80 
LUMMER. 
that, is, its law of radiation. Thus the black body is con- 
nected to the whole material world. 
Kirchhoff appreciated the great importance of knowing 
the radiation laws of the black body. In his celebrated 
paper in 1860 he wrote: “The radiant energy of the black 
body is a function only of the wave-length and the tempera- 
ture, to know which is of highest importance. The experi- 
mental research is attended with very great difficulties, but it 
is to be hoped that some one will in the future succeed in 
determining this function, which surely is of a simple form, 
for all the functions known up to the present time, which do 
not depend at all on special properties of the bodies, are 
simple ” 
Knowing these laws as we now do, the law of Kirchhoff 
has grown from a qualitative to a quantitative one. In order 
to know the radiation laws of the black body or, as we will 
call it, black radiation, it was necessary to realize this curious 
body, which does not exist ready-made. In fact, until one 
was constructed by W. Wien and myself in 1895, it was not 
possible to reach the high standpoint of today. According to 
its definition, the black body must absorb every radiation and 
not allow any wave to be reflected or transmitted. Lamp black 
and platinum black come near to this definition, but they 
cannot be heated to high temperatures. Rut, since reflection 
depends on the refractive index of the two media, it cannot 
be zero for all the incident frequencies or waves; thus a 
really black surface cannot exist. 
In considering bow to make a reflecting substance — for 
example, bright platinum — free from reflection, and thereby 
make it a black radiator (platinum in thick plates transmits 
no radiation), W. AVien and I came to the conclusion ee that 
a cavity of uniform temperature must emit, through a small 
aperture in the wall, black radiation corresponding to the 
temperature of the cavity ” By such an arrangement the 
black body is realized with an almost theoretical perfection. 
Rut it is indeed remarkable that this realization failed to be 
attained for nearly forty years, when we read in Kirchhoff’s 
original memoir the following statement: “If a space is en- 
