After the laws of the motions of the planets had been determined, it was only natural that men should ask themselves what forces were concerned in these motions. The ancient philosophers had proposed the idea that the sun attracts all heavenly bodies, but the suggestion had not been accepted by the world at large. However, after the discoveries of Kepler, the English, philosopher Newton advanced the theory that there is in the universe an attractive force which influences all matter, beyond the limits of known space. He further proved that the intensity of this force varies directly with the product of the attractive masses, and inversely, with the square of the distances between them--that is, the greater the bodies the greater the attraction; the greater the distance between them, the smaller the attraction. This law of gravitation has been verified by repeated experiments, and, taken in connection with the astronomical theories of Copernicus and Kepler, has made celestial mechanics what they are today. By the aid of the law of gravitation, many astronomical predictions have been fulfilled. Among the most famous is the following incident: In the early part of the last century, astronomers noticed that the motions of the planet Uranus did not agree with those derived from calculations based upon the law of gravitation. About 1846, two investigators, M. Leverrier, of France, and Mr. Adams of England, stated, as their opinion, that the discordance between theory and observation in the case of the motions of Uranus, was due to the attraction of a planet, not yet known, and they calculated by means of the law of gravitation, the size and orbit of the unknown planet. In the fall of 1846, this planet was actually discovered and named Neptune. It was found to harmonize with the predictions made by the astronomers before its discovery. During the days of Newton, the question was raised if the celestial bodies outside of the solar system obey the law of gravitation. Among the stars, there are some which are called double stars, and which consist of two stars so near to each other that the telescope alone can separate them to the eye. In 1803, after twenty years of observation, William Herschel discovered that some of these couples were revolving around each other with various angular velocities. The son of William Herschel continued this work, and many years later, he discovered that the laws of motion of these double stars are the same as those that prevail in the the solar system.[A] This result indicated not only the universality of the law of gravitation, but also the probability that all heavenly bodies are in motion. [Footnote A: History of the Inductive Sciences, Whewell, 3rd ed. Vol. I, pp. 467-469.] [Sidenote: The invention of the spectroscope laid the foundation of the new astronomy.]