560 ANNUAL REPORT SMITHSONIAN INSTITUTION, 102!] 



detail the methods of securing the necessary data : Magnitude, proper 

 motion, parallax, class of spectrum, and radial velocities for the 

 stars in 252 well-selected regions. In the first and second reports 

 (1911) Kapteyn was able to announce the formation of a committee 

 to share the responsibility of advancing this plan. Its membership 

 included Gill, Pickering, Hale, Kustner, Schwarzchild, Dyson, 

 Adams, and Kapteyn, and it is sad to stale that with Kapteyn 

 one-half of its members have already gone forever. Yet the whole 

 astronomical world is so convinced of the need of such a cooperative 

 plan that it will undoubtedly be continued. 



Next to the motions of the stars, their distribution in space 

 interested Kapteyn most keenly. In this connection he derived the 

 two well-known laws: The density law and the luminosity law, 

 the former giving the density of stars per unit of volume and the 

 change in the density with distance from the sun, and the latter, 

 the percentage of stars equal in luminosity to the sun, and of those 

 ten times, one hundred times, etc., as bright or as faint. Both are 

 statistical laws; they do not give the distance and brightness of the 

 individual stars, but how many stars there are at a certain distance 

 and of a certain brightness. By successive steps these researches 

 led Kapteyn to a conception of the distribution of the stars in space ; 

 they indicate that the stars are contained in a nearly ellipsoidal uni- 

 verse with an axial ratio of 5.1, with a decrease in the density away 

 from the center and with the sun at a distance of about 650 parsecs 

 from the center. In his last long paper on the subject, which with 

 the modesty of the really great, was called " A first attempt at a 

 theory of the arrangement and motion of the sidereal system," Kap- 

 teyn had the satisfaction of giving a beautiful exposition of his life 

 work. If a longer life had been granted to him, undoubtedly we 

 would have seen him elaborate his beloved subject; yet, as it is, it 

 must have been a great satisfaction to him to reach this goal. 



At about the time Kapteyn was spending his vacations in Leiden 

 for the purpose of making his determinations of stellar parallaxes, 

 he became acquainted and was soon on terms of warm friendship 

 with the man who was then the leader in practical astronomy, 

 David Gill, director of the observatory at the Cape of Good Hope. 

 The story of the Cape Photographic Durchmusterung is well known 

 to every astronomer. The difficulties met by Gill and Kapteyn 

 would have disheartened most men. Kapteyn's famous letter of 

 1886 to Gill, offering his help in the following words, " However, 

 1 think my enthusiasm for the matter will be equal to (say) six 

 or seven years of such work" has been widely quoted. It took 

 about double that time, yet his enthusiasm did not fail, and the 

 Cape Photographic Durchmusterung was completed with a thor- 



