164 REPORT—1868. 
when the light is feeble, the lines as they appeared in the still feebler light 
of the atmosphere near the sun’s edge were carefully compared with the 
same lines in the spectrum of the umbra. The lines in the former case, 
though they appeared slightly stronger, were not so in a degree that could 
be accepted as an explanation of the more marked increase of strength 
which they presented in the spectrum of the umbra. There seemed, there- 
fore, to be evidence of a peculiarity due to the light of the umbra itself. 
In the spectrum of the umbra, which was sufficiently extended to show 
all the lines in Kirchhoff’s maps, no lines were detected which were not also 
present in the spectrum of the sun’s normal surface, nor were any lines 
observed to be wanting. 
The increase of thickness did not take place in the same proportion for all 
the lines. ‘The lines C and F, due to hydrogen, appeared to be increased but 
in a very small degree, not more so than would be due to the feebler inten- 
sity of the light. 
There is a small group of lines a little less refrangible than 4, at 1601 to 
1609 of Kirchhoff’s scale, and which in his map are marked as coincident 
with chromium, which were increased in a very marked degree. The lines 
D appeared in a small degree broader, as if by the addition of a faint and 
narrow nebulosity at both edges (see fig. 3. Pl. V.). The group of lines at 
B was stronger, also the lines 6 and E and many lines found by Kirchhoff 
to be coincident with lines of iron. The absence of sensible increase in F 
was marked, in comparison with the greater strength of a line or lines, on 
the less refrangible side of F, at about 2066-2 to 2067-1 of Kirchhoff’s scale. 
No bright lines were detected in the spectrum of the umbra. 
It may be permitted to refer to some of the conditions of the solar surface 
by which the phenomena observed might be brought about*. 
A cooler state of the heated vapours by which the dark lines of the solar 
spectrum are produced would diminish the radiation from the gas itself, and 
thus leave more completely uncompensated the absorption by the gas of the 
light from behind it. Such a cause would produce increased blackness of the 
lines, but would not account for more than a very slight apparent increase of 
breadth. The greater breadth of the lines may point to a condition of the 
solar vapours in which their power of absorption embraces, for each line, a 
greater range of wave-length. Such an alteration we know to occur in 
hydrogen as its tension increases. It may therefore be due to an increase 
of the density of the vapours existing within the umbra. 
We do not know from how a great a depth below the layer of bright gra- 
nules the light came that we haye now under consideration. Probably it 
was emitted, for the most part, by that part of the sun which Mr. Dawes 
has named the cloudy stratum. 
We have at present no certain knowledge of the true nature of a solar 
spot. Telescopic observation would seem to suggest that it consists essen- 
tially of the unveiling, by the withdrawal and dissipation of the layer of 
bright granules, of that part of the sun which is immediately beneath the 
granules, and of which we obtain some glimpses through the pores, which are 
always present, and are of different degrees of blackness. 
The absence of bright lines from the spectrum of the umbra may show 
that no considerable part of the light which emanates from the umbra of a 
spot is due to luminous gas. This negative evidence, however, is probably 
* [The absence of increase in the lines C and F may show that the absorption by hydro- ; 
gen is not materially increased, or it may be caused by the bright iines of prominences — 
lying over the umbra of the spot,—January 1869.] id 
