And now we may proceed to explain the subject of this chapter.
Up to the year 1846, when Leverrier made his great discovery, it was believed that Uranus was the most distant planet revolving around the sun. Uranus itself was discovered by Sir John Herschel in England in the year 1781. As this planet takes eighty-four years to go round the sun, its complete revolution had not yet been observed in 1846; in spite of this, however, the course of Uranus was calculated and known very precisely, because the attractive force of the sun was known; and all the disturbances that might influence the planet were taken into account.
But notwithstanding all nicety of calculations, the real course of Uranus would not at all agree with the one computed. At that time already, long before Leverrier's discovery, the idea arose that beyond Uranus, in a region where the human eye could, in spite of all telescopes, discover nothing, there must probably exist a planet which changed the course of Uranus. Bessel, a great astronomer, who unfortunately for science died too soon, was already on the point of finding out by computation the unknown disturber. But he died, shortly before Leverrier's discovery. As early even as 1840, Maedler, in the city of Dorpat, in Russia, wrote a fine article on this as yet unseen disturber.
Leverrier, however, began the task and finished it. He computed with an acuteness that was admired by all men of science. He investigated whereabout in the heavens that intruder must be situated, so as to be able to trouble Uranus to such an extent; how fast this disturber itself must move in its orbit, and how large must be its mass.
We live to see the triumph of Leverrier's being able to discover with his mental eye, by means of computation only, a planet at a distance of millions of miles from him.
Therefore let us say: Honor science! Honor the men that cultivate it! And all honor to the human intellect which sees farther than the human eye!