in the Theory of Heat. | 245 
be so placed that no straight line can be drawn from any point 
of opening 1 to any point of opening 8 without passing through 
the screen. Let the opening 3 be now closed with a black sur- 
face, which I will call “surface 3.” The whole system is then 
supposed to possess the same temperature; there is therefore im 
this case equilibrium as regards the heat. This equilibrium is 
supported by rays which, proceeding from surface 3, suffer 
reflexion on the plate, pass through opening 1, and fall on the 
body C. ‘These rays are polarized in the plane of incidence of 
the plate, and contain, according to the thickness of the plate, 
sometimes more of one, sometimes more of another kind of ray. 
Let the surface 3 be removed and replaced by a circular mirror 
whose centre is situated at the spot where the plate reflects an 
image of the centre of the opening 1; then the rays emitted by 
surface 3 will no longer fall on body C, but instead of them 
those reflected from the mirror will fall upon it, and the equili- 
brium of the temperature remains unchanged. If we reflect that 
it does not matter what thickness the plate possesses, or in what 
position we turn it round the axis of the pencil determined by 
passing through openings | and 2, we arrive, by means of similar 
considerations, at the conclusion that the power of emission of the 
black body C, considered with respect to a given wave-length 
and a given plane of polarization, is quite independent of the 
constitution of this body. A conclusion which naturally arises 
from this proposition is, that a// rays which a black body emits 
are completely unpolarized. | 
If we imagine that in the foregoing arrangement the body 
C is not black, but of any other colour, the following equation is 
found by similar reasoning :— 
senate OA nua ae a 
This equation indicates that the relation between emission and 
absorption remains constant for all bodies. The equation may 
obviously be written 
B= App Pet Get Oo Say ae) 
or 
I will now notice some remarkable conclusions derived from 
my proposition. If we heat any body, a platinum wire for ex- 
ample, gradually more and more, it first emits only dark rays ; 
at the temperature at which it begs to glow, red rays begin to 
appear; at a certain higher temperature yellow rays are seen ; 
then green rays, until at last it becomes white-hot, 2, e. emits all 
