

312 Life, Researches, and Discoveries of F. W. Bessel. 



proofs of the imperfections of our instruments and the inade- 

 quacy of our methods, was one which combined, with distinct 

 grounds of a priori probability in favor of its proximity to our 

 system, peculiar advantages for the application of the mode of 

 observation contemplated. The proper motion is remarkably 

 large, — the greatest, with one exception, yet observed. This of 

 itself affords some presumption of proximity. Another, less 

 equivocal, is found in the fact, that it is demonstrably a binary- 

 double star, whose orbitual motion is remarkably rapid when 

 compared with the apparent angular distance of its individuals, 

 indicating a large angular dimension of the orbit mutually de- 

 scribed about each other as seen from the earth. As regards its 

 adaptation for micrometric observation, two minute stars, at the 



respective 

 pair, situa 



permit 



place without effecting a change of apparent distance from one 

 or other of them ; so that the maximum rapidity of change with 

 respect to one shall correspond to the minimum and near evan- 

 escence of such change with respect to the other, a very impor- 

 tant circumstantial character of the reality of any observed move- 

 ment supposed to arise from parallax. 



The only obstacle in the way of the detection of any minute 

 parallactic motion, by means of these stars, consisted in the diffi- 

 culty of measuring distances so large as 8' or 12' to the precision 

 of a very small fraction of a second. Thanks to the perfect 

 workmanship of the Munich opticians, this precision was render- 

 ed attainable by the heliometer constructed for the Konigsberg 

 Observatory, and there erected in 1829. The observations in 

 question were made in the years 1837-1840 ; and their result 

 as is well known, has satisfied every astronomer of the reality of 

 the parallax attributed by Bessel to this star, and of the near ap- 

 proximation to its true amount. 



The discussion of these observations involved considerations 

 of very great delicacy, chiefly turning on the effect of tempera- 

 ture on the focal distances and metallic mounting of the lenses, 

 as well as on an infinity of minute considerations as to the effects 

 of refraction, &c, and of instrumental errors on the measures oi 

 angles of such magnitude in various positions with respect w 

 the vertical. Every thing of this nature has been made tne 

 subject of minute and careful inquiry in four very elaborate |*- 



pens 



sen 



ipeak. The first of these enters in its fullest extent urt 

 sral theory and formulae of an equatorially mounte a n 

 The second is devoted to a special application of 

 theory to the Konigsberg instrument. Among the remark 



ometer. 



