mountains of the Moon (1780), our satellite appears to have occupied him but little. The observation of volcanoes (1787) and of a lunar eclipse are his only published ones. The planets _Mercury_, _Venus_, _Mars_, and _Jupiter_, although they were often studied, were not the subjects of his more important memoirs. The planet _Saturn_, on the contrary, seems never to have been lost sight of from the time of his first view of it in 1772.