MOUNT WILSON OBSERVATORY. 185 



series of spectra of elements found in this way; the discovery by Stromberg that 

 the asymmetry found to exist in the velocity-distribution of stars is a special 

 case of a more general phenomenon which applies to all celestial objects, includ- 

 ing the globular star-clusters and the spiral nebulae; the investigation of Scares 

 on the luminosity and density functions of stars; the further results of van 

 Maanen on the internal motions of spiral nebulae and the strong evidence 

 they afford of rotation or motion outward along the spiral arms, in excellent 

 agreement with the theoretical work of Jeans ; the extension of the method of 

 spectroscopic parallax determinations to the helium stars and the improve- 

 ments made in the treatment of those of later types; the discovery by Hubble 

 of a new Magellanic cloud, and his application of his theory of the source of 

 luminosity to the non-galactic nebulae; the continued successful use of the 

 thermo-couple in measurements of the heat radiation of the stars and planets 

 by Pettit and Nicholson ; studies of the masses of stars by Russell ; the inves- 

 tigations of Merrill on the spectra of the Me-type stars, and of Joy and 

 Sanford on the spectra of many other variables and spectroscopic binaries; 

 the work of King on the temperature classification of the spectral lines of 

 numerous elements and on ionization effects in the electric furnace; the 

 determination by Babcock from the Zeeman effect of the ratio of the charge 

 on the electron to its mass; and the progress made by St. John in his important 

 studies of the displacements of lines in the spectrum of the center and the 

 limb of the sun, pressures in the solar atmosphere, and the shift of spectral 

 lines required by the theory of relativity. Many of these investigations 

 are in continuation of those made in recent years, and all of them have a 

 direct bearing on the larger problems toward the solution of which the Obser- 

 vatory has been working. 



Several extensive investigations, which have been in progress during 

 recent years, have now been brought to a conclusion or have reached the 

 limit set by the observational material at present available. To the first 

 category belongs the great investigation by Scares of the photographic magni- 

 tudes of the stars in Kapteyn's Selected Areas. This work, which was begun 

 13 years ago, has involved a complete study of the methods of photographic 

 photometry as applied to a large reflecting telescope, a revision of the magni- 

 tude scale, the determination of standard magnitudes in 139 areas, and finally 

 the measurement of position and magnitude for about 60,000 stars. The 

 completion of this undertaking places at the disposal of astronomers a fund 

 of material applicable to many problems relating to the laws of stellar dis- 

 tribution and luminosity, and marks a notable stage of progress toward the 

 realization of the plans of the great astronomer to whom the conception of 

 the plan of the Selected Areas is due. 



Three other investigations which may be regarded as brought to completion 

 for the present are those of van Maanen on the internal motions in spiral 

 nebulae; of Merrill on the radial velocities of the long-period variable stars; 

 and of Babcock on the Zeeman effect for the elements iron, vanadium, and 

 chromium. Further studies of the motions in spiral nebulae must await the 

 accumulation of photographs separated by an adequate interval of time, and 

 the results of Merrill and Babcock are so nearly complete for the material 

 at their disposal that few additions may be expected in the immediate future. 

 With this completion of the observational data, the chief importance of further 



