OF SOME OF THE STAES AND NEBULAE. 
553 
the spectrum of the umbra to about the same degree of invisibility the wedge had to be 
moved until the part marked 20 came before the eye. A photometric examination of 
the wedge shows that the light intercepted at 10 is to that intercepted at 20 as 1 to 8. 
It may then be concluded that about three-fourths of the light which formed the spec- 
trum of the umbra was really due to the umbra of the spot. 
There were several bright granules on the umbra, but the spectra of these were seen 
distinct from that of the umbra. Each bright point as it came upon the slit gave a 
narrow spectrum like a bright thread extending along the dark spectrum of the 
umbra. 
There still remained two sources of uncertainty. 1. In consequence of the mode in 
which the spectrum is formed, under similar conditions of the instrument, the dark lines 
should appear rather thicker when the light is feebler. 2. The increased thickness of 
the lines in the compound spectrum might be due to the light of the umbra, or to that 
of our atmosphere. 
The uncertainty on both these points was removed by observing the feebler spectrum 
of the illuminated atmosphere near the sun’s limb. The lines in this spectrum, though 
they appeared very slightly stronger, were not so in a degree that could afford an expla- 
nation of the very marked increase of strength which most of them presented in the 
spectrum of the umbra. It seemed, therefore, satisfactorily determined that the light 
from the umbra had really suffered a more powerful absorption. The term umbra is 
used to include the cloudy stratum and the darker nucleus into which Mr. Dawes has 
shown the umbra of a spot may be usually resolved. It is probable that nearly the 
whole of the light under examination came from the part of the umbra known as the 
cloudy stratum. It was not possible to distinguish the spectra of these portions of the 
umbra. 
The spectroscope was sufficiently powerful to show all the lines which are given in 
Kirchhoff’s maps. 
I carefully examined the spectrum of the umbra with that of the adjoining parts of 
the solar surface from A to G, but I was not able to detect any line of absorption in the 
spectrum of the umbra which was not also present in that of the sun’s normal surface, 
or that any ordinary solar line was wanting in that of the umbra. 
The increase of thickness, however, did not appear to take place in the same proportion 
for all lines. The lines C and F, due to hydrogen, appeared increased but very slightly, 
if indeed they were any thicker than would be due to a spectrum of feebler intensity. 
I incline to the opinion that these lines are not sensibly altered. 
There is a small group of lines a little less refrangible than b, at 1601 to 1609, of 
Kirchhoff’s scale, and which in his map are marked as coincident with lines of chromium, 
which was especially noticeable from increased thickness. That this circumstance was 
not connected with any peculiarity of the spot under examination is shown by a similar 
observation having been made on other spots. 
Fig. 3, Plate XXXIII. represents the appearance of the double line D in the spec- 
4 g 2 
