VISIBLE AND NEAR-VISIBLE RADIATION 135 



Table 6. — Terms Relating to a Substance in Homogeneous Solution in a 

 Solvent Contained in a Cell with Plane, Parallel Sides Perpendicular 

 to the Direction of Propagation, the Propagation through the Cell 

 and Solution Being Rectilinear (56, cf. page 177) 



Let 



TboI = transmission of a given cell containing the solution. 



Tgov = transmission of the same (or a duplicate) cell containing pure solvent. 

 Then* 



T = T^oi/Tsov = Tsoi/Tsov = transmittancy 



t = \/T = specific transmissivityt 



where c = concentration of the solution 

 and b = thickness of the solution 

 i = — loge t = specific transmissive exponent. 

 k = — logiot = specific transmissive index. | 



* The symbols T, t, i, and k are distinguished from the terms T, t, i, and k, of Table 5, in hand- 

 writing and typewriting by using the underscore, and in printing by the use of bold face type, 

 t This relation is known as Beer's law. In many cases it is only approximate. 

 X This term has been commonly designated as extinction coefficient. 



THE BLACK BODY 



Throughout the discussion so far, we have tacitly assumed a basis 

 for determining the energy associated with a given radiation. The 

 actual method by w'hich we arrive at such a specification is directly 

 dependent upon our concept of a black body. The black body has been 

 in fact the key to the development of our understanding of radiation. 

 Since we measure temperature on an absolute thermodynamic basis, 

 we may relate our observations of the energy absorbed by a body to its 

 rise in temperature. Since the black body absorbs totally all the radi- 

 ation falling upon it, it becomes our logical starting point. Knowing 

 the heat capacity of a black body, it may be placed in a beam of radi- 

 ation and the rise of temperature observed for a given length of time. 

 Thus the rate of rise in temperature must be related to the rate of supply 

 of energy. Investigation of the radiation emitted by black bodies of 

 different temperatures in this way showed that the amount of energy 

 emitted per unit time was greater for greater temperatures. In fact, 

 the radiation was found to increase as a high power of the temperature. 

 Since, however, this disclosure indicates that bodies all radiate and that 

 the radiation depends upon the temperature, the black body which we 

 have chosen as a detector must itself be regarded as a radiator. Thus, 

 our measurement becomes one dependent upon net gain or loss, this 

 exchange depending upon the temperature of the source, the temperature 

 of the detector, and the temperature of the surroundings. Thus, Stefan 

 observed that the total radiation emitted by a black body is given by the 

 expression 



