foote: gray radiation 



575 



As a concrete example we have plotted in figure 1 data for a 

 black body at absolute temperatures 1200°, 1300°, 1500°, and 

 1600°. The upper four straight lines represent Dr. Nutting's 

 method of plotting. The lower four straight lines represent 

 the method originally suggested by Lummer. The lower four 

 lines converge at # = 1200° absolute, while, as Dr. Nutting 

 concludes, the upper four do not converge. It may be pointed 

 out that no one ever contended they did converge to a 

 common point. Indeed the points of intersection are given in 

 table 1. 



TABLE 1 



Temperatures at Which Intersections Occur on the Basis of Dr. Nutting's 



Method of Plotting 



When the data for equilibrium radiation are properly plotted 

 there is no "approximation" to the "stigmatic condition." It 

 is a definite intersection as clearly shown in the earlier paper 

 referred to. 



In conclusion it is desirable to point out the following facts: 

 (1) A gray body is by definition one for which at a true tempera- 

 ture § the ratio of the intensity of its emission to that of a black 

 body at temperature §, at any given monochromatic wave 

 length, is independent of the wave length chosen. (2) There 

 are two types of log isochromatic curves, one referring to ratio 

 of intensities, the other to the actual values of intensity. The 

 former type of curve is the one of interest in the present discus- 

 sion. (3) The intersection of log isochromatics for the radia- 

 tion from a non-black body spectrophotometrically compared 

 with that from a black body is not a criterion for grayness of 

 the non-black body. (4) The temperature at which intersec- 

 tion occurs is not the true temperature of the non-black body, 



