124 Parallax of 61 1 Cygni, deduced from 



ferences have been corrected by their proportional part of the 

 variation of distance. This variation is deduced from the devia- 

 tion of the sum of the distances on the various plates from the 

 mean of all the plates. See Table XVI, columns four to seven 

 inclusive. 



Equations of condition thus formed are in Table XVII. At the 

 foot of each page of this table will be found the normal equa- 

 tions, the deduced values of the unknowns, with their probable 

 errors and the probable error of one equation of unit weight. 



Eight different stars combined in five pairs were used for the 

 determination of the parallax by measures of distance. 



34. Whereas heretofore only measures of distance were used 

 for parallax, in the present research measures of angle have also 

 been used. For it has been recently shown* in the case of eight 

 stars among the Pleiades whose average distance is 2160", rang- 

 ing from 631" to 3160", that the displacement on the arc of a 

 great circle by reason of the probable error of observation is but 

 little larger in measures of angle than of distance. This re- 

 search on the measures of 61 1 Cygni has shown in addition that, 

 for the seven stars whose measurement of position-angle I have 

 used, the probable error for one plate of unit weight is ±o".i49 

 in angle, whereas for the eight stars used in distance it is ±o ,f .igi. 

 The average distance of these stars is only 1956", ranging from 

 1075" to 2754". This fully j ustifies the use of position-angle for 

 parallax determination from the Rutherfurd photographic plates 

 in the present and in future reductions. 



35. In the use of measures of position-angle the seven stars 

 were selected irrespective of distance but as equally distributed 

 in angle as possible. 



Several methods of reduction suggested themselves, but the 

 following was adopted. The observed angles were corrected as 

 described in paragraphs 16-21. The "reduced angles," obtained 

 from adding together columns six and seven of Table VI and the 

 " variation " of paragraph 21, are printed in Table XVIII. 



If x' and y' be introduced for error of the adopted mean of 

 the angles and of proper motion respectively, and the parallax 

 coefficient 



#8 Pi + #9 Pi 



* On the Permanence of the Rutherfurd Photographic Plates, by Harold Ja- 

 coby. Page 282. 



