304 
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. 
B 
Kies 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 envelope © 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 GC 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 
