THE TOTAL SOLAR ECLIPSE OP APRIL 6, 1875. 
149 
and owing to the small dispersive power, and therefore relatively great breadth of the 
slit, the measurement can only be regarded as a first approximation. Turning now 
again to our photographs, and calculating the angular distance between the first and 
second ring of protuberances, we find that distance to be 3' 15". We conclude, 
therefore, that this second ring is due to hydrogen. We, therefore, 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 Hy, 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 on our 
photographs. The distance between this series and the one we have found reason 
for identifying with Hy is very little greater than that between H /3 and Hy. 
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 3957 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 endeavoured to find out 
both by photography and fluorescence whether hydrogen possesses a fine in that part 
of the spectrum. We have not at present come to any definite conclusion. In 
vacuum tubes prepared by Geissler containing hydrogen a strong line more 
refrangible than H is seen, but these same tubes show between Hy and HS 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 atmo- 
spheric 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 fines. We are going on with experiments to settle this point. 
Should it turn out that the fine is not due to hydrogen, the question 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 fines are always 
reversed in the penumbra and immediate neighbourhood of every important sunspot, 
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 fight 
of the protuberances is in great part due to an ultra-violet fine 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 fine. 
