354 GREAT MEN OF SCIENCE 



law states that the most intense wave-length radiated by a 

 completely black body is inversely proportional to its abso- 

 lute temperature. 



There now only remained the question of how the longer 

 and shorter waves always present along with the most in- 

 tense radiation are distributed over the spectrum; in other 

 words, the question of the exact distribution of the energy 

 of radiation of a black body at any temperature over the 

 whole spectrum. The answer to this question could not be 

 given by the then existing knowledge and theory. In par- 

 ticular, new observations were necessary; the whole distribu- 

 tion of energy in the spectrum had to be examined in detail 

 by means of the thermopile or similar apparatus, before 

 further conclusions could be drawn. Here it was a matter 

 of finding as a radiating body a trustworthy completely black 

 body, and not only such a substance as soot or the like. 



But Kirchhoff had already shown how to make such a 

 body; it was necessary to use a hollow space having a small 

 aperture; the aperture would behave like the surface of a 

 completely black body. For if a ray of light of any wave- 

 length falls from outside upon the hole, it enters the cavity, 

 and if the internal walls of this are anything but perfectly 

 reflecting, which can be easily arranged, at the worst only a 

 very small part of the energy received escapes again from the 

 hole, and this part can be still further diminished by making 

 the hole smaller or by enlarging the hollow space. This 

 blackness of an aperture opening into a hollow space can be 

 seen at any cellar or in the pupil of the eye. But if the aper- 

 ture behaves like a completely black body as regards ab- 

 sorption, it must do the same, according to Kirchhoff's law, 

 also for emission. If, therefore, the hollow space is heated 

 to a measured temperature, the aperture will always radiate 

 exactly like a perfectly black body. This radiation can then 

 be decomposed by the spectroscope, and its energy dis- 

 tribution over the spectrum measured. 



