MOUNT WILSON SOLAR OBSERVATORY. 253 



An application has been made of the spectroscopic method of 

 deriving parallaxes to the question of the existence of two classes of 

 M and later K-type stars. The investigations of Russell and Hertz- 

 sprung, based on measured parallaxes, have pointed to the conclusion 

 that such stars are divided into two groups, one of very small and the 

 other of very large magnitude, with an entire absence of intermediate 

 values. In view of the small number of measured parallaxes available, 

 it has been thought possible that this result might be due to the lack of 

 data for the intermediate stars. The spectroscopic evidence, how- 

 ever, based on a larger number of stars, confirms the existence of the 

 two groups, separated by an interval of about 7 magnitudes within 

 which no stars have been found. 



A remarkable feature of the high-luminosity or giant stars, as 

 they have been termed, of the M type of spectrum is the abnormal 

 intensity of the hydrogen Unes. In stars Uke a Orionis and Antares 

 these are of an intensity comparable with that in the sun, while in the 

 low-luminosit}'^ stars of the same type they are scarcely visible. Other 

 marked differences have been found between the spectra of the two 

 groups, particularly in the relative behavior of the enhanced and the 

 low temperature lines of various elements. It seems probable that quite 

 different stages of stellar evolution are represented in these two groups. 



The Intensity of the Continuous Spectrum in Large and Small Proper-Motion Stars. 



A comparison of the spectra of stars of the same type of spectrum 

 but of widely different proper motions, made about three years ago, 

 showed that the large proper-motion stars have a relatively much more 

 intense continuous spectrum in the more refrangible region. This 

 effect was found to vary with spectral type, being nearly or wholly 

 absent in the A-type stars, and increasing progressively through the 

 F, G, and K types. A continuation of the investigation by Mr. 

 Monk, with the aid of a large amount of additional spectroscopic 

 material, gives results in harmony with those found previously. It has 

 also been possible to derive a relationship between absolute magnitude 

 and the intensity of the continuous spectrum by the aid of magnitude 

 determinations made in the way already described. 



The Spectra of Cepheid Variables. 



A well-known characteristic of the Cepheid type of light variation is 

 the excess of photographic over visual range in brightness. Periodic 

 changes in the mean color of the radiating surface are thus indicated, 

 and it is a natural inference that periodic changes in the spectrum must 

 also occur. Observations by Mr. Pease in 1914 estabhshed the nature 

 of the change for one of these variables, RS Bootis, and indicated its 

 probable character for the whole class of Cepheids. An actual change 

 of type was found to take place, ranging from B9 at maximum to FO at 

 minimum. 



