136 REPORTS OF INVESTIGATIONS AND PROJECTS. 



groups or streams into which Kapteyn and Eddington have separated the 

 stars, and contribute in other important ways to the sohition of the problem 

 of sidereal distribution. The photographs would also serve, quite as well as 

 though made with no other end in view, for the study of stellar evolution. 



This possibility illustrates the mutual advantages of the arrangement I 

 have recently concluded with Professor Kapteyn. Astronomers are well 

 acquainted with his remarkable work, not only in the solution of cosmical 

 problems, but in organizing a cooperative attack involving the use, for a 

 common end, of telescopes in many parts of the world. Since faint stars, 

 as well as bright ones, must be included in any investigation of the general 

 laws of stellar distribution, it is evident that an attempt to cover the entire 

 sky, which would involve the minute study of many millions of stars, is out 

 of the question, at least within any reasonable period of time. 



Accordingly, Kapteyn has selected a certain number of areas, uniformly 

 distributed over the heavens. The brightness, proper motion, parallax, spec- 

 tral type, and radial velocity of all stars within these areas will be deter- 

 mined, through the joint efforts of many observatories. There has been 

 little difficulty in providing for the work on the bright stars, but the instru- 

 mental means available do not suffice for the fainter ones. For this reason 

 I have long felt that the large reflecting telescopes of this Observatory should 

 provide some of the necessary observations of the fainter stars, if this could 

 be done in such a way as to contribute effectively toward the solution of our 

 own problem of stellar evolution. 



During the past year I have devoted much time to the preparation of a 

 working program for our 60-inch reflector. Since all the lines of research 

 with this instrument should converge on our principal problem, cooperation 

 with Professor Kapteyn, who is concerned with the very different question 

 of stellar distribution, might appear to be excluded. A single illustration 

 will show, however, that this is not the case. Kapteyn believes that the stars 

 of the Hyades may belong to a true physical system, since their proper 

 motions, and probably their parallaxes, are in good general agreement. Hith- 

 erto, however, he has been unable to find whether their radial velocities also 

 correspond, since no large telescope could be employed to measure them. 

 Should the 60-inch reflector prove them to do so, a common origin may 

 with confidence be assigned to the group. If these stars could be shown to 

 have commenced their evolutional career at the same period and under sim- 

 ilar conditions, a minute study of their spectra might throw light on the 

 possible relationship between a star's mass and its spectral t3-pe — a question 

 of fundamental importance. In many such cases the measurement of radial 

 velocities, parallaxes, and stellar magnitudes will be essential in connection 

 with our spectroscopic work. The Solar Observatory is neither a physical 

 laboratory nor an observatory for the study of the astronomy of position. 

 Experience has shown, however, that it can not depend upon other institu- 



