106 PRODEEDINGS OF THE AMERICAN ACADEMY. 



tensities of radiation reaching the surface from outside [corresponding 

 to different temperatures of the body S]. For in general the absorp- 

 tion coefficient has to be measured under one set of conditions when it 

 is desired to apply the results to another set. 



Hence it may seem that Kirchhoflf's Law holds very well for one set 

 of bodies and very poorly for another set, if the absorption coefficient 

 is regarded as a constant. The preceding analysis shows, however, 

 that carefully stated, the law holds as well for one substance as an- 

 other, as well for gases and liquids as for solids. 



In closing I should like to acknowledge my indebtedness to Professor 

 G. W. Pierce of Harvard University for helpful criticism. 



Jefferson Physical Laboratory, 

 Harvard University, 

 Cambridge, Mass. 



