Ball — Researches on Annual Parallax. 



359 



The equations of condition are now formed in tlie usual manner. 



The true mean distance at the epoch is assumed to be 73"-06414 - x ; 

 the true value of the proper motion in declination is assumed to be 

 3"'016 + x' ; the correction to be applied to an observed difference of 

 declination for parallax ct is - [9-92229] cos (© - 121° 39') Hzj. The 

 correction to be applied for a possible difference k between the coeffi- 

 cients of aberration of the two stars is- [9-92229] sin (© - 121° 39') k. 



Solving the epilations, I obtain 



X = + 0"-0227 ± 0"-0194, 

 x' = - 0"-0878 ± 0"-0705, 

 OT = + 0"-4676 ± 0"-0321, 

 K = + 0"-07l9 ± 0"-0314. 



The sum of the squares of the absolute terms is 6*172, while the 

 sum of the squares of the residuals is but 1-223, the weights being of 

 course taken into account in each case. 



The probable error of one observation is + 0"-1298, whence the 



Fig. 2.- 



-Parallax in Declination of 61 (B) Cygni. Ordinates indicate Parallax 

 of 0"'47. Dots indicate Observations. 



probable error of the mean of four observations which form a complete 

 series for one night is + 0"-0649. 



The agreement of the present parallax of 0"-4676 with that of 



