188 PROCEEDINGS OP THE AMERICAN ACADEMY 



screw, the portion visible on the C line did not remain constant, but 

 either extended or contracted of a small quantity ; the maximum por- 

 tion visible on the C line never being more than one-fourth the width 

 of the slit, while sometimes it was not seen at all on this line. 



The portion of the protuberance projected on the spectrum was con- 

 siderably more brilliant than the spectrum itself, and about one-third 

 only of its whole length was visible. As the slit was carried along it, 

 the visible parts became invisible near the C line, and invisible parts 

 appeared on the spectrum ; and the area of the visible portion either 

 contracted or extended, when seen in different parts. 



I had been observing this phenomenon for eight or ten minutes, 

 when, while, looking at it with the slit wide open, the flame suddenly 

 vanished, at lO** 30", no traces of it remaining. As no motion of any 

 kind, no extension, no contraction, could be perceived before or at the 

 moment this phenomenon took place, and as the light did not go out of 

 it gradually, but as suddenly as a flash of liglitniug, it does not seein 

 that a change of position was the cause of its disappearance, but rather 

 because the light which rendered it visible abandoned it in an instant. 



According to theor}^ this protuberance was moving rajiidly away 

 from the earth at the moment of the observation, as it was projected 

 upon the less refrangible side of the spectrum ; yet this would fail to 

 explain its sudden disappearance, since for this it should have moved 

 out of sight with an inconceivable velocity. 



For over half an hour I watched attentively the same spot in 

 expectation of seeing the flame reappear ; employing for this a narrow 

 and a wide slit in succession, but with entirely negative results. I saw 

 no more traces of it, although the small aigrette-shaped flames of the 

 chromosphere, which wei'e still visible, indicated the exact place where 

 it had vanished, and where very probably it still existed, but now as a 

 dark protuberance. 



On several occasions I have seen the light abandon a protuberance 

 gradually, but never so suddenly and on such a grand scale ; and 

 sometimes I have seen also the lij^ht gradually illuminating protuber- 

 ances which were invisible before, something after the manner of 

 clouds in our atmosphere lighting up and fading into darkness by the 

 appearance or disappearance of the sun. Of course, the illumination 

 of dark solar protuberances cannot be conceived as being due to the 

 reflection of light, as in the case of the clouds in our atmosphere : it 

 is the protuberance itself which is rendered luminous by some ciiange 

 taking place in it. These observations would seem to indicate that on 

 the sun there are sometimes dark and non-luminous protuberances, 



