JOSEF STEFAN AND LUDWIG BOLTZMANN 351 



furnished a possibility, that in further investigating the 

 pecuHarities of emission, one might become independent of 

 the particular qualities of the hot bodies emitting the radia- 

 tion, and so be able to investigate the influence of tempera- 

 ture, for itself alone. The point was to determine this 

 influence in the case of a completely 'black' body, that is to 

 say, one which completely absorbed radiation of all wave- 

 lengths. Any body which does this has the maximum 

 possible power of absorption, and in this respect all black 

 bodies are alike, no matter of what they consist; for none 

 can absorb more than everything. But in this case, Kirch- 

 hoff's law tells us that all black bodies must be alike as 

 regards radiation also, and must also be possessed of the 

 maximum possible radiating power, which will now no 

 longer depend upon anything but the temperature. The 

 emission of a completely black body must be investigated 

 as regards its dependence upon the temperature, in order 

 to find out the essential features of this dependence. 



If we are to measure the whole radiation emitted by a 

 body, the infra-red always present must not be overlooked; 

 we cannot therefore make use of the eye; it is necessary to 

 allow the radiation proceeding from the body to fall upon a 

 black and sufficiently sensitive thermometer, such for 

 example as a blackened thermopile, and to measure the 

 corresponding rise in temperature; this gives us the correct 

 measure of the total radiation, since this is completely ab- 

 sorbed by the black surface of the measuring instrument, the 

 whole of its energy being turned into heat, and thus into a 

 measurable form. 



Measurements of this kind had already been published in 

 Stefan's time (1879) from various sides. Stefan collected 

 them all and made a critical comparison of them, paying 

 particular attention to a possible effect of the heat con- 

 ductivity of the air, which had in part been overlooked. 

 Good measurements, for example by Tyndall, existed, in 



