TEMPERATURE AND STRUCTURE OP THE SUN. 79 
We must read the papers of that time in order to recall 
the revolution which was caused by Kirchhoffs theory and 
the astonishment ol the old school, which believed the sun 
to be a cold body surrounded by a hot ocean. The excite- 
ment and the discussions caused by Kirchhoff’s new solar 
theory were as great as when Roentgen published his dis- 
covery oi the X-rays or when Sir William Herschel dis- 
covered new kinds of solar rays. “Where we have no light 
at all, we have the most heat,” Sir William Herschel wrote 
in his first publication, in 1800. Since that time the radia- 
tion of the sun has been studied continuously down to the 
present time, culminating in the famous researches of Mr. 
Langley, whose acquaintance I was proud to make when I 
was in your country over thirteen years ago. 
Of so great importance were the conclusions from Kirch- 
hoff’s law with respect to spectrum analysis that the real 
weakness and strength of the law of absorption and emis- 
sion were overlooked. 
Let me first call your attention to the real importance of 
Kirchhoff’s law before discussing the limitations of his con- 
clusions. In my opinion, Sir William Thomson and so 
many other scientists need not have questioned the priority 
of Kirchhoff’s law, even in its precise form, for they all over- 
looked the importance to this law of the definition of the 
black body. You all know that Kirchhoff formulated his 
law as follows: 
a, t 
where S is the radiation of the “black” body, E and A the 
power of emission and absorption of any other body, all 
symbols referring to the same temperature and the same 
spectral region. This means that the ratio of the emission 
to the absorption of any arbitrary body is equal to the radia- 
tion of the black body, and when we know the radiation 
laws of the black body, we only need to determine the absorp- 
tive power of another body at every temperature and for 
every wave-length in order to know its power of emission 
