THE PARALLAX OF FIXED STARS. 161 



double stars, but he did not succeed in establishing any 

 parallax. 



In the year 1837, the late Professor Bessel applied this 

 method to determine the parallax of the double star 61 

 Cygni. This is a small star, hardly exceeding the sixth 

 magnitude, which had been pointed out for special ob- 

 servation by the circumstance of its having a very large 

 proper motion, viz., more than five seconds per year, be- 

 ing a more rapid motion than had been detected (until 

 recently) in the case of any other star, on which account 

 it had been suspected to be comparatively near our sys- 

 tem. Bessel repeatedly measured the distance of this star 

 from two other stars in its neighborhood, both of them 

 very minute, and therefore presumed to be very distant : 

 and he continued his observations every month, when 

 practicable, for three years. The distances were measured 

 by means of a magnificent heliometer, which Fraunhofer 

 of Munich, had recently executed for the observatory of 

 Konigsberg. It appeared that in January of each year, 

 the distance of 61 Cygni from one of the stars of compar- 

 ison was one third of a second less than the mean dis- 

 tance ; while in June it was one third of a second greater 

 than the mean. This effect is precisely such as should be 

 produced by the motion of the earth about the sun, caus- 

 ing an apparent displacement of the nearer stars as com- 

 pared with those which are more remote, and as no other 

 explanation of the phenomenon seems admissible, these ob- 

 servations are considered as settling the long vexed ques- 

 tion of parallax. In 1841, Professor Bessel received the 



