Among modern astronomical discoveries none has been regarded as more important than that of Neptune, the outermost known planet of the solar system. It was a rich reward to the watchers of the sky when this new planet swam into their ken. This discovery was hailed by astronomers as "the most conspicuous triumph of the theory of gravitation." Long after Copernicus even, the genius of philosophers was slow to grasp the full conception of a spherical earth and its relations with the heavenly bodies as presented by him. So it was also with the final acceptance of Newton's demonstration of the universal law of gravitation (1685), whereby he showed that "the motions of the solar system were due to the action of a central force directed to the body at the centre of the system, and varying inversely with the square of the distance from it." After making this discovery, Newton himself, with the aid of others, especially of the French mathematician Picard, labored for years to verify it, and still further verification was necessary before it could be fully comprehended and accepted by the scientific world. The discovery of the asteroids or small planets revolving in orbits between those of Mars and Jupiter, aided in confirming the Newtonian theory, which the discovery of Uranus, by Sir William Herschel (1781), had done much to establish. From the time of Sir William Herschel the science of stellar astronomy, revealing the enormous distances of the stars--none of them really fixed, but all having real or apparent motions--was rapidly developed. The discovery of stellar planets, at almost incalculable distances, still further changed the aspect of the heavens as viewed by astronomers, and when the capital discovery of Neptune was made those men of science were well prepared for studying its nature and importance. These matters, as well as the simultaneous calculation of the place of Neptune by Adams and Leverrier, and its actual discovery by Galle, are set forth by Sir Oliver Lodge in a manner as charming for simplicity as it is valuable in its summary of scientific learning.