binary stars proved that the two stars attracted each other with a force which varies inversely as the square of their distance. Even under the most favourable circumstances the observations are very difficult; they cannot be made with the same accuracy as is attained in observing the movements of the planets; they have not even the value which antiquity will often confer on an observation which has not much else in its favour. There are probably many different suppositions which would explain all that has yet been observed as to the motions of the binary stars. Gravitation is but one of those suppositions. Gravitation will no doubt carry with it the prestige acquired by its success in explaining phenomena in the solar system. I do not know that any one has ever seriously put forward any other explanation except gravitation to account for the movements of the binary stars, nor is any one likely to do so while gravitation can continue to render an account of the observed facts; but all this is very different from saying that the discovery of the binary stars has proved that the law of gravitation extends to the stellar regions.
Except for what the binary stars tell us, we should know nothing as to the existence or the non-existence of the law of gravitation beyond the confines of the solar system. Does Sirius, for instance, attract the pole star? We really do not know. Nor can we ever expect to know. If Sirius and the pole star do attract each other, and if the law of their attraction be the same as the law of attraction in the solar system, it will then be easy to show that the effect of this attraction is so minute that it would be entirely outside the range of our instruments even to detect it. Observation in such a matter is hopeless. If
