Febetjaey 4, 1910] 



SCIENCE 



169 



for such substances as porcelain, magnesia 

 and glass, as shown by Coblentz's curves 

 (Fig. 2). 



The curves of wave-length and radiant 

 energy which are shown are, with slight 



modifications, taken from work of Lummer 

 and Pringsheim and of Dr. Coblentz. The 



curve for the ideal, or black body radiator, 

 gives a picture of the total energy and 

 its distribution over the different wave- 



lengths. It is the peculiarity of the black 

 body to radiate more energy of any given 

 wave-length than does any other body at 

 the same temperature. Therefore, in case 

 of all substances acting as thermal radi- 

 ators, the black body will always give the 

 greatest brilliancy. Since this body at the 

 same time radiates a maximum in all wave- 

 lengths, it will be surpassed in light effi- 



ciency by any substance which is a rela- 

 tively poor radiator in the invisible or non- 

 luminous part of the spectrum. 



In the energy curves shown it is to be 

 noticed that the visible part of the energy 

 is practically only that between 0.4 and 0.8 

 thousandths of a millimeter. Consider the 

 black lines in Fig. 3 for a moment. These 

 show the emission of a black body at centi- 

 grade temperatures noted on the curves. 

 Evidently the energy emitted rises very 

 rapidly with the temperature; i. e., as the 

 fourth power of the absolute temperature. 

 It will be noted also that the point of maxi- 

 mum energy or wave-length corresponding 

 to maximum energy shifts gradually to- 



