176 Mr. J. N. Lockyer, Researches in Spectrum [Dec. 12, 



naturally looked for the third photographic hydrogen line, which is 

 generally called h, but we found no protuberance on our photographs 

 corresponding to that wave-length. Although this line is always 

 weaker than H7, its absence on the photograph is rather surprising, if 

 it be not due to the fact that the line is one which only comes out at 

 a high temperature. This is rendered likely by the researches of 

 Frankland and Lockyer (" Proc. Roy. Soc," vol. xvii, p. 453). 



" We now turn to the last and strongest series of protuberances 

 shown od our photographs. The distance between this series and the 

 one we have found reason for identifying with H7 is very little greater 

 than that between H|8 and H7. Assuming the distances equal, we 

 conclude that the squares of the inverse wave-lengths of the three 

 series are in arithmetical progression. This is true as a first approxi- 

 mation. We then calculated the wave-length of this unknown line, 

 and found it to be approximately somewhat smaller than 3,957 tenth- 

 metres. No great reliance can be placed, of course, on the number, 

 but it appears that the line must be close to the end of the visible 

 spectrum. 



" In order to decide, if possible, what this line is due to, we endea- 

 voured to find out both by photography and fluorescence whether 

 hydrogen possesses a line in that part of the spectrum. We have not 

 at present come to any definite conclusion. In vacuum tubes pre- 

 pared by Geissler containing hydrogen, a strong line more refrangible 

 than H is seen, but these same tubes show between H7 and H<3, other 

 lines known not to belong to hydrogen, and the origin of the ultra- 

 violet line is therefore difficult to make out. We have taken the spark 

 in hydrogen at atmospheric pressures, as impurities are easier to 

 eliminate, but a continuous spectrum extends over the violet and part 

 of the ultra-violet, and prevents any observation as to lines. We are- 

 going on with experiments to settle this point. 



" Should it turn out that the line is not due to hydrogen, the ques- 

 tion will arise what substance it is due to. It is a remarkable fact 

 that the calculated wave-length comes very close to H. Young has 

 found that these calcium lines are always reversed in the penumbra 

 and immediate neighbourhood of every important sun-spot, and calcium 

 must therefore go up high into the chromosphere. We draw attention 

 to this coincidence, but our photographs do not allow us to draw any 

 certain conclusions. 



" At any rate, it seems made out by our photographs that the 

 photographic light of the protuberances is in great part due to an 

 ultra-violet line which does not certainly belong to hydrogen. The 

 protuberances as photographed by this ultra-violet ray seem to go up 

 higher than the hydrogen protuberances, but this may be due to the 

 relative greater length of the line." 



In my remarks upon calcium I have already referred to the fact that 



