tJie late Balfoiw Stewart. 



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has passed through different thicknesses of the transparent 

 bodies in various directions. What Stewart established 

 for the sum of these streams, Kirchhoff proved for each of 

 them individually. The reasoning thus becomes much 

 more complicated, but the proof more complete and 

 definite. 



In other words : Stewart had shown that the law of 

 equality between radiation and absorption could explain all 

 phenomena. Kirchhoff went further and proved that 

 the law is the only one which can account for the facts. 

 The existence of internal radiation was completely esta- 

 blished by Stewart's researches, and he also considered the 

 connexion between internal radiation and refractive indices. 

 Considering the problem at first only as one of two dimen- 

 sions, he took the internal radiation to be proportional to 

 the refractive index, but later corrected the result and con- 

 cluded, as was also done by Kirchhoff, that when three dimen- 

 sions are taken into account, the intensity of internal radia- 

 tion must vary as the square of the refractive index. 

 Although Stewart generally speaks only of the sum of all 

 thermal radiation, he shows that the laws he deduces must 

 be true for each individual wave length. From this there 

 was only one step to the complete generalisation of spectrum 

 analysis. He has told the author of this notice, how he 

 tried an experiment which was to lead him still further in 

 the same direction. Salt was placed in a spirit flame and 

 the yellow flame was looked through a plate of rock salt. 

 Believing the yellow colour of the flame to be due to lumi- 

 nous salt, he expected the rock salt to absorb an appreciable 

 quantity of the light, which, however, was found not to be 

 the case. We now know that salt is decomposed in the 

 flame and that the yellow flame is due to luminous 

 sodium vapour, which explains the failure of Stewart's 

 experiment. 



In February, i860, Stewart communicated to the Royal 



