60 REPORTS ON INVESTIGATIONS AND PROJECTS. 



ASTRONOMY. 



REPORT ON THE SOLAR OBSERVATORY, MOUNT WILSON, CALIFORNIA.* 



By George E. Hale, Director. 



Two events of the past year, of special importance in their bearing 

 upon the future work of the Solar Observatory, call for mention at the 

 opening of this report. 



The first of these is the gift of $45,000, by Mr. John D. Hooker, of Los 

 Angeles, to meet the cost of a mirror of 100 inches aperture and 50 feet 

 focal length, for a great reflecting telescope. Such an instrument, which 

 will collect about 2.7 times as much light as our 60-inch reflector, will 

 permit the work of the Solar Observatory to be very greatly extended. 

 Our plan of research, which is intended to provide for an effective attack 

 upon the problem of stellar evolution, requires varied apparatus and the 

 most powerful telescopes obtainable. 



As stated in my previous report, the attack is along three converging 

 lines, involving the study of the sun as a typical star ; the study of stars 

 and nebulae and of their relationship to the sun and to one another ; and 

 the interpretation of both solar and stellar phenomena by means of care- 

 fully chosen laboratory experiments. In the laboratory work, our lumi- 

 nous sources are so far under control that we can usually adapt them to 

 the requirements of the particular investigation. In other words, if a light 

 source is only feebly luminous, we can ordinarily increase its brightness 

 to such a degree as to permit its analysis by the most powerful spectro- 

 graphs. In the case of the stars, however, we have no such means of 

 controlling the conditions. It too often happens that the very stars which 

 present the most interesting and important peculiarities — perhaps for the 

 reason that they resemble the "missing link" of the naturalist, in view 

 of their promise' of uniting some broken chain of evidence — are so faint 

 that an adequate analysis of their Hght is impossible. A loo-inch 

 reflector, with its immense light-collecting power, will obviously be of the 

 greatest advantage in just such cases. A similar illustration is afforded 

 by the spiral nebulae. Only a iew of these remarkable objects are large 

 enough, and bright enough, to lie well within the range of existing photo- 

 graphic telescopes. Even in these the changes in form are so slow that 

 none have ever been detected by existing means. Hundreds of thousands 

 of smaller nebulae of this type, although the remarkable fact of their exist- 

 ence and spiral form has been discovered with the Crossley reflector, are 

 so minute on the photographic plate that little can be learned of their true 



* For the year ending September 30, 1906. Grant No. 314. $150,000 for construction 

 and maintenance. (For previous reports see Year Book No. 3, pp. 154-174, and Year 

 Book No. 4, pp. 56-77.) 



