190 COSMOS. 



and Flamstead believed that they had discovered parallaxes 

 of several seconds, whereas they had confounded the proper 

 motions of the stars with the true changes from parallax. 

 On the other hand, the ingenious John Michell {Phil. Trans. 

 1767, vol. lvii., p. 234-264) was of opinion that the paral- 

 laxes of the nearest fixed stars must be less than 0"-02, and 

 in that case could only "bedjhe perceptible when magnified 

 12,000 times." In consequence of the widely-diffused opin- 

 ion that the superior brilliancy of a star must invariably in- 

 dicate a greater proximity, stars of the first magnitude, as, 

 for instance, Vega, Aldebaran, Sirius, and Procyon, were, 

 with little success, selected for observation by Calandrelli 

 and the meritorious Piazzi (1805). These observations must 

 be classed with those which Brinkley published in Dublin 

 (1815), and which, ten years afterward, were refuted by 

 Pond, and especially by Airy. An accurate and satisfactory 

 knowledge of parallaxes, founded on micrometric measure- 

 ments, dates only from between the years 1832 and 1838 



Although Peters,* in his valuable work on the distances 

 of the fixed stars (1846), estimates the number of parallaxes 

 hitherto discovered at 33, we shall content ourselves with re 

 ferring to 9, which deserve greater, although very different, 

 degrees of confidence, and which we shall consider in the 

 probable order of their determinations. 



The first place is due to the star 61 Cygni, which Bessel 

 has rendered so celebrated. The astronomer of Konigsberg 

 determined, in 1812, the large proper motion of this double 

 star (below the sixth magnitude), but it was not until 1838 

 that, by means of the heliorfteter, he discovered its parallax. 

 Between the months of August, 1812, and November, 1813, 

 my friends Arago and Mathieu instituted a series of numer- 

 ous observations for the purpose of finding the parallax of 

 the star 61 Cygni, by measuring its distance from the zenith. 

 In the course of their labors they arrived at the very correct 

 conclusion that the parallax of this star was less than half a 

 second.f So late as 1815 and 1816, Bessel, to use his own 



* Struve, Astr. Slell., p. 104. 



t Arago, in the Connaissance des Temps pour 1834, p. 281 : " Nous 

 observances avec beaucoup de soin, M. Mathieu et moi, pendant le 

 mois d'AoUt, 1812. et pendant le mois de Novembre suivant, la hauteur 

 angulaire de l'etoile audessus de l'horizon de Paris. Cette hauteur, a 

 la seconde epoque. ne surpasse la hauteur angulaire a la premiere que 

 de 0"-66. Une parallaxe absolue d'une seule seconde aurait necessaire- 

 ment amene eutre ces deux hauteurs une difference de l" - 2. Nos ob* 

 servations n'indiqueut done pas que le rayon de l'orbjte terreste, que 



