SPECTRUM ANALYSIS OF THE HEAVENLY BODIES. 163 
formed by the light from the bright central parts of the solar disk. I have 
as yet failed to detect any difference. 
2. It appeared possible that a view of the red prominences visible during 
a solar eclipse might be obtained by reducing the light of our atmosphere 
by means of prismatic dispersion ; for, under these circumstances, if the red 
prominences give a spectrum of bright lines, these lines would remain but 
little diminished in brightness, and might become visible. Observations with 
this object in view have been made during the last two years by different 
methods, but hitherto without success. 
3. The third investigation, in which some success has been obtained, was 
with a view to gain, from an examination of the spectra of the umbre and 
penumbre of spots, some information as to the nature of these phenomena. 
I had already made some observations in this direction, when, in August 
1866, I received a note from M. Faye, in which he suggests to me the 
prismatic examination of solar spots with reference especially to the theo- 
retical views of the sun proposed by him. 
My first observations were made with a small spectroscope, which was so 
arranged that the imago of the sun was formed upon the slit after its heat 
had been enfeebled by reflection from a prismatic solar eyepiece. The prin- 
cipal solar lines were observed to be stronger in the spectrum of the light 
from the umbra. 
IT use the term “umbra” to describe the whole of the dark part within the 
penumbra, since it was not’ possible in these observations to distinguish the 
two distinct parts of which, in most spots, it is seen to consist, namely, the 
nucleus and the cloudy stratum, which were first described by Mr. Dawes. 
In October 1866, Mr. Lockyer, who had independently made similar ob- 
servations, presented a paper to the Royal Society. He observed the lines 
to be thicker where they crossed a spot. 
It was not until April 15, 1868, that a favourable opportunity occurred 
to observe the spectrum of a spot with the powerful spectroscope described 
in this paper. The spectroscope was rotated until the length of the slit was 
in the direction of the length of the spot. When the middle of the umbra 
was brought upon the slit, its spectrum appeared as a feebly illuminated 
band upon the bright solar spectrum. The band appeared divided into two 
parts by the spectrum of the bright prominence, which at the time ex- 
tended nearly across the umbra. It was obvious that a part only of the 
light which appeared to form the spectrum of the umbra came from that 
region of the sun. The imperfect transparency of our atmosphere causes 
it to become strongly illuminated when the sun shines upon the earth; and 
the brilliant light which is seen to be radiated by it near the sun’s limb is 
also radiated by that part of the atmosphere which is between the observer 
and the sun. 
Tn order to determine how much of the light came from the atmosphere, I 
made use of a graduated wedge of neutral-tint glass, which was first inter- 
posed before the eye when the atmosphere near the limb was upon the slit. 
The wedge was moved until the lines ceased to be distinguishable. When 
the umbra was upon the slit, the wedge was again moved so as to bring the 
Same part of the spectrum to the same degree of obscurity, as nearly as could 
be judged by the inability of the eye to distinguish the lines. In this way’ 
it was found that, roughly, about three-fourths of the light which formed 
the spectrum of the umbra was really due to that part of the sun. 
_ As, in consequence of the way in which the spectrum is formed, under 
similar instrumental conditions the dark lines should appear rather thicker 
n2 
