I87IJ 



Light, 



109 



represent the appearances assumed by the F and C lines respectively, at the 

 times indicated below each figure, during an observation on the afternoon of 

 September 22nd. The point where these changes of wave-length occurred was 

 at the western edge of the penumbra. The C and F lines were reversed in 

 some portion or other of the group nearly every time Professor Young observed 

 it. On September 22nd, the sodium lines were both reversed for several hours, 

 while D 3 appeared as a dark shade. On September 28th, again, at 4 p.m., the 

 southern nucleus of the group (which at this time contained four large umbras, 



Fig. 15. Fig. 16. 



m a 



/ 



Z.35 2.38 &/U 



Z¥3 



2.4S 



ZM 



besides many small ones), reversed all of the following lines, viz. : C ; T)j ; 

 D 2 ; D 3 ; 1474 ; 61 ; 62 ; 63 ; 64 ; F ; 2796 ; and h. All of these were con- 

 spicuous, except 1474 ; D 3 and & 3 especially so, and the latter (a nickel line) 

 showed considerable changes of wave-length, alternate increase and diminution, 

 which were not shared by its magnesian neighbours, b lf b 2 , and & 4 . At 

 4.05 p.m. the brilliance of the F line increased so greatly that it occurred to 

 Professor Young to widen the slit, and to his great delight he saw upon the 

 disk of the sun itself a brilliant cloud in all its structure and detail identical 

 with the protuberances around the limb. Indeed, there were two of them, 

 and there was no difficulty in tracing out and delineating their form. Fig. 17 



represents them as they were from 4.05 



Fig. 



faded out at both ends. 



to 4.10 ; Fig. 18 gives the form at 

 4.15-20. They were then considerably 

 fainter than at first. During the 

 intervening ten minutes the other lines 

 of the spectrum were examined, and it 

 was found that the form could be 

 distinctly made out in all the hydrogen 

 lines, even in h ; but that the reversal 

 of the other lines, including D 3 , was 

 confined to the region immediately 

 over the spot-nucleus, where the smaller 

 but brighter cloud terminated abruptly; 

 or, rather, originated. The larger one 

 When the clockwork of the equatorial was stopped, 



Fig. 18. 



the luminous cloud took 167 seconds of time to traverse the slit which was 



