268 



Sir James Dewar 



[Jan. 16, 



assumption of ordinary " D " light, but the actual radiation measured 

 is very different from this, and the consequent correction to be 

 applied is doubtful. 



^ In this connection it is as well to recall the altering wave length 

 of the region of maximum radiation as the temperature of the source 

 is changed. This is illustrated by Figs. 15 and 16. The first set of 



40 



. Percentage 

 Reflected. 

 30 (Nwm«jl^Incid«* "D'jjghtJ 



Reflection at a Plane Surface: 

 Altering Proportion, with 

 Increasing Refractive Index .- . 



-b-(£rr) a ;( Fresnel ) 



-lotalutber? 



Index or Refraction 



Fig. 14. 



?5 



curves are from Lummer and Pringsheim's results. The second, at 

 lower temperatures, are calculated from Planck's formula — 



A"* X C 



!»U T -1 



(as given in the Smithsonian Physical Tables, 1920, p. 247). 



At a temperature of 1650° Abs. the maximum theoretical wave 

 length emitted is of the order 2 /x, while by the time a source is 



