78 
LUMMER. 
scientific problems and their solutions. I mean the radia- 
tion-laws of the black body, which were introduced into 
physics as a definition by Kirchhoff, the discoverer and 
founder of spectrum analysis. We will lose no time, for time 
is valuable this evening, in discussing the dispute which 
arose among the scientists of different nations over the 
priority of the discovery of KirchhofFs famous law of ab- 
sorption and emission of light. 
Even before Kirchhoff ’s time the law, 
E . . 
— = constant 
A 
was known, where E denotes the emissive power of any body 
and A its absorptive power. But this law has no real im- 
portance if we do not refer E and A, as Kirchhoff did, to 
the same temperature and spectral region. It is only in the 
form, 
= const] 
*-A J A, T 
that this law has a scientific value. In this form it says that 
a body at a certain temperature, T, and in a certain spectral 
region, emits those rays, which it absorbs, at the same 
temperature and of like wave length. The theoretical de- 
velopment of this law became the basis upon which spectrum 
analysis was built. Starting from this law, Kirchhoff and 
Bunsen looked for the coincidence of dark Fraunhofer lines 
in the solar spectrum with bright spectrum lines emitted by 
terrestrial substances. For the same reason Kirchhoff con- 
structed his famous spectroscope of high resolving power, 
with which, after much tedious labor in mapping many 
thousand spectrum lines, he could assert that iron and other 
terrestrial substances are found on our sun. The probability 
of a coincidence of sixty lines being due to chance is (Vs) 00 . 
Furthermore, since Kirchhoff succeeded in inverting the 
sodium lines by sending through the sodium flame the light 
of an arc lamp, he assumed the sun to be a highly heated 
body surrounded by radiating gases and vapors. 
