MOUNT WILSON OBSERVATORY. 225 



the center of such clusters in less than an hour. Moreover, the style 

 of mountmg and design of dome adopted for the Hooker telescope 

 permit it to work much farther south than the 60-inch, and thus to 

 deal with the rich collection of globular clusters south of the equator. 



Among the numerous cluster problems calling for early attack with 

 the Hooker telescope are the detection of the faintest possible stars 

 and the measurement of their magnitudes and colors ; the determination 

 of spectral types, and of the radial velocities of individual stars and of 

 clusters as a whole; a search for spectroscopic e\'idence of possible 

 axial rotation of globular clusters; and the development and applica- 

 tion of all available means of determining the distances of clusters, 

 particularly the method of spectroscopic parallaxes. 



The possibility of dealing with many problems relating to the distri- 

 bution and evolution of the faintest stars depends upon the establish- 

 ment of photographic and photovisual magnitude scales. Below the 

 twelfth magnitude the only existing scale of standard visual or photo- 

 visual magnitudes is the Mount Wilson sequence, already carried by 

 Seares to magnitude 17.5 with the 60-inch telescope. The extension 

 of this scale to still higher magnitudes, and its application to the study 

 of the faintest stars within its range, is an important task for the 

 Hooker telescope. There is good reason to believe that this instrument 

 will bring into \'iew hundreds of millions of stars beyond the reach of the 

 60-inch. The giants among these Mill form for us the outer boundary 

 of the galactic system, while the dwarfs will be of almost equal interest 

 from the evolutional standpoint. Such questions as the condensation 

 of the faintest stars toward the galactic plane, the color of the most 

 distant stars, and the final settlement of the long inquiry regarding the 

 possible absorption of light in space, are among those to be dealt 

 with in the photometric field. 



It is clear that such activities as those already enumerated will 

 heavily tax the capacity of the Hooker telescope. But certain special 

 studies of exceptional promise must also be undertaken if possible. 

 One of these, which will be greatly facilitated by the large aperture, is 

 the determination of the spectral-energy curves of stars of various 

 classes, for the purpose of measuring their surface temperatures. This 

 is obviously of great importance in a general study of stellar evolution. 

 Other special investigations dealing with variable and temporary 

 stars, the peculiarities of variable nebulae, and the spectra of the com- 

 ponents of close double stars wdll be discussed in future reports. 



In all of this work, the desirability of cooperating -vsath other insti- 

 tutions and of limiting our observations, so far as may be practicable, 

 to those which can be undertaken to exceptional advantage by our 

 staff and equipment, must be constantlj^ borne in mind. Several 

 important cooperative arrangements have already been made with 

 other observatories, and others are now in prospect. 



