BURNS— RELATIVE AND ABSOLUTE PARALLAXES. 499 



results published by Campbell in L. O. B., 6, 132, 191 1, and re- 

 peated in column 4 of this table. Columns 3 and 5 give the number 

 of stars used to derive the values in columns 2 and 4, respectively. 

 It is likely that the mean magnitude of the larger group of stars of 

 the same class is the fainter, but the difference will be small. Camp- 

 bell's results were derived from a comparison of the tau components 

 with the radical velocities freed from the sun's motion. The values 

 just determined were derived by comparing total observed radial 

 velocity and total proper motion. Since the two methods give the 

 same results as nearly as could be expected from so few data, no 

 advantage in point of accuracy can be claimed for either. The use 

 of total motions has the advantage of being less laborious. 



TABLE I. 

 Average Computed Parallaxes. 



If now the observed relative parallaxes were at hand for all the 

 stars in either of these groups, the difference between the mean ob- 

 served and computed parallax would give the mean parallax of the 

 comparison stars. While we do not have the parallaxes of all these 

 stars, there are a sufficient number to permit the computation of a 

 preliminary value of the correction which must be applied to reduce 

 mean relative parallax to absolute. In the following discussion the 

 data were confined to stars of magnitude 5.0 and brighter. Stars 

 fainter than 5.0 have been selected for parallax observation almost 

 entirely on the basis of proper motion, and stars of large total motion 

 have been observed to the exclusion of others. But the most serious 

 defect of the data for faint stars lies in the fact that the motions of 

 the faint parallax stars are directed too nearly across the line of sight. 

 The data which were used are free from this difficulty. 



PROG. AMER. PHIL. SOC, VOL. LX, GG, MARCH l8, 1922. 



