MOUNT WILSON OBSERVATORY. 215 



INVESTIGATIONS IN PROGRESS. 



It is naturally difficult in any statement of the scientific activities of the 

 Observatory to make a distinction between the results already accompHshed 

 in many fields of investigation and those which depend upon a continuation 

 of the work in progress. A summary of the principal researches upon which 

 the members of the staff are immediately engaged is of value, however, both 

 for the direct information which it conveys regarding the problems under 

 consideration and for the insight afforded into the interrelationship of differ- 

 ent investigations and the manner in which one may develop from another. 



SOLAR RESEARCH. 



The regular work in direct photography and with the spectroheliograph 

 is being continued as usual, and especial attention is being given to active 

 areas on the sun's surface. Apart from the value of these photographs as 

 records of the solar activity, exceptional interest attaches to them in view 

 of Dr. Abbot's investigations of the correlation between the solar constant 

 and sun-spots and solar disturbances. 



The study of the polarities of sun-spots is especially interesting because 

 of the approach of the new cycle. A continuous record is being maintained 

 of all visible spots, and the search for new invisible spots by means of their 

 magnetic fields is a regular feature of the work. Closely allied with this 

 is Hale's extensive study of the nature of the magnetic field in sun-spots as 

 revealed in the spot spectrum and of the hypothesis of the nature of sun- 

 spots to which it gives rise. During the present spot minimum this work 

 consists mainly in the measurement and discussion of the photographs 

 obtained several years ago, but with increasing spot activity the accumulation 

 of new material will be resumed. 



Radiometric observations of the sun's surface are being continued, and 

 similar observations on sun-spots will form an important part of the solar 

 program as the number of spots increases. 



In the field of solar spectroscopy numerous investigations are in progress. 

 Among these, reference may be made to the new series of photographs for 

 a study of the sun's general magnetic field, the continuation of the work on 

 the solar rotation, determinations of the wave-lengths of lines in the solar spec- 

 trum on the international system, and an extensive study of the displacements 

 of lines between the center and limb of the sun. The last two of these investi- 

 gations bear directly upon the question of the gravitational shift of the spectral 

 lines, which forms so essential a part of the generalized theory of relativity. 



NEBULAR AND STELLAR RESEARCH. 



Direct photography of nebulae is being continued, both for purposes of 

 classification and to aid in studies of luminosity, and photographs of the larger 

 spirals are being accumulated to serve as a basis for future measurements of 

 internal motion in these objects. The important question of the source of 

 luminosity in nebulae will be further studied by spectroscopic observations and 

 measurements of color indices. 



Trigonometric measurements of parallax are being made for a selected list 

 of objects, including stars of large and small proper motion, planetary nebulae, 

 and distant companions of stars of known motion. 



The principal photometric work in progress on the stars of the Selected 

 Areas is the continuation by the method of exposure ratios of the determina- 



