Prof. Besscl on the Parallax of the Star 61 Cygni. 69 



When the atmosphere has been unusually unsteady, I have, however, 

 made more numerous repetitions ; although, by this, I fear the result 

 has not attained that precision which it would have possessed by 

 fewer observations on more favourable nights. This unsteadiness 

 of the atmosphere is the great obstacle which attaches to all the 

 more delicate astronomical observations. In an unfavourable cli- 

 mate we cannot avoid its prejudicial influence, unless by observing 

 only on the finest nights ; by which, however, it would become still 

 more diflicult to collect the number of observations necessary for an 

 investigation. The places of both stars, referred to the middle point 

 of the double star, are for the beginning of 1838. 



Distance. Angle of Pos. 



a 46r'-617 201° 29' 24" 



b 706 -279 109 22 10 



As the instrument gives, at the same time, the distance and angle 

 of position, 1 have always observed both. But the position circle is 

 divided only into whole minutes ; which, in the distance of the first 

 star, have the value of 0''"134 ; in that of the second, 0""205. More- 

 over, other causes exist which may render the observation of the 

 angle of position less certain than that of the distances. 1 have, 

 accordingly, considered the first of these as of less consequence in 

 so delicate an investigation, and concentrated my attention, as far as 

 I could, upon the latter. 



The following tables [which M'ill be found in the Monthly No- 

 tices of the Society, vol. iv. No. 17.] contain all my measures of di- 

 stance, freed from the effects of refraction and aberration, and re- 

 duced to the beginning of 1838. In these reductions, the annual va- 

 riations employed of both distances are =: +4""3915 and — 2"'825; 

 which 1 have deduced (on the supjiosition that the stars a and b 

 have no proper motions) from the mean motions of both stars of 

 61 Cygni, which M. Argelander had latel)' found by comparison of 

 my determination (from Bradley's observations) for 1755, with his 

 own for 1830. In the meantime, we cannot regard these variations 

 of distance as the true variations ; because the stars compared may 

 have proper motions, and, also, because it is not known whether the 

 mean of the motions of both stars of 61 Cyr/ni appertains to its 

 centre, and whether this (motion) is proportional to the time. In 

 what follows, let us denote the true variations of the distances by 

 + 4"-3915 + a' and — 2''-825 + /)', the mean distances for the be- 

 ginning of 1S35 by a and /d ; the time, reckoned from this begin- 

 ning, by t; the difference of the constants of the annual jjarallax of 

 61 Cygni, and of the comjiarison- stars a and b, by a" and />" ; and, 

 lastly, the coefficients of the parallax depending on the place of the 

 earth Ijy a. Then the expressions of the distances at the beginning 

 of 1838 arc— 



For the star « = a + / u'-\- a a" 

 For the star b = ft + tft'+ a ft" 



These expressions, as they were at the time of each observation, I 



