MOUNT WILSON SOLAR OBSERVATORY. 251 



The spectrograph has been used without modification during the 

 year. An important addition, however, is the recent installation of a 

 new system of temperature control designed by Mr. Babcock. A 

 series of nickel resistance-coils distributed throughout the prism-box 

 acts in conjunction with a Wheatstone bridge and galvanometer to con- 

 trol the main heathig-circuit in the spectrograph case. The temperature 

 is maintained constant within less than 0°1 C, and the system has 

 been found to be most reliable and satisfactory in its action. 



Radial Velocities. 



The measurement and reduction of the photographs of spectra have 

 given determinations of the radial velocities of 141 stars based on 

 three or more plates. Numerous other stars are nearly completed. 

 The number of spectroscopic binaries found is comparatively small, 

 amounting to only 8. This is due in part to the fact that the stars 

 observed are mainly of types G, K, and M, among which the propor- 

 tion of spectroscopic binaries is known to be small, and in part to the 

 very low dispersion emploj^ed for some of the fainter stars, which pre- 

 vents the detection of sUght variations. 



Among the more interesting results we may refer to the following: 

 Most of these stars have large proper motions, and 

 many have measured parallaxes. Two are of excep- 

 tional interest. The star B.D. + 18?3423 is of type 

 estimated as A9 and has a radial velocity of —250 

 km., which appears to be constant. The star recently 

 discovered by Barnard near B.D.-f4?3560, with a 

 proper motion of over 10" annually, has a radial 

 velocity of —94 km. and is of type Mb. A third star, referred to in 

 last year's report, is A.Oe. 14318, with a radial velocity of -|-306 km. 

 A companion distant from it 5' in declination has the same proper 

 motion, parallax, and velocity, the two stars thus forming a system 

 which moves in space at the rate of 580 km. per second. 



A spectroscopic binary of exceptional interest is the eighth-magnitude 

 star distant 80" from Castor. The first photograph of the spectrum 

 of this star showed double lines with a separation indicating a relative 

 velocity of about 220 km. The spectrum is Md with bright hj^drogen 

 lines. 



Classification of Stellar Spectra. 



The method of classification of spectra based on the relative intensi- 

 ties of certain Unes which was developed by Mr. Kohlschiitter has been 

 employed in determining the types of the stars photographed during the 

 year. The introduction of a few modifications into the method and a 

 revision of the scale of intensities used in the estimates now render it 

 possible for an experienced observer to make determinations of spectral 

 type with, a probable error of less than 0.1 division of the Harvard 



