[Sidenote: Philosophical import of Newton's discoveries.] Such was the physical meaning of Newton's discoveries; their philosophical meaning was of even greater importance. The paramount truth was resistlessly coming into prominence--that the government of the solar system is under necessity, and that it is mathematically impossible for the laws presiding over it to be other than they are. Thus it appears that the law of gravitation holds good throughout our solar system. But the heliocentric theory, in its most general acceptation, considers every fixed star as being, like the sun, a planetary centre. [Sidenote: Unity of idea in the construction of the universe.] Hence, before it can be asserted that the theory of gravitation is truly universal, it must be shown that it holds good in the case of all other such systems. The evidence offered in proof of this is altogether based upon the observations of the two Herschels on the motions of the double stars. Among the stars there are some in such close proximity to each other that Sir W. Herschel was led to suppose it would be possible, from observations upon them, to ascertain the stellar parallax. While engaged in these inquiries, which occupied him for many years, he discovered that many of these stars are not merely optically in proximity, as being accidentally in the same line of view, but are actually connected physically, revolving round each other in regular orbits. The motion of these double suns is, however, in many instances so slow as to require many years for a satisfactory determination. [Sidenote: Gravitation of double stars.] Sir J. Herschel therefore continued the observations of his father, and with other mathematicians, investigated the characteristics of these motions. The first instance in which the true elliptic elements of the orbit of a binary star were determined was given by M. Savary in the case of chi Ursæ Majoris, indicating an elliptic orbit of 58-1/4 years. But the period of others, since determined, is very much longer; thus, in sigma Coronæ, it is, according to Mr. Hind, more than 736 years. From the fact that the orbits in which these stars move round each other are elliptical, it necessarily follows that the law of gravitation, according to the inverse square, holds good in them. Considering the prodigious distances of these bodies, and the departure, as regards structure of the systems to which they belong, from the conditions obtaining in our unisolar system, we may perhaps assert the prevalence of the law of gravitation throughout the universe.