In 1835 F. B. G. Nicolai, director of the observatory at Mann heim, in discussing the motion of Halley's comet, considered the possibility that it was acted upon by an ultra-Uranian planet, the existence of which was made probable by the disagreement be tween the older and more recent observations.
In 1838 Airy showed in a letter to the Astronomische Nach richten that not only the heliocentric longitude, but the tabulated radius vector of Uranus was largely in error, but made no sugges tions as to the cause. In 1843 the Royal Society of Sciences of GEittingen offered a prize of 5o ducats for a satisfactory working up of the whole theory of the motions of Uranus, assigning Sep tember 1846 as the time within which competing papers should be presented. It is also recorded that Bessel, during a visit to England in 1842, in a conversation with Sir John Herschel, ex pressed the conviction that Uranus was disturbed by an unknown planet. He went so far as to set his assistant Fleming at the work of reducing the observations, but died before more was done. The question had now reached a stage when it needed only a vigorous effort by an able mathematician to solve the problem. Such a man was found in John Couch Adams, then a student of St. John's College, Cambridge, who seriously attacked the prob lem in 1843, the year in which he took his bachelor's degree. He soon found that the observations of Uranus could be fairly well represented by the action of a planet moving in a radius of twice the mean distance of Uranus, which would closely correspond to Bode's law. During the two following years he investigated the possible eccentricity of the orbit, and in September 1845 com municated his results to Professor James Challis. In 1845, about the ist of November, Adams also sent his completed elements to Airy, stating that according to his calculations the observed irreg ularities in the motion of Uranus could be accounted for by the action of an exterior planet, of which the motions and orbital elements were given. It is worthy of note that the heliocentric longitude of the unknown body as derived from these elements is only between one and two degrees in error, while the planet was within half a degree of the ecliptic. Two or three evenings assidu ously devoted to the search could not therefore have failed to make the planet known. Adams's paper was accompanied by a comparison of his theory with the observations of Uranus from 178o, showing an excellent agreement. Airy in replying to this letter inquired whether the assumed perturbation would also ex plain the error of the radius-vector of Uranus, which he seemed to consider the crucial test of correctness.
lished communication on the subject was made to the French Academy on the loth of November 1845, a few days after Adams's results were in the hands of Airy and Challis. A second memoir was presented by Leverrier in 1846 (June 1). His investigation was more thorough than that of Adams. He first showed that the observations of Uranus could not be accounted for by the attraction of known bodies. Considering in succession various explanations, he found none admissible except that of a planet exterior to Uranus. Considering the distances to be double that of Uranus he then investigated the other elements of the orbit.
The following are the elements found by Adams and Leverrier : The longitude of the planet was 57' on Oct. I, 1846.
The close agreement of these elements led Airy to suggest to Challis, on the 9th of July 1846, a search for the planet with the Northumberland telescope. He proposed an examination of a part of the heavens 3o° long in the direction of the ecliptic and o° broad, and estimated the number of hours' work likely to be employed in this sweep. The proposed sweeps were commenced by Challis on the 29th of July. The plan required each region to be swept through twice, and the positions of all the known stars found to be compared, in order that the position of the planet might be detected by its motion. On the 31st of August Lever rier's concluding paper was presented to the French Academy, and on the i8th of September he wrote to John G. Galle (181 2— 1910), then chief assistant at the Berlin observatory, suggesting that he should search for the computed planet, with the hope of detecting it by its disk, which was probably more than 3" in diameter. This letter, probably received on the 23rd of September, was communicated to J. F. Encke, the director of the observatory, who approved of the search. H. L. d'Arrest, a student living at the observatory, expressed a wish to assist. In the evening the search was commenced, but it was not found possible to detect any planet by its disk. Star charts were at the time being pre pared at the observatory under the auspices of the Berlin Academy of Sciences. It was suggested by d'Arrest that this region might be covered by one of the charts. Referring to the chart, which was lying in a drawer, it was found that such was the case. Com paring the stars on the chart one by one with the heavens it was found that an eighth magnitude star now visible was not on the chart. This object was observed until after midnight, but no certain motion was detected. On the following evening the object was again looked for, and found to have moved. The existence of the planet was thus established. It was afterwards found that Challis had observed the planet on Aug. 4, but had failed to detect it.