230 CARNEGIE INSTITUTION OF WASHINGTON. 



knowledge of the distribution of stars in space (p. 252). Another method, 

 based upon the relative intensities of different parts of the continuous 

 spectrum, should also prove to be of practical importance (p. 253). It 

 may be applied spectroscopically, or with the aid of direct photographs 

 taken through color screens. A new method of determining star- 

 colors which has been developed during the year should be of special 

 value in this connection (p. 248). 



The spectroscopic method of determining parallaxes has afforded a 

 valuable confirmation of the conclusions of Russell and Hertzsprung 

 regarding the existence of giant and dwarf stars (p. 253). The 

 further development of this investigation will be of importance in its 

 bearing on the problem of stellar evolution. Here, too, the quantita- 

 tive method of classifying stellar spectra, which is based upon the 

 ratios of the intensities of certain lines, will prove of value (p. 251). 



The true significance of the change of color with brightness, long 

 recognized as characteristic of variable stars of the Cepheid class, has 

 now been detected. All stars of this class hitherto investigated are 

 found to go through periodic variations in spectral type, passing by 

 gradual changes of line intensity from one type to another. The 

 chief value of this discovery will not be confined to its bearing on the 

 nature of these variable stars, but is likely to he in its apphcation to the 

 interpretation of the spectral changes which accompany stellar 

 evolution (p. 253). 



Star clusters offer exceptional opportunities for certain researches, 

 especially those which require the comparative study of stars at 

 the same distance from the earth. A comprehensive study of the 

 magnitudes and colors of stars in globular and open clusters has already 

 yielded some interesting results. These include proof of the existence 

 of giant red stars in globular clusters and evidence that light is not 

 appreciably scattered in its passage through space (p. 249). 



A general investigation of nebulse, which we have long planned 

 to undertake, will be fully organized as soon as the 100-inch telescope 

 is completed. In spite of the many demands on the 60-inch reflector, 

 it has been possible during the year to give more attention than for- 

 merly to work of this character. 



Direct photographs of interesting nebulse are taken regularly at the 

 principal focus of the 60-inch reflector and occasionally on the larger 

 scale afforded by the Cassegrain combination (p. 244). In spite of the 

 evidence of internal motions suggested by the appearance of spiral 

 nebula?, no change in their form has been detected until this year. 

 Differential measures of many condensations in the spiral M 101, made 

 with the stereocomparator on two photographs separated by an 

 interval of five years, yielded distinct evidence of rotation. As this 

 has been confirmed by two measurers, using Mount Wilson and Lick 

 photographs, its reality appears to be established. A series of photo- 



