2 Prof. Kirchhoff on the Relation between the Radiating and 



is sufficiently charged with electricity, or when it is phospho- 

 rescent or fluorescent. Such cases are, however, here excluded. 



When a body encounters rays from without, it absorbs a por- 

 tion of them and converts it into heat. But, besides this species 

 of absorption, there may, under certain circumstances, be others, 

 as, for example, when a body is phosphorescent or fluorescent. 

 It is, however, here assumed that all absorbed rays are converted 

 into heat. 



§ 1. Before the body C (fig. 1), imagine two screens, S„ 

 S ? , to be placed, containing openings 1 and 2, Yig. 1. 



whose dimensions must be regarded as infi- s» * . 



nitely small in comparison with their distance 



apart, and each of which has a middle point. 



Through these two openings a pencil proceeds 



from the body C. Of this pencil let that part 



be considered which consists of waves, the ~~Z^Z 



length of which lies between X and X -f dk, and Qy 



let this be divided into two component parts 



polarized in the perpendicular planes a and b passing through 



the axis of the pencil. Let the intensity of the part polarized in 



a be Ec?\ : E is then the radiating power of the body. 



Conversely, a pencil of rays polarized in plane a, and having 

 waves of the length X, falls on the body C through the openings 

 2 and 1. Of this, part is absorbed by the body, the rest being 

 partly reflected and partly transmitted. Let the ratio of the 

 absorbed to the incident rays be called A ; then A will represent 

 the power of absorption of the body. 



The magnitudes E and A depend on the nature and tempera- 

 ture of C, on the position and form of the openings 1 and 2, on 

 the magnitude X, and on the position of the plane a. It will be 

 shown that the ratio of E to A is independent of the nature of 

 the body ; it will thence necessarily follow that it cannot be 

 affected by the position of the plane a, and its independence of 

 the position and form of the openings 1 and 2 will thence be 

 easily deduced, so that it only remains to be determined how far 

 it depends on the temperature of C and the wave-length X. 



The proof I am about to give of the law above stated, rests on 

 the supposition that bodies can be imagined which, for infinitely 

 small thicknesses, completely absorb all incident rays, and neither 

 reflect nor transmit any. I shall call such bodies perfectly black, 

 or, more briefly, black bodies. It is necessary in the first place 

 to investigate the radiating power of bodies of this description. 



§ 2. Let C be a black body. Let its radiating power (gene- 

 rally indicated by E) be called e. It will be shown that e 

 remains the same when C is replaced by any other black body of 

 the same temperature. 



