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a black body, but not black because its true temperature is slightly highet 

 than a black body. 



A uniformly heated inclosure is the nearest approximation to our ideal 

 black body. 



Fig. 2. 



Consider a body A within a heated inclosure B, Fig. 2, both at the 

 same temperature throughout. A receives a certain amount of thermal 

 radiation from the wall of the enveloj^e C and radiates to C an equal 

 amount if they are in temperature equilibrium. Also A radiates a certain 

 amount to D and receives the same amount, if D is at the same tempera- 

 ture. Since A, on the whole, neither gains nor loses it radiates to D the 

 same amount it receives from C, consequently the radiation from A towards 

 D is the same as that from C towards D. Not only is the quantity the 

 same but also the quality, for the coefficient of absorption depends upon 

 the quality (i. e., it is different for different parts of the spectrum), so 

 that if C and A radiate the same amount they must radiate the same qual- 

 ity. If the spectral distribution of A were different from C its coeffi- 

 cient of absorption would be different and therefore it would not radiate 

 the same quantity. Hence any other body within B and at same tem- 

 perature would radiate the same as A so that no detail could be detected, 

 i. e., the objects could not be distinguished from one another or the walls 

 of the inclosure. 



Moreover any body outside of B at the same true temperature could 

 not radiate more energy than A, consequently, A is a complete radiator 

 or a perfectly black body when within B, and it also follows that the 

 Interior of B radiates as a perfectly black body. A piece of polished 



