The feeling of wonder, admiration, and enthusiasm aroused by this intellectual triumph was overwhelming. In the world of astronomy reminders are met every day of the terrible limitations of human reasoning powers; and every success that enables the mind’s eye to see a little more clearly the meaning of things has always been heartily welcomed by those who have themselves been engaged in like researches. But, since the publication of the _Principia_, in 1687, there is probably no analytical success which has raised among astronomers such a feeling of admiration and gratitude as when Adams and Le Verrier showed the inequalities in Uranus’s motion to mean that an unknown planet was in a certain place in the heavens, where it was found. At the time there was an unpleasant display of international jealousy. The British people thought that the earlier date of Adams’s work, and of the observation by Challis, entitled him to at least an equal share of credit with Le Verrier. The French, on the other hand, who, on the announcement of the discovery by Galle, glowed with pride in the new proof of the great powers of their astronomer, Le Verrier, whose life had a long record of successes in calculation, were incredulous on being told that it had all been already done by a young man whom they had never heard of. These displays of jealousy have long since passed away, and there is now universally an _entente cordiale_ that to each of these great men belongs equally the merit of having so thoroughly calculated this inverse problem of perturbations as to lead to the immediate discovery of the unknown planet, since called Neptune. It was soon found that the planet had been observed, and its position recorded as a fixed star by Lalande, on May 8th and 10th, 1795. Mr. Lassel, in the same year, 1846, with his two-feet reflector, discovered a satellite, with retrograde motion, which gave the mass of the planet about a twentieth of that of Jupiter. FOOTNOTES: [1] Bode’s law, or something like it, had already been fore-shadowed by Kepler and others, especially Titius (see _Monatliche Correspondenz_, vol. vii., p. 72). BOOK III. OBSERVATION 10. INSTRUMENTS OF PRECISION—STATE OF THE SOLAR SYSTEM. Having now traced the progress of physical astronomy up to the time when very striking proofs of the universality of the law of gravitation convinced the most sceptical, it must still be borne in mind that, while gravitation is certainly the principal force governing the motions of the heavenly bodies, there may yet be a resisting medium in space, and there may be electric and magnetic forces to deal with. There may, further, be cases where the effects of luminous radiative repulsion become apparent, and also Crookes’ vacuum-effects described as “radiant matter.” Nor is it quite certain that Laplace’s proofs of the instantaneous propagation of gravity are final.