26 



tain the spectrograph, now increased in length 

 from 1 8 to 30 feet. The gain resulting from its 

 greater stability and from the constancy of tem- 

 perature of the grating at the bottom of the well 

 was plainly apparent in the new photographs of 

 spot spectra, which brought out details previously 



unrecognized. 



SOLAR VORTICES AND MAGNETIC FIELDS. 



The development of the spectroheliograph had 

 also advanced another step. The dark hydrogen 



June 2 



6 h io m AM 



June 3 



P.M 



linn- J, 

 5 1 ' i ."' 1' 



lllllr 3 

 5 I' 22'" I'M 



FIG. 19. Hydrogen Flocculus sucked into Sun-spot, June 3, 1908. 



flocculi, first photographed at the Yerkes Observa- 

 tory in 1903, had hitherto been recorded only with 

 the blue hydrogen lines. In 1908 the new 7 red- 

 sensitive plates of Wallace, applied to the pho- 

 tography of the sun's disk with the red (Ha) line of 

 hydrogen, gave results of great interest. In the 



