176 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Mr. C. G. Abbot, Director of the Smithsonian Astrophysical Observatory, 

 visited Mount Wilson in the spring before going to Algiers, where he con- 

 tinued the daily observations of the solar constant of radiation begun there 

 last year. Mr. F. E. Fowle has carried on the regular solar-constant w^ork 

 at Mount Wilson during Mr, Abbot's absence. 



INVESTIGATIONS IN PROGRESS. 



SOI^AR RESEARCH. 



Direct Photography of the Sun. 



As in previous years, direct photographs of the sun have been taken on all 

 clear days with the Snow telescope. The absence of spots during a large 

 part of the year makes this series of less interest than usual, but the plates 

 serve to record faculae, and also the structure of the photosphere, when the 

 seeing is good enough for this purpose. 



Work with the s-foot Spectroheuograph. 



The routine work with the 5-foot spectroheliograph has been continued 

 on every clear day, but the low state of the solar activity has greatly reduced 

 the number of calcium (Hj) flocculi showing interesting structure. The 

 plates have served mainly for the measurement of the areas of the flocculi 

 and prominences, in continuation of the work of former years. 



The use of the 5-foot spectroheliograph for the photography of the hydro- 

 gen flocculi has been criticized by M. Deslandres on the ground that the 

 camera slit includes the whole of the Ha line, so that the images representing 

 the higher and lower levels are superposed. In view of the fact that the 

 vortex structure of the Ha flocculi is beautifully shown by this instrument 

 (with which it was first detected), the validity of the criticism may be ques- 

 tioned. The existing evidence indicates that these flocculi probably follow 

 the lines of force of the magnetic field produced by the spot vortex. If so, 

 they do not lie in a single plane, but on a surface of revolution more or less 

 conical in form. Hence they should be best shown by an instrument which 

 combines in a single image the flocculi of different levels. For certain stud- 

 ies of these flocculi, high-dispersion instruments, permitting the isolation of 

 any desired part of the Ha line, are of course required. But in such im- 

 portant investigations as those of Professor Stormer,* photographs showing 

 the flocculi of different levels projected upon a plane are essential. 



A second criticism by M. Deslandres is better founded. He points out 

 that the camera-lens and the prisms of this instrument are unnecessarily 

 large, in view of the fact that the collimator-lens forms a reduced image of 

 the concave mirror of the Snow telescope at a distance of 65 inches (1.6 m.) 

 from its optical center. This important fact was overlooked in designing 

 the spectroheliograph. As the reduction in the diameter of the beam is suf- 

 ficient to permit it, the second prism has been cut down to a height of 4.5 



* See page 182. 



