8 



SEE— DETERMINATION OF THE 



[January 5, 



lip by Boss of the Dudley Observatory, Albany, New York. By 

 the recent study of several thousand of the brighter stars included 

 in his Preliminary General Catalogue, Professor Boss has deduced 

 their proper motions with a high degree of accuracy. Campbell 

 found from i8o of these stars resembling our sun in spectral type 

 that their average cross proper motion in the sky, from the values 

 derived by Boss, was about o.ii second of arc per annum, while 

 at the same time their average speed in the line of sight shown by 

 the spectrograph at Lick Observatory was 8.9 miles per second, or 

 two hundred and eighty million miles a year. Having the average 

 motion in the line of sight, in absolute units, and the average cross 

 proper motion in seconds of arc, it is easy to find how far away a 

 base line of 280 million miles would have to be to subtend an angle 

 of O.II of a second of arc. It turns out to be ninety-two light-years. 

 In this way it is possible to get the average distances of large 

 groups of stars. Here are some of the results found by Campbell. 



This table contains the most important results of the Campbell- 

 Boss method of obtaining average distances for large groups of 

 stars. It need scarcely be remarked that its significance can hardly 

 be overrated. But whilst the average values given are quite trust- 

 worthy, the method is of course inapplicable to the individual stars; 

 and if their distances are to be found recourse would have to be 

 had to the standard method of paralla.xes, or to the spectroscopic 

 method in the case of visual binaries. 



