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550 for the observatory. Bessel, who had then completed his Fundamenti Astronomiæ, gave him some calculations of stars not observed since Bradley that he had identified, and the problem of determining the latitude of the observatory from observation of the pole star not yet worked out. The publication of the results of these labors in the Königsberg observations introduced the author to the scientific world as Bessel's most capable pupil. Other similar calculations followed, and Argelander showed a growing devotion to practical astronomy.

The observation of an occultation of the Pleiades in August, 1820, was regarded by Argelander as his first astronomical observation; and he held a vivid recollection of its incidents in his later years. In October following he was appointed, on Bessel's nomination, assistant at the observatory. Bessel had conceived his plan for a Durchmusterung, or sounding of the sky, and had begun upon it, and wanted another astronomer. He employed Argelander to assist him in reading and writing down the micrometric indications on the circle. While waiting for this work to begin, Argelander made observations on setting stars, which were used by Bessel in completing his refraction tables to zenith distances 85° and 89°, and in observing the comet of 1821. His first independently published paper, written in 1821, as a thesis for the degree of Ph. D., was on Flamstead's Astronomical Observations, and largely concerned the errors of his instruments. A memoir on the comet of 1811 was published shortly afterward. It recorded fuller observations, and covering more time, than had been made before of any comet. Relying upon Bessel's methods for the comets of 1807 and 1815, and realizing that the value of labors of the kind lay in treating the observations exhaustively, he went back to the very beginning of them. Some unexpected difficulties met in the calculations, involving among other things apparent disagreements with the law of gravitation, led him to suppose that some abnormal force worked upon the comet—a hypothesis that was not without influence on Bessel's views concerning the repulsive force of comets' tails. Although Bessel further developed these views in his work on Halley's comet, and in the controversy with Encke, respecting the resisting medium, Argelander was afterward inclined to modify his own opinions, and to hold the question open whether the movements of comets pointed to the operation of foreign forces upon them. On the faith of this paper Argelander was given a license to teach in the university.

The place of Observer at the Observatory of Abo, Finland, having been made vacant by the death of Walbeck, Bessel was applied to to name a successor. He recommended Argelander, who was appointed in April, 1823. Argelander's journey to