Fig. 2. 



Absorbing Powers of different Bodies for Light and Heat. 3 



Imagine the body C enclosed in a black covering, of which the 

 screen Sj forms a part. Let the second screen, S 2 , be black 

 also, and let the two be united all round by black walls (fig. 2). 

 Let the opening 2 be in the first place closed by 

 a black surface, which I shall call surface 2, and 

 let the whole system be kept at a constant tem- 

 perature by being completely enclosed in a co- 

 vering impermeable to heat, as, for example, in 

 a perfectly reflecting surface. Then, since the 

 temperature of the body C remains constant, 

 the intensity of the incident rays (which, ac- 

 cording to hypothesis, it entirely absorbs) must 

 be equal to that of the emitted rays. Now imagine surface 2 

 to be removed and replaced by a portion of a perfectly reflecting 

 spherical mirror having its centre in the middle point of open- 

 ing 1. The equilibrium of the temperature of the system will 

 be undisturbed, and the equality of the intensities of the emitted 

 and incident rays must therefore still subsist. As, however, the 

 body C emits the same rays as before, it follows that the inten- 

 sity of the rays that impinge on it must be the same in both 

 cases. By the removal of surface 2, those rays are withdrawn 

 from the body which proceeded from that surface through open- 

 ing 1. In the place of the rays so withdrawn, the mirror applied 

 at 2 reflects back on the body the rays which proceeded from it 

 through the openings 1 and 2*. Whence it may be concluded 

 that the intensity of the pencil which proceeds from the body 

 C through the openings 1 and 2, is equal to the intensity of the 

 pencil which at the same temperature proceeds from surface 2 

 through opening 1. But this is independent 'of the form and 

 constitution of the body C. The alleged law is therefore proved 

 when all the rays of the pencils compared have the same wave" 

 length X, and are polarized in the same plane a. llegard, how- 

 ever, to the possible diversity of these rays renders somewhat 

 more complex considerations necessary. 



§ 3. In the arrangement represented in fig. 2, 

 imagine a small plate P (fig. 3) placed between 

 openings 1 and 2, which in the visible rays 

 displays the colours of thin plates, and which, 

 partly on account of its extreme thinness, and 

 partly on account of its material constitution, 

 neither absorbs nor emits any perceptible quan- 

 tity of rays. Let this plate be so placed that 

 it cuts the pencil passing between 1 and 2 at the 



* The effect of the diffraction of the rays by the edges of opening 1 is 

 here neglected. This is allowable if openings 1 and 2, though infinitely 

 small in comparison with their distance apart, be considered as very great 

 in comparison with the length of a wave. 



