194 FOOTE AND FAIRCHILD : GRAY BODY RADIATION 



1. Are there materials which show true intersections of the 

 logarithmic isochromatics? 



2. Do these intersections furnish an exact measure of the true 

 temperature? 



Carbon was found to show the intersections referred to. The 

 conclusion was accordingly made that carbon is gray and that 

 the temperature corresponding to the point of the intersection is 

 the true temperature of the carbon. 



It is the purpose of this note to point out that these conclu- 

 sions can not be drawn from the experimental data. It will be 

 shown that even though the logarithmic isochromatics of vari- 

 ous wave lengths do intersect in one common point, this point 

 of intersection is no immediate indication whatever of the true 

 temperature of the non-black radiator, and is no proof that the radi- 

 ator is even approximately gray. 



The radiation of a black body is compared spectrophotometri- 

 cally with that of a non-black body. 



J i = intensity of radiation of wave length X from black body. 



J 2 = intensity of radiation of wave length X from non-black 

 body. 



6 = absolute temperature of black body. 



T = absolute true temperature of non-black body. 



A = emissivity coefficient of non-black body which in gen- 

 eral is a function of both T and X. 



The question of A as a function of T is not considered in the 

 present note. We shall accordingly assume A to be a function 

 of X only. 



(1) J 1 =c,X e M 



{Wieri's law for black body.) 



(2) J 2 = d \~ b Ae~*r 



(Analogous law for non-black body.) 

 Whence 



^ ^jr x ^ A+c i(j-i) 



