460 Lord Rayleigh on the Character of the 



experienced. On electrifying, the needle would take up a 

 certain position and would remain there as long as the charge 

 was kept up ; on reversal, it would move off to a new and 

 perfectly definite position about 6 to 7 mm. away, and remain 

 there, &c. H. A. R. 



C. T. H. 



LIII. On the Character of the Complete Radiation at a given 

 Temperature. By Lord Rayleigh, Sec. M.S., Professor of 

 Natural Philosophy in the Royal Institution*. 



BY complete radiation is here meant the radiation which 

 would ultimately establish itself in an enclosure, whose 

 walls are impervious, and are maintained at a uniform tem- 

 perature. It was proved by Stewart and KirchhofF that this 

 radiation is definite, not only in the aggregate, but also in its 

 various parts ; so that the energy radiated with wave-fre- 

 quencies between n and n + dn may be expressed by 



F(n)dn, (1) 



where, for a given temperature, F(n) is a definite function 

 of n. The reservation implied in the word ultimately is ne- 

 cessary in order to exclude radiation due to phosphorescence 

 or to chemical action within the enclosure. The radiation 

 commonly characterized, so far at any rate as its visible ele- 

 ments are concerned, by the term white, is supposed to be 

 approximately similar to the complete radiation at a certain 

 very high temperature. 



As remarked by Kirchhoff*, the function F, being indepen- 

 dent of the properties of any particular kind of matter, is 

 likely to be of a simple form ; and speculations have naturally 

 not been w^anting. Within the last two years the subject has 

 been considered by W. Michelsonf and by H. F. Weber J 

 The former, on the basis of an a priori argument of a not 

 very convincing character, arrives at the conclusion that at 

 temperature 6 the radiation between the limits of wave-length 

 X and X + dX may be expressed 



l^d\=Be-^-f(e)e~'^'X-'P-'dX. ... (2) 

 According to Stephan the total radiation is proportional to 6*. 



* Communicated by the Author. 



t Journal de Physique, t. vi. Oct. 1887 ; Phil. Mag. xxv. p. 425. 



X Berlin Sits. Ber. 1888. 



