574 



foote: gray radiation 



confusion of terminology. Lummer and Pringsheim, 4 Benedict, 6 

 and Foote and Fairchild 6 have been considering always the ratio 

 of intensities at two temperatures, while Dr. Nutting uses the 

 intensity at a single temperature. It is unfortunate that the 

 term "log isochromatics" has been applied to both types of 

 curves for fifteen years. If one carries through Dr. Nutting's 

 derivation upon the same basis as that used by the other writers, 



Fig. 1. Plot of log isochromatics for a black body- 

 it is clear that the log isochromatics for black bodies and for a 

 black body compared with a gray body always satisfy the "stig- 

 matic condition" over the entire range of wave lengths from 

 X = to X = °° . Furthermore, the condition of grayness of 

 one or both of the radiators necessitates the intersection of the 

 log isochromatics. On the contrary the intersection of the log 

 isochromatics does not prove that the material compared is 

 gray. All of these points were brought out in the earlier paper 

 referred to. 7 



4 Lummer and Pringsheim. Verh. d. D. Phys. Ges., 3: 36. 1901. 

 6 Benedict. Ann. d. Phys., 47: 641. 1915. 

 6 Foote and Fairchild. Loc. cit. 

 3 Foote and Fairchild. Loc. cit. 



