( 300 ) 



Is it possible to derive from these numbers, even though only 

 approximately, values for the radiating power per unit of surface? 

 The two influences mentioned above which determine the colours of 

 the self-luminous celestial bodies may be subjected to a calculation, 

 if we disregard the specific properties of the composing substances 

 and treat them as abstract theoretical cases. In other words we can 

 investigate the radiation of a perfectly black body and in the absorp- 

 tion neglect the selective absorption in lines and bands in order only 

 to examine the general absorption. As a first approximation this may 

 be deemed sufficient. 



In this calculation we have made use of the measurements of 

 A. KöNiG on the relative quantities of the elemental colours red, 

 green and blue as functions of the wavelength in white sunlight. Ii 

 for an other source of light we know the relation of the brightness 

 with regard to the former source as a function of the wavelength, we 

 can calculate the quantities of the red, green and blue in this second 

 source of light. If we call the numbers of König R (A), G {X), B {X), 

 which are chosen so that 



r72(A)fa= 1000 I G{X)dX=l()0^ i B(k)dX—lOO(i 



and a f{X) represents the brightness of another source of light, then 



^f{X) R{X) dX Cf{X) G{X) dX and Cf{X) B{X) dX 



represent the quantities of R, G, and B occurring in this light. As 

 the impression of brightness of a source of light is almost proportional 

 to the quantity of red, this calculation gives at the same time a 

 measure for the optical brightness. 



The radiation of a black body may be represented by : 



c 



X e dX 

 where T is the absolute temperature and « and c constants. For two 

 sources of light of different temperatures the relation of the inten- 

 sities is : 



c/J_ 1_\ 6 6' 



//n ~^^^ ^"^ ^ 1ft ^ 

 f{X) = e zz:<? = 10 , 



if h =z c( I and b' = 0.43 b. As unit for X we adopt 0,00 J mm; 



jf'o is supposed to be given, then b' is a function of the variable 

 tem})erature T only and may be called the degree of glowing with 

 regard to tlie glowing of a body at a temperature 7\. If we adopt 



