162 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1'J16, 



resulting from his campaign of spectroscopic observing in both the 

 Northern and Southern Hemispheres, begun about the year 1898. 

 From this work he finds the sun to be moving at about 19.5 kilome- 

 ters (12 miles) per second in its course among the stars. 



Eapid progress is being made in measuring the radial motions of 

 the fainter stars at Mount Wilson Solar Observatory. The 60-inch 

 reflecting telescope there has been employed, under the direction of 

 Dr. W. S. Adams, for this purpose since 1910, and the new 100-inch 

 reflecting telescope also will soon be available. Other great reflectors 

 are being prepared for Canada and for Argentina, and will doubtless 

 be joined in this work. As it is a slow business at best, observers of 

 the radial motions of the fainter stars will generally confine their 

 measurements to what are termed Kapteyn's selected areas. 



Prof. Kapteyn, of Groningen, Holland (who has just been deco- 

 rated for his astronomical work by the Emperor of Germany with 

 the Prussian order Pour le Merite, at the same time with a group 

 of generals, marshals, and kings), has been engaged for many years 

 in a general study of the motions and distances of the stars. His 

 studies are continually thwarted by lack of information about the 

 fainter stars, which are so numerous that they will never be all 

 observed individually. Hence Kapteyn has proposed that attention 

 be devoted to 206 selected areas all over the sky, each about 1| 

 degrees square, so that samples of the stars so chosen may have 

 their positions, motions, brightness, distances, and spectral classes 

 determined within a reasonable time. The distances will always be 

 the weak point, but progress will be rapid along the other lines. 



Now, let us see how knowledge of proper motions, radial motions, 

 and distances can be combined when all three are known, as in the 

 case of a few individual stars. FroiA the distance and proper 

 motion together we learn that the star appears to move at right 

 angles to the line of sight at a certain rate in miles per second. 

 The proper motion also indicates in which direction this cross motion 

 is taking the star. The spectroscope indicates that the star is 

 approaching or receding at a certain rate. By combining the two 

 components — the apparent cross and radial motions — the actual speed 

 and direction of the star's motion becomes fully determined. Apply- 

 ing next a correction for the known motion of the solar system, the 

 star's own peculiar motion with respect to the whole system of stars 

 is at length found. 



STAR GROUPING. 



As the distance is so weak a link in this chain, several devices have 

 been employed to strengthen it, and these depend in one way or an- 

 other on star grouping. First of all, there are a good many pairs 

 of stars which have been shown by telescopic observations to be 



