DEVELOPMENTS IN ELECTROMAGNETISM BLOCH. 231 



analysis, to admit, to a closer degree of approximation, that the ether, 

 at least slightly, is similar to ordinary matter, that it may propagate 

 a disturbance with a velocity greater than that of light, that it does 

 not remain perfectly stationary when matter traverses it, etc. New 

 experiments must be added to the purely electro-optic ones of Michel- 

 son, Rayleigh, Brace, and Troughton before we will be able to build 

 these theories. 



III. ELECTROMAGNETISM AND RADIATION. 



The difficulties just described are not the onl}^ ones which the 

 modern theory of electromagnetism encounters. Perhaps the gravest 

 ones arise in adapting it to the experimental facts of radiation. We 

 know that thermal radiation in equilibrium in a constant-temperature 

 chamber, and called " black radiation," has a density independent of 

 the particular body producing it. It is a function only of the wave- 

 length X and the absolute temperature T. Our theoretical knowl- 

 edge of this density, ii^, is expressed by the well-known laws of 

 Kirchoff, Stefan-Boltzmann, and Wien.^ Our experimental knowl- 

 edge is expressed by the formula of Planck, 



5r XT 



~0< AX ^ 



u.=c^X \e — 1] 



This equation satisfies not only the three theoretical laws but also 

 corresponds to the observed distribution of energy in the spectrum 

 of a black body. This formula reduces for large values of XT to 

 the earlier one of Rayleigh, 



u^ = cJc,-TIX'. 



Now, the electromagnetic theory seems to lead almost inevitably to 

 Kayleigh's formula for all wave-lengths in flagrant contradiction to 

 experimental facts. The second formula, indeed, does not give a 

 maximum to the radiation distribution curve and makes the total 

 radiation in-finite. This consequence, which the researches of Lord 

 Rayleigh^ and Jeans ^ made extremely probable, has been ren- 

 dered certain by those of Lorentz.* According to the latter's re- 



1 These laws rest only on the Doppler-Fizeau principle, thermodynamical reasoning and 

 the pressure of radiation, principles which may be held as well proven if not as ex- 

 perimental facts. 



2 Rayleigh, Phil. Mag., vol. 2, p. 539, 1900. 



3 Jeans, 1. c, vol. 10, p. 91, 1905; vol. 17, p. 229, 1909; vol. 17, p. 77?., 1909; vol. 18, 

 p. 209, 1909. 



* Lorentz, Revue generale des Sciences, p. 14, 1909 ; La theorie du rayonnement (The 

 theory of radiation), Rapports au Congres de Bruxelles de 1911, publies par Langevin 

 and de Broglie. 



