244. Prof. Kirchhoff on a New Proposition 
of heat from without by an absolutely non-conducting medium. 
Under these circumstances the temperature of the body C cannot 
alter; the sum of the intensities of the rays which it emits must 
therefore be equal to the sum of the intensities of the rays which 
it absorbs ; and because it absorbs all those that fall upon it, the 
sum of the intensities of the rays it emits must be equal to the 
sum of the intensities of the rays which fall upon it. If, now, 
we suppose the followmg change: The “ surface 2” is removed 
and replaced by a circular mirror which reflects all the rays fall- 
ing upon it, and whose centre is in the middle of opening 1. 
The equilibrium of the heat must still be kept up; the sum of 
the rays which fall on the body C must still remain equal to the 
sum of the rays which it emits. But, as it emits just as much 
as before, the quantity of rays which the mirror reflects upon 
the body C must be equal to that which the surface 2 emitted. 
The mirror produces an image of opening 1, which is coincident 
with opening 1. For this reason, just those rays come back to 
the body C, after one reflexion from the mirror, as the body C 
would have emitted through the openings | and 2 if this last one 
had been open; and the intensity of these rays is equal to the 
intensity of the rays which the surface 2 sent back through the 
opening 1. This last intensity however, is, evidently indepen- 
dent of the nature of the body C; and hence it follows that the 
intensity of the pencil of rays which the body C radiates through 
the openings 1 and 2, is independent of the form, position, and 
constitution of the body C; supposing of course that this body 
is black, and that its temperature is a given one. According to 
this, however, the qualitative composition of the pencil of rays 
might become different if the body C were replaced by another 
black body of the same temperature. This is, however, not the 
ease. If I call e the power of emission of this black body com- 
pared with a certain wave-length and a given plane of polariza- 
tion—that, therefore, which I have called E under the suppo- 
sition that C is a body of any kind—then e is independent of 
the nature of the body C, if it only be black. In order to render 
this evident, a further arrangement is necessary. Into the pencil 
of rays which passes from the opening 1 towards the surface 2, 
Jet us suppose a sinall plate placed, which is of so: slight a thick- 
ness that it shows in the visible rays the colours of thin plates ; 
let it be so placed that the pencil of rays is incident at the polar- 
izing angle; let the material of the plate be so chosen that it 
neither absorbs nor emits a sensible amount of rays; let the 
envelope joining the screens 8, and S, be so shaped that the 
image which the plate reflects of the surface 2 lies in the envelope. 
At the position, and of the size of this image, let an opening in the 
cuvelope be made; tuis I will call “opening 3.” Let a screen 
