550 THE POPULAR SCIENCE MONTHLY. 



tions for the observatory. Bessel, who had then completed his 

 Fundamenti Astronomic, gave him some calculations of stars 

 not observed since Bradley that he had identified, and the prob- 

 lem of determining the latitude of the observatory from obser- 

 vation of the pole star not yet worked out. The publication of 

 the results of these labors in the Konigsberg observations in- 

 troduced the author to the scientific world as Bessel's most capa- 

 ble 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 ob- 

 servation ; 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 em- 

 ployed 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 con- 

 cerning 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 Fin- 



