RECENT ADVANCES IN SCIENCE 197 



twentyfold what it was in 1902 ; further parallaxes to the 

 extent of three or four hundred will be published very shortly 

 from the Allegheny and Leander McCormick Observatories. 

 It is now possible to discard all the visual determinations 

 except the best of those made with the heliometer, as modern 

 photographic methods give results with a much smaller prob- 

 able error. In addition, a large number of determinations by 

 the spectroscopic method are accumulating at the Mount 

 Wilson Observatory, and their publication is being temporarily 

 deferred merely until the method of reduction has been made 

 definitive. Shortly we may expect the publication of hun- 

 dreds, if not of thousands, of spectroscopic determinations. 

 The number of accurate determinations of stellar apparent 

 magnitudes, both visual and photographic, also shows a 

 remarkable increase since 1902 : in fact, no really accurate 

 photographic determinations of magnitude were available 

 before 1902. Much work has been done of recent years at 

 Greenwich in this country, at Harvard and Mount Wilson in 

 America, and at Potsdam in Germany, towards increasing our 

 knowledge in these directions. The difference between the 

 photographic and visual magnitudes is called the colour-index 

 of a star, and can be correlated with its spectral type. Our 

 knowledge of the spectral types of stars is mainly due to the 

 valuable work done at the Harvard College Observatory, and 

 it will be enormously increased when the publication of the 

 valuable Henry Draper catalogue (the first volume of which, 

 covering oh to 4/t of right ascension, has already appeared) is 

 completed. Further, the accurate determinations of the 

 proper-motions of the stars show a great increase, as the 

 interval of time since the first reliable determinations of posi- 

 tion increases. The Greenwich and Albany Observatories 

 have done much towards increasing the number of accurate 

 proper-motions. 



In view of this enormous increase in the material available 

 for statistical discussion, Prof. Kapteyn has commenced a 

 revision and extension of some of his earlier investigations. 

 These earlier results were mainly applicable to the mean of the 

 whole stellar system : an attempt is now being made to ex- 

 tend them for different galactic latitudes, over fainter stars 

 and over the several spectral classes. In Groningen Publica- 

 tions, No. 29, 191 8, he discusses the mean parallaxes of stars 



