218 CARNEGIE INSTITUTION OF WASHINGTON. 



association with Professor Kapteyn, have led to a material modification 

 of our original view. The independent and striking discovery by 

 Campbell and Kapteyn of an important relationship between stellar 

 speed and spectral type ; the demonstration by Hertzsprung and Rus- 

 sell of the existence of giant and dwarf stars ; the successful application 

 of the 60-inch reflector by van Maanen to the measurement of minute 

 parallaxes of stars and nebulae; the important developments of Shap- 

 ley's investigation of globular star-clusters; the possibilities of research 

 resulting from Seares's studies in stellar photometry; and the remark- 

 able means of attack developed by Adams through the method of 

 spectroscopic parallaxes, have naturally and inevitably led to a very 

 considerable extension of our original program. Indeed, the success of 

 these and other investigations, and the importance of profiting by 

 them, have resulted in a seeming reduction of emphasis on the more 

 purely physical aspects of our attack. This is apparent rather than 

 real, however, as the new advances will actually permit us to carry 

 this attack well beyond some of its original objectives. Especially 

 is this true at the present time, when the recent commissioning of the 

 Hooker telescope and the forthcoming extension of our laboratory 

 facilities are affording better means of utilizing such enlarged oppor- 

 tunities. 



The investigations in solar physics, which formerly held the chief 

 place in our research program, have also developed along unexpected 

 lines. We did not foresee at the outset that solar magnetic phenomena 

 would one day become a subject of our inquiry, demanding special 

 instrumental facilities and throwing light on the complex question of 

 the nature of sun-spots and other solar problems of long standing. It is 

 obvious that these researches, together with those on the solar rota- 

 tion and the motions of the solar atmosphere, so admirably developed 

 by Adams and St. John, must be carried to their logical conclusion, 

 and utilized, in the fullest possible degree, for the interpretation of 

 stellar and nebular phenomena. 



The discovery of solar magnetism, like many other Mount Wilson 

 results, was the direct outcome of a long series of instrumental de- 

 velopments. At their beginning in 1890, the most powerful solar 

 spectrograph applied to the study of sun-spots was an instrument of 

 42.5 inches focal length, attached to a 12-inch equatorial refractor 

 and giving a solar image 2 inches in diameter. For many visual 

 purposes such a combination is excellent, but it is poorly adapted for 

 photography. A much larger solar image and a spectrograph of much 

 greater linear dispersion are essential. In fact, the spectrograph must 

 be made the prime element in the combination, and the telescope so 

 designed as to serve as a necessary auxiliary. Thus, through the suc- 

 cessive steps embodied in the horizontal Snow telescope, with its 18-foot 

 spectrograph, and the 60-foot vertical tower telescope, with its 30-foot 



