Yll 



of electric currents. A suggestion made in the latter paper to use 

 closed rings of iron for the determination of the coefficient of induc- 

 tion has led, at the hands of Stoletow and Rowland, to important 

 practical results. 



Kirchhoff's name is most generally known in connexion with his 

 researches on the relation between the absorptive and the emissive 

 properties of bodies. The explanation of the Fraunh of er lines which 

 are derived from these researches, and the work done jointly with 

 Bunsen on the discontinuous spectra of gaseous bodies, gave such an 

 impulse to the study of radiation that a whole science — that of 

 spectrum analysis — developed as a historical sequence to Kirchhoff's 

 work. It is a carious instance of an abstruse calculation giving rise 

 to extended experimental investigations which have in reality very 

 little connexion with it ; for it is only a small fraction of spectrum 

 analysis in which the connexion between radiation and absorption is 

 made use of at all. There is really no d priori reason why we could 

 not have known as much as we do now of the spectra of different 

 bodies without being acquainted with the important law proved by 

 Kirchhoff. 



A discussion has arisen as to how far Kirchhoff's work was antici- 

 pated by that of Balfour Stewart. The latter had experimented on the 

 radiation and absorption of heat, and had drawn some important 

 conclusions from his experiments. Stewart's work is conclusive in 

 showing that if we assume the ratio of the emissive to the absorp- 

 tive power to be the same for all bodies and only a function of the 

 temperature and wave-length, all facts can be satisfactorily accounted 

 for. Kirchhoff, without being acquainted with Stewart's researches, 

 went further, and proved that the law just stated is the only one 

 consistent with thermodynamical equilibrium. KirchhofPs paper has 

 been objected to as being too elaborate in the method of its proof, but 

 no simpler proof has ever been given, and it would be difficult to lay 

 a finger on a single sentence of this classical paper which could be 

 removed or shortened without detriment to the logical sequence of the 

 argument. 



In connexion with these theoretical researches and in order to trace 

 the existence of terrestrial elements in the sun, Kirchhoff prepared 

 a drawing of the solar spectrum. Unfortunately an arbitrary scale 

 was used, and the prisms were occasionally shifted, so that the map 

 was soon superseded by Angstrom's, in which the lines were directly 

 referred to wave-lengths. It seems of interest, however, to point 

 out as a proof of the acuteness of Kirchhoff's observing power and 

 the perfection of the optical adjustments, that the amount of detail 

 given in his map is exactly that given by calculation as possible with 

 the resolving power which he used. No work could be more trying 

 than that of drawing a map of the solar spectrum reaching the 



