THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOIkRJkAL OF SCIENCE. 



— 



M>R I® '{SIXTH SERIES.] 



APRIL 1922. 



T 



LXXVI. Note on the Theory of Radiation. By C. G. Darwix, 

 Fellow and Lecturer of Christ's College, Cambridge *. 



HE theory of optics and the thermodynamic theory 

 of radiation both deal with the electromagnetic 

 changes of free space, and } r et their methods of treatment 

 are so widely different that hardly any point of contact is 

 to be found in their fundamental definitions and procedure. 

 In optics the primary description is in terms of the electric 

 and magnetic forces, and an arbitrary field is specified by 

 giving those forces at all times and places. On the other 

 hand ; in thermodynamics the analysis is entirely different, 

 since it is in terms of radiant energy, passing through every 

 point in every direction and sorted out according to its 

 frequency. Now, though the spectroscope makes it fairly 

 clear what is meant by "the energy in a certain range of 

 frequencies,*' yet a rigorous definition is lacking, and even 

 the conception of streams of radiation going simultaneously 

 in all directions does not lend itself at once to the ordinary 

 energy formulae of electromagnetic theory. It appeared to 

 me, therefore, that it might be useful to place on record the 

 formal connexion between the optical and the thermodynamic 

 descriptions of an arbitrary field of radiation ; its outline 

 must have been present in the minds of most writers on 

 radiation, but I have not seen it in any work on the subject. 



* Communicated by the Author. 

 PHI. Mag. S. 6. Yol. 43. No. 256. April 1922. 2 T 



