in the Theory of Heat. 243 
A difference of quality is noticed amongst the rays of light, not 
only in respect to the colour, but also in respect to the state of 
polarization. Hence not only have we to distinguish the heating 
rays according to the wave-lengths, but we have also to divide 
rays of one wave-length into those variously polarized. If we 
take into consideration these various kinds of rays of heat, the 
conclusions which we had drawn concernimg the relation between 
the powers of absorption and emission cease to be binding. 
Whether this relation is still found to exist when these variations 
are taken into consideration, is a question which has, as yet, not 
been decided either by theoretical considerations or by an appeal 
to experiment. I have succeeded in fillmg up this gap; and I 
have found that the proposition concerning the ratio between the 
power of emission and the power of absorption remains true, 
however different the rays which the bodies emit may be, as long 
as the notions of emissive and absorptive powers be confined to 
one kind of ray. 
_ The proposition which I have discovered may be thus more 
precisely defined :—Let a body C be placed behind two screens 
S, and S,, in which two small openings are made. Through these 
openings a pencil of rays proceeds from the body C. Of these 
rays we consider that portion which corresponds to a given wave- 
length A, and we divide this into two polarized components, 
whose planes of polarization are two planes a and 0 at right 
angles to each other, passing through the axis of the pencil of 
rays. Let the intensity of the polarized component a be HE 
(emissive power). Now suppose that a pencil of rays, having a 
wave-length =), and polarized in the plane a, falls through the 
openings 2 and J upon the body C. ‘The fraction of this pencil 
which is absorbed by the body C is called A (absorptive power). 
Then the relation 5 
nature of the body C, and is alone determined by the size of the 
openings 1 and 2, by the wave-length >, and by the tempera- 
ture. I will point out the way in which I have proved this pro- 
position. I began by considering that bodies are conceivable 
which, although very thin, absorb all the rays which fall upon 
them, or which have the capacity of absorption =1. I call such 
bodies perfectly black, or simply black. I first mvestigated the 
radiation of such black bodies. Let C be a black body. The 
body C is supposed to be enclosed in a black envelope, of which 
the screens 8, and S, are a part, and the two screens are sup- 
posed to be connected by a black surface surrounding all. 
Lastly, let the opening 2 be closed by a black surface, which I 
will call “surface 2.” The whole system is to be considered to 
possess the same temperature, and to be protected against loss 
R2 
is independent of the position, size, and 
