140 



INFRA-RED EMISSION SPECTRA. 



EMISSION FROM A PARTIAL RADIATOR. 

 We have now to consider a problem which has heretofore not been 

 discussed in connection with the radiation from the moon. Planck's 

 formula for the intensity of radiation at a given wave-length X in the spec- 

 trum of a complete radiator is 



E x ^c 1 X- B (^ XT i)- 1 



For any radiating body the emission cf> x = A x E x , where A is the absorption 

 coefficient. But A x =iR x (for "transparent media," i.e., non-metals), 

 where R is the reflecting power, and hence <> X =E X (i R^). 



185 



Defl in mm 

 150 



6 7 8 9 10/1 

 Fig. 98. Emission spectrum of copper oxide and of quartz, 325 C (Rosenthal). 



It follows therefore that a selectively reflecting body like quartz will 

 be a partial radiator 1 and that if we compare its radiation curves with 

 that of a complete radiator, there will be minima of emission in the region 



Partial radiators may be divided into so-called "gray bodies" and bodies having a 

 highly selective emission. Quartz belongs to the latter class. 



