ASTRONOMICAL, PROBLEMS CURTIS. 337 



spectrum has opened up to astronomy a field so vast that we scarcely 

 dare to-day even to demark its boundaries. Few are the fields of 

 astronomical research where the work in radial velocities is not 

 making itself felt, and to-day we are furnished with the interesting 

 spectacle of the oldest astronomy of position and the newer astron- 

 omy of the spectrum drawing closer and closer together for the 

 solution of problems of sidereal structure. In order to determine 

 the motion of our sun through space many analyses have been made 

 of the minute proper motions of the stars across our line of vision, 

 but all such determinations are subject to some uncertainty because 

 of the fact that the true distances of the stars whose proper motions 

 are used in the analyses are, in general, very imperfectly known. 

 On the other hand, the spectrograph gives us the velocity of a star in 

 the line of sight, a velocity which, in stars possessing good spectral 

 lines, is accurate within a few tenths of a kilometer per second, and 

 which is entirely independent of the distance of the star from our 

 system. For this reason it should be possible to determine from 

 the radial velocities of a considerable number of stars well dis- 

 tributed over the entire sky a much more accurate value of the amount 

 and the direction of the movement of the solar system through space. 



For a complete solution of this problem, at which Dr. Campbell 

 and his associates have been working during the past 15 years, it was 

 necessary that radial velocities be secured for the stars in the South- 

 ern Hemisphere. This need was laid before Mr. D. O. Mills, who, in 

 1902, generously gave the funds necessary for the installation on 

 Mount San Cristobal, Santiago, Chile, of a 37-inch reflecting tele- 

 scope with the necessary spectrographs equipment, and in 1905 ad- 

 vanced further funds to continue the southern work for five years 

 longer. A further extension of the work for two years has been made 

 possible through funds supplied by Mr. Ogden Mills, son of the late 

 D. O. Mills. 



Up to date about 7,200 spectrograms have been taken at Mount 

 Hamilton and 3,700 by the D. O. Mills expedition at Santiago, on 

 nearly 1,400 stars. The northern portion of the program is nearly 

 completed, and two years more should see the southern portions of 

 the work essentially finished, though decades could well be used in 

 investigating the " by-products " which have appeared in the course 

 of the work, and other decades for the much-needed extension of 

 these researches to fainter stars. To give one instance only, at the 

 Santiago station alone 48 spectroscopic binaries have been announced 

 up to May, 1909, and to work up these binary systems adequately 

 and compute their orbits would necessitate at least three years' work. 

 One in every five or six of the northern stars examined has proved to 

 be a binary, and nearly one in five of the stars observed by Prof. 



97578°— sm 1910 22 



