86 L. Trouvelot—Sudden extinction of the 
When the slit was set wide open, so as to allow the whole 
protuberance to be seen between its jaws, the comma-shaped 
flame appeared perfect, and showed plainly its texture. But 
when it was observed with a narrower slit it became partly 
invisible on the C line; only a short and jagged portion being 
seen in it on the red side. When the slit was carried along the 
protuberance by means of its screw, the portion visible on the 
C line did not remain constant, but either extended or con- 
tracted of a small quantity; the maximum portion 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 considerably more brilliant than the spectrum itself, and 
about one-third only of its whole iength 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. 
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 lightning, it does not seem that a change of posi- 
tion was the cause of its disappearance, but rather because the 
light which rendered it visible abandoned it in an instant. 
According to theory, this protuberance was moving rapidly 
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 unconceiv- 
able velocity. 
For over half an hour I watched attentively the same spot in 
