148 Parallax of 61 2 Cygni, deduced from 



Incidentally it should be noticed that the assumed annual 

 change in distance of 61 2 from 61 1 was — o".i28, the negative 

 sign being used to indicate separation. From the value of y in 

 Table XIX the correction to this is o".027. Thus the Ruther- 

 furd plates for an interval of only 2.6 years give : 



Annual increase of distance = o // . 101 ± // .03o. 



In like manner, the assumed increase of position-angle being 

 o°.370 and Table XX, giving as a correction a further increase 

 of o°.i5i, there results : 



Annual increase of angle = o°. 521 =h°.o8i. 



But what is of greater significance, there is also a difference of 

 parallax : 



(61 1 Cygni — 61 2 Cygni) = + o".072± ".028 



11. This result is so surprising and yet the difference of paral- 

 lax obtained from angles and from distances is so accordant 

 within the limits of their respective probable errors that I have 

 deemed it advisable to use the very excellent series of measures 

 of the distance of these two stars given by Wilsing in Sitzungs- 

 berichten* der Konigl. Preuss. Akademie der Wissenschaften, 

 1893, Bd. 40, to see what value of the difference of parallax, if 

 any, might be deduced from them. 



42. Using Auwers' values of the right ascension and declina- 

 tion of 61 1 Cygni for 1875.0 and his constants of precession we 



have : 



a = 2i h 2 m o s .6 ) . 



d = 38"i2'.48".6i for lS9I -° 



I have taken p = ii4°.692 for 1873 with an annual variation of 

 o .5O which gives p — i23°.5 for 1 891.0 quite accurately enough ; 

 as an error of a whole degree would, demonstrably, have no sen- 

 sible effect upon the deduced value of parallax. Therefore : 



S 3 = [9-9849] sin 35o°.8 = — 0.154 

 #4 = [9 9181] sin 27i°.3 = — 0.828 



With these and the values of P s and P 4 computed for each date 

 of Wilsing's platesf were formed the parallax coefficients 



which are found in the equations of condition of Table XXI. 



*Since given more fully in PMicationen des Astrophysikalischen Observator- 

 iums zu Potsdam, Nr. 36, Bd. XI, 1897. 

 \8itzungsberichten, etc., pages 883-4. 



