164 REPORTS OF INVESTIGATIONS AND PROJECTS. 



prism, and an 8-inch plane grating can be used in this spectroheliograph. 

 Thus a wide range of dispersion is available for different classes of work. 

 In one arrangement of the apparatus, two spectra of the same region of the 

 sun are formed side by side, with light from a single collimator slit. Thus 

 simultaneous photographs of the same region can be taken with light from 

 the red and violet edges of the Ha line, for example. Such photographs are 

 being used in an investigation of the possible effect of anomalous refraction 

 on the flocculi. 



In harmony with the results of Deslandres, photographs of the flocculi 

 made with different parts of the Ha line show different structure correspond- 

 ing to different levels. The vortices are well photographed when the camera- 

 slit is set so as to include the edges of the line. But M. Deslandres is not 

 thereby justified in drawing the following conclusion: 



'Tapparition des plages faculaires noires, annoncee depuis 1903, est due, 

 au moins pour une large part, non a des particularites dans le pouvoir emissif 

 ou absorbant de I'hydrogene, mais a une simple cause instrumentale, a un 

 defaut primordial du spectroheliographe qui, ayant une fente de largeur 

 constante, ne pent isoler completement une raie de largeur variable." 

 (Comptes Rendus, 10 Mai, 1909.) 



If the Ha line is due to absorption, the dark flocculi photographed with 

 light from its edges represent regions of increased absorption. Moreover, 

 the possibility of photographing such regions (even if it depended solely 

 upon variations in the width of the line, which is not the case) should not 

 be regarded as a "defect" of the spectroheliograph, but rather as one of its 

 most valuable properties. 



The dark filaments, as well as the bright flocculi, are given by the light 

 from the center of the line. A study of the width, intensity, and structure 

 of the Ha line in various parts of the sun's disk has been made in the third 

 order of the grating. 



Although the vortex structure (except as it appears in the larger filaments) 

 is better shown near the edges than at the center of Ha, it is easily photo- 

 graphed with the camera slit of the 30-foot spectroheliograph set at the 

 center of H(3 or Hy. Moreover, H(3 now gives admirable photographs of the 

 vortices with the 5-foot spectroheliograph. Hy, however, fails to bring out 

 the finer stream-lines, and therefore does not give the complete structure, 

 though many of the stronger stream-lines appear. Further investigations 

 are in progress. 



Spectra of Sun-spots. 



In my last annual report it was stated that the work at that time accom- 

 plished on the spectra of sun-spots indicated the probable existence in these 

 regions of strong magnetic fields. This investigation has been continued and 



