Connexion between Radiation and Absorption. 99 



In a paper* which should be studied by the historical inquirer, 

 Stewart himself protests against some of KirchhofPs remarks, 

 and to my judgment makes out his case. In his excellent 

 Handbuch dev Spectroscopies recently published, Prof. Kayser, 

 with evident desire to be impartial, gives Stewart much, but 

 not all, of the credit that I would claim for him. But, so far 

 as I have seen, neither Stewart himself nor any of his critics 

 favourable or unfavourable have cited the paragraph upon 

 which he mainly relies. It may be of service to readers who 

 are unlikely to see the original, if I reproduce it here, exactly 

 as it stood : — 



" 20. A more rigid demonstration may be given thus : — 

 Let AB, BC be two contiguous, equal, and similar plates in 

 the interior of a substance of indefinite extent, kept at a 

 uniform temperature. The accumulated radiation from the 

 interior impinges on the upper 

 surface of the upper plate ; let 

 us take that portion of it which 

 falls upon the particles A, in the 

 direction DA. This ray, in 



passing from A to B will have ■ 



been partly absorbed by the sub- 

 stance between A and B ; but 



the radiation of the upper plate 



being equal to its absorption C 



(since its temperature remains 



the same), the ray will have been just as much recruited by 

 the united radiation of the particles between A and B, as 

 it was diminished in intensity by their absorption. It will 

 therefore reach B with the same intensity as it had at A. 

 But the quality of the ray at B will also be the same as its 

 quality at A. For, if it were different, then either a greater 

 •or less proportion would be absorbed in its passage from B 

 to C, than was absorbed of the equally intense ray at A, in 

 its passage between A and B. The amount of heat absorbed 

 by the particles between B and C would therefore be different 

 from that absorbed by the particles between A and B. But 

 this cannot be; for, on the hypothesis of an equal and 

 independent radiation of each particle, the radiation of the 

 particles between B and C is equal to that of the particles 

 between A and B, and their absorption equals their radiation. 

 Hence the radiation impinging on B, in the direction of DB, 

 must be equal in quality as well as quantity to that impinging 

 upon A; and, consequently, the radiation of the particles 

 between A and B must be equal to their absorption, as regards 

 * Phil. Mag. vol. xxv. p. 354 (1863). 

 H 2 



