MOUNT WILSON SOLAR OBSERVATORY. 



269 



4 minutes in an east-and-west line. Identification of 20 spectra is 

 possible, though some are composite and represent several close stars. 

 Spectra of some stars of Barnard's visual magnitude 15 are included. 



Taking as units five divisions of the Harvard scale, we have 1 star 

 of Ao, 6 of A5, 9 of Fo, 3 of F5, and 1 of Go. 



The mean of this determination is practically the same as that given 

 by the 19 stars mentioned in last year's report. The two results com- 

 bined, making a total of 39 stars, give the mean class of the cluster as Fq. 



PROFESSOR KAPTEYN'S INVESTIGATIONS. 



The papers by Professor Kapteyn mentioned in the last report as 

 being nearly ready for the press, have been printed as Contributions 

 Nos. 82 and 83 under the titles "On the Individual Parallaxes of the 

 Brighter Galactic Helium Stars in the Southern Hemisphere, together 

 with Considerations on the Parallax of Stars in General," and ''On the 

 Change of Spectrum and Color Index with Distance and Absolute 

 Brightness. Present State of the Question." As a sequel to the first 

 of these, a beginning has been made upon the investigation of the 

 galactic helium stars between longitudes 150° and 216°, including 

 mainly the constellation of Orion. 



A great difficulty lies in the fact that the bulk of these stars is very 

 near the vertex of motion. As a consequence, the derivation of sys- 

 tematic corrections to the proper motions Boss's Preliminary General 

 Catalogue becomes very troublesome. On the other hand, such correc- 

 tions are all-important, for, as a rule, the proper motions near the vertex 

 are very small. Much labor was spent 

 on this question, with the result that the 

 accompanying corrections were applied 

 to the proper motions in declination. 



The last of these is the most uncer- 

 tain. It is, however, of little importance 

 for the present purpose. The very large 

 correction near declination —30° may well seem startling, but as 

 several independent determinations all agree in yielding a large posi- 

 tive value, its reality seems well established. With the derivation of 

 these corrections, the last link in the chain of arguments is furnished 

 by which it is shown that the stars above and below galactic longitude 

 216° belong to two different groups. Particulars of further results 

 are omitted, as they may still be considerably changed by a more 

 detailed investigation. 



The further elaboration of the subject was temporarily interrupted 

 in favor of another, more urgent, investigation undertaken jointly by 

 Professor Kapteyn and Mr. Adams. Mr. Adams finds that the radial 

 velocities of F, G, K, and M stars having small proper motions are 



