the Light emitted by Incandescent Zinc Oxide. 25 



sufficiently sensitive thermopile ; or the intensities may be 

 expressed in terms of those of the comparison-lamp, wave- 

 length for wave-length ; or, finally, one may express the 

 curve for each temperature in terms of the radiation from 

 platinum at the same temperature, wave-length for wave- 

 length. Our knowledge of the spectrum of the incandescent 

 lamp was not accurate enough to enable us to adopt the first 

 of these methods, which is moreover not so well adapted for 

 the graphic expression of colour as is the second one. The 

 third method offers the advantage of a direct comparison 

 between the light from platinum and from the oxide under 

 like conditions of incandescence. 



In figs. 7, 8, and 9 the second method has been adopted. 



Isothermal Curves for the Radiation from Zinc Oxide and Platinum ; 

 showing three stages in the development of the radiating-power of the 

 former substance. Ordinates are intensities in terms of those of corre- 

 sponding wave-lengths in the spectrum of the comparison-lamp. 



Fig. 7. 



Fio-. 8. 



Fig. 9. 



005 



ABC D E F G A B C D E F G A B C B E F G 



Fig. 7 gives typical curves for a temperature below 700°, at 

 which stage the radiation from the oxide is weaker throughout 

 the spectrum than that from platinum. Fig. 8 contains the 

 corresponding curves for 848°, at which temperature the 

 green, blue, and violet are stronger in the spectrum of the 

 oxide than in that of the platinum. In fig. 1) the spectrum 

 of the film of the oxide, which has been maintained at 1013° 

 for ten minutes, is compared with the platinum spectrum for 

 the same temperature. The measurements of the oxide are 



