260 COSMOS. 



1816, Bessel, to use his own -words, " had arrived at no avail 

 able result.'' 20 The observations taken from August, 1837, to 

 October, 1838, by means of the great heliometer erected in 

 1829, first led him to the parallax of 0"'3483, which corresponds 

 with a distance of 592200 mean distances of the earth, and 

 a period of 9^ years for the transmission of its light. Peters 

 confirmed this result in 1842, by finding 0"-3490, but sub- 

 sequently changed Bessel's result into O v> 3744 by a correction 

 for temperature. 21 



tends an angle of 0"'5 only when it is observed at a distance 

 of 412000 times its length. Therefore the star 61 Cygni is 

 situated at a distance from our earth at least equal to 412 

 thousand times 39 millions of leagues." 



20 Bessel, in Schum. Jahrb. 1839, s. 39-49, and in the 

 Astr. Nachr., 110. 366, gave the result 0"-3136, as a first 

 approximation. His later and final result was 0"*3483. (Astr. 

 Nachr., no. 402, in bd. xvii. s. 274.) Peters obtained by 

 his own observations the following, almost identical, result, 

 of 0"-3490. (Struve, Astr. stell, p. 99.) The alteration which, 

 after Bessel's death, was made by Peters in Bessel's cal- 

 culations of the angular measurements, obtained by the 

 Konigsberg heliometer, arises from the circumstance that 

 Bessel expressed his intention (Astr. Nachr., bd. xvii. s. 267) 

 of investigating further the influence of temperature on 

 the results exhibited by the heliometer. This purpose he had 

 in fact partially fulfilled in the first volume of his Astronomische 

 Unter suchung en, but he had not applied the corrections of 

 temperature to the observations of parallax. This application 

 was made by the eminent astronomer Peters (Erganzungsheft 



zu den Astr. Nachr., 1849, s. 56), and the result obtained, 

 owing to the corrections of temperature, was, 0"'3744 instead 

 of 0"'3483. 



21 This result of 0"'3744 gives, according to Argelander, as 

 the distance of the double star 61 Cygni from the sun, 550900 

 mean distances of the earth from the sun, or 45576000 

 miles, a distance which light traverses in 3177 mean days. 

 To judge from the three consecutive statements of parallax 



