Absorbing Powers of different Bodies for Light and Heat. 1 1 



which consists of waves whose length lies between X and X + d\ t 

 and let it be divided into two components polarized in a and its 

 perpendicular. Let that part of the first component which is 

 not absorbed by C, and which therefore reaches the black cover- 

 ing in which the body C is enclosed, be M!d\. Of the rays that 

 proceed from the covering and are incident on C, a certain por- 

 tion reach surface 2 through opening 1. The body C thus ori- 

 ginates a pencil of rays which is incident on surface 2 through 

 opening 1 . Of this pencil consider that part which consists of 

 waves whose length lies between X and \ + d\, and let this be 

 divided into two parts polarized in plane a and its perpendicular. 

 Let the intensity of the first component be M.d\, Then is 



M = M'. 



This follows from the proposition established in the last sec- 

 tion, by applying that proposition to all the pencils which sur- 

 face 2 and all the elements of the black covering exchange with 

 each other through the medium of the body C, and then sum- 

 ming the equations so obtained. 



§ 11. Let the arrangement represented in fig. 3, and described 

 in § 3, be again taken, C, however, being no longer a black body, 

 but one of any kind. In both the cases described in that section 

 the equilibrium of temperature must still subsist, and therefore 

 the vis viva that is withdrawn from the body by the removal of the 

 surface 3, will be equal to the vis viva it receives by the applica- 

 tion of the concave mirror. Let the letters used in § 3 have the 

 same meaning as in that section, and let E and A have the sig- 

 nification given them in § 1. Then the vis viva withdrawn from 

 the body C by the removal of surface 3 is, according to § 7, 



roo 

 d\erA. 



■I 



The vis viva which the body receives by means of the concave 

 mirror consists of three parts : — The first due to the rays emitted 

 by C itself; this is 



1 



= 1 d\Er 2 A. 



The second due to the rays which, having proceeded from the 

 part of the black covering opposite the mirror, have passed 

 through the plate P, and have been reflected, first by the mirror, 

 and then again by P. This, according to § 9, is 



= 1 d\er{l-?')A. 



The third and last part is due to the rays which have fallen on C 



