MOUNT WILSON OBSERVATORY. 207 



all of those in Orion, Scorpio, and Centaurus. Apparently the hypo- 

 thesis is confirmed by practically all the observational and statistical 

 data bearing on the motions and distribution of the nearby stars. 



Variable Stars. 



As a by-product of the determination of the distances of globular 

 clusters, the parallaxes of 140 isolated Cepheid variables have been 

 derived by Mr. Shapley. The accuracy depends on that of the 

 observed apparent magnitudes and on the rehabiUty of the period- 

 luminosity curve; unless some quite unsuspected systematic error is 

 present, the accuracy is at least as high as that of trigonometric paral- 

 lax measures of the nearer stars, although the Cepheids, with few 

 exceptions, are beyond the sphere of practical trigonometric work. 

 Most of the Cepheids at present known are intermediate in distance 

 between the naked-eye stars and the nearer globular clusters and 

 Milky Way clouds. The study of the distribution in space reveals 

 Httle that is new. Some of the faint shorter-period Cepheids are 

 outside the equatorial segment that is devoid of globular clusters. 

 The longer-period Cepheids supply a valuable but provisional deter- 

 mination of the distance of the sun north of the galactic plane. 



Mrs. Shapley has continued her computations of the orbits of 

 eclipsing binaries. The systematic observations of the variables in 

 the Orion nebula have been continued by Mr. Shapley, and the colors 

 of a considerable number of these variables and their comparison 

 stars have been measured by Miss Davis, who has assisted in the 

 work on clusters and variables throughout the year. A sequence of 

 comparison stars for the novae in the Andromeda nebula has also 

 been measured. In various globular clusters Miss Davis has found 

 some new variable stars, probably of the Cepheid type. Mr. Benioff 

 has undertaken the determination of photographic light curves with 

 the 10-inch portrait lens of a group of Cepheid variables in the star- 

 clouds of Sagittarius. The variations in the velocity, spectrum, and 

 light of the Cepheid variable SU Cassiopeiae have been discussed by 

 Mr. Adams and ]\Ir. Shapley. 



STELLAR SPECTROSCOPY. 



In continuation of the plan of observations outlined in the annual 

 report of last year, a large proportion of the stellar spectroscopic 

 work has been devoted to determinations of the absolute magnitudes 

 and the radial velocities of certain selected lists of stars. Especial 

 attention has been given to the stars listed in the American Ephemeris, 

 since among these are many of the most luminous objects, intrinsically, 

 of which we have any knowledge. Since the radial velocities of most 

 of these stars are known, comparatively few measurements of velocity 

 have been made. A sufficient number, however, has been determined 



