In 1841 the difficulties of the problem presented by these residual perturbations of Uranus excited the imagination of a young student, an undergraduate of Cambridge--John Couch Adams by name--and he determined to make a study of them as soon as he was through his _tripos_. In January, 1843, he was graduated as senior wrangler, and shortly afterward he set to work. In less than two years he reached a definite conclusion; and in October, 1845, he wrote to the astronomer-royal, at Greenwich, Professor Airy, saying that the perturbations of Uranus could be explained by assuming the existence of an outer planet, which he reckoned was now situated in a specified latitude and longitude. We know now that had the astronomer-royal put sufficient faith in this result to point his big telescope at the spot indicated and begin sweeping for a planet, he would have detected it within 1-3/4º of the place assigned to it by Adams. But anyone in the situation of the astronomer-royal knows that almost every post brings absurd letters from ambitious correspondents, some of them having just discovered perpetual motion, or squared the circle, or proved the earth flat, or discovered the constitution of the moon or of ether or of electricity; and in this mass of rubbish it requires great skill and patience to detect such gems of value as may exist. Now this letter of Adams's was indeed a jewel of the first water, and no doubt bore on its face a very different appearance from the chaff of which I have spoken; but still Adams was unknown: he had been graduated as senior wrangler, it is true, but somebody must be graduated as senior wrangler every year, and a first-rate mathematician is not produced every year. Those behind the scenes--as Professor Airy of course was, having been a senior wrangler himself--knew perfectly well that the labeling of a young man on his taking his degree is much more worthless as a testimony to his genius and ability than the general public is apt to suppose. Was it likely that a young and unknown man should have solved so extremely difficult a problem? It was altogether unlikely. Still, he should be tested: he should be asked for explanations concerning some of the perturbations which Professor Airy had noticed, and see whether he could explain these also by his hypothesis. If he could, there might be something in his theory. If he failed--well, there was an end of it. The questions were not difficult. They concerned the error of the radius vector. Adams could have answered them with perfect ease; but sad to say, though a brilliant mathematician, he was not a man of business. He did not answer Professor Airy's letter.