1870.] 



Observations of the Sun. 



355 



very largely strengthened my former observations, so that the many hours 

 I have spent in watching phenomena, now perfectly familiar to me, have 

 not been absolutely lost. 



The negative results which Dr. Frankland and myself have obtained in 

 our laboratory-work in the matter of the yellow bright line, near D, in the 

 spectrum of the chromosphere being a hydrogen line, led me to make a 

 special series of observations on that line, with a view of differentiating it, 

 if possible, from the line C. 



It had been remarked, some time ago, by Professor Zollner, that the 

 yellow line was often less high in a prominence than the C line; this, how- 

 ever, is no evidence (bearing in mind our results with regard to magne- 

 sium). The proofs I have now to lay before the Royal Society are of a 

 different order, and are, I take it, conclusive : — 



1. With a tangential slit I have seen the yellow line bright below the 

 chromosphere, while the C line has been dark ; the two lines being in 

 the same field of view. 



2. In the case of a bright prominence over a spot on the disk, the C 

 and F lines have been seen bright, while the yellow line has been invisible. 



3. In a high-pressure injection of hydrogen, the motion indicated by 

 change of wave-length has been less in the case of the yellow line than in 

 the case of C and F. 



4. In a similar quiescent injection the pressure indicated has been less. 



5. In one case the C line was seen long and unbroken, while the yellow 

 line was equally long, but broken. 



The circumstance that this line is so rarely seen dark upon the sun 

 makes me suspect a connexion between it and the line at 5015 Angstrom, 

 which is also a bright line, and often is seen bright in the chromosphere, 

 and then higher than the sodium and magnesium lines, when they are 

 visible at the same time ; and the question arises, must we not attribute 

 these lines to a substance which exists at a higher temperature than those 

 mixed with it, and to one of very great levity ? for its absorption line 

 remains invisible, as a rule, in spot spectra. 



I have been able to make a series of observations on the fine spot which 

 was visible when I commenced them on the 10th instant, not far from the 

 centre of its path over the disk. At this time, the spot, as I judged by 

 the almost entire absence of indications of general absorption in the pen- 

 umbral regions, was shallow, and this has happened to many of the spots 

 seen lately. A few hours' observation showed that it was getting deeper 

 apparently, and that the umbrae were enlarging and increasing in number, 

 as if a general downsinking were taking place ; but clouds came over, and 

 the observations were interrupted. 



By the next day (April 11) the spot had certainly developed, and now 

 there was a magnificently bright prominence, completely over the darkest 

 mass of umbra, the prominence being fed from the penumbra or very close 

 to it, a fact indicated by greater brilliancy than in the bright C and F lines, 



VOL. XVIII. 2 E 



