344 Sir John Lubbock’s Address. 
descent metals. In order to get some fixed measure by which 
they might determine and record the lines characterizing any 
given substance, it occurred to them that they might use for 
comparison the spectrum of the sun. They accordingly ar- 
ranged their spectroscope so that one-half of the slit was lighted 
by the sun, and the other by the luminous gases they pro- 
posed to examine. It immediately struck them that the bright 
lines in the one corresponded with the dark lines in the other— 
the bright line of sodium, for instance, with the line or rather 
lines D in the sun’s spectrum. The conclusion was obvious. 
glorious moment when that thought flashed across them, and 
even by itself well worth all their labor. 
ut why is the bright line of a sodium flame represented by 
a black one in the spectrum of the sun? To Angstrém is due 
the theory that a vapor of gas can absorb luminous rays 
of the same refrangibility only which it emits when highly 
heated; while Balfour Stewart independently discovered the 
same law with reference to radiant heat. 
This is the basis of Kirchhoff’s theory of the origin of Fraun- 
hofer's lines. In the atmosphere of the sun the vapors of 
various metals are present, each of which would give its char- 
acteristic lines, but within this atmospheric envelope is the still 
more intensely heated nucleus of the sun, which emits a bril- 
liant continuous spectrum, containing rays of all degrees of re- 
frangibility. When the light of this intensely heated nucleus 
is transmitted through the surrounding atmosphere, the bright 
lines which would be produced by this atmosphere are seen as 
ark ones. 
Kirchhoff and Bunsen thus proved the existence in the sun of 
hydrogen, sodium, magnesium, calcium, iron, nickel, chromium, 
manganese, titanium and cobalt; since which Angstrém, Thalen 
and Lockyer have considerably increased the list. 
But it is not merely the chemistry of the heavenly bodies on 
which light is thrown by the spectroscope; their physical 
structure and evolutional history are also illuminated by this 
wonderful instrument of research. 
It used to be supposed that the sun was a dark body envel- 
oped in a luminous atmosphere. The reverse now appears to 
be the truth. The body of the sun, or photosphere, is intensely 
brilliant ; round it lies the solar atmosphere of comparatively 
cool gases, which cause the dark lines in the spectrum; thirdly, 
a chromosphere,—a sphere principally of hydrogen, jets of 
which are said sometimes to reach to a height of 100,000 miles 
or more, into the outer coating or corona, the nature of which 
is still very doubtful. : 
Formerly the red flames which represent the higher regions 
