COBLENTZ: CONSTANTS OF SPECTRAL RADIATION 11 



tion, introducing all the known correction factors which can 

 effect the observed spectral energy curves. These factors are 

 (1) corrections for the selective reflection of the silver mirrors, 

 the fluorite prism, and the fluorite window which covers the 

 vacuum bolometer; and (2) the corrections for the variation in 

 reflecting power with angle of incidence upon the silver mirrors 

 and upon the fluorite prism. 



In the isothermal spectral energy curves the position, X^a^,, of 

 the maximum emission, E^^xj is computed by taking the wave- 

 lengths, Xi and Xo corresponding to equal emissivities Ei = Ei, 

 on the assumption that the observed energy curve fits the Planck 

 equation: 



£'x = C,X-«(e-'^^-l)-^ (1) 



from which follows 



(2) 



^ ^ a(l ogX2-logX:)XiX2 _ XiX2[log(l-e-'-^A^^)-log(l-e---A-^)] 

 a^ (X2 — Xi) log e a^ (X2 — Xi) log e 



The second term in this equation can usually be abbreviated 

 since terms involving Xi are usually negligible. For values of 

 Xo which are less than about 4/x the term log (1 -e~''^^''^) may be 

 expanded into a series and (by dropping all terms but the first) 

 may be used in the form — e"'''/^'^ log e. 



In this equation a = 5, a^ = 4.9651 and Co = a^'KjT. For 

 computing the second term correction factors to X,„, the value of 

 C2 = 14,500 was used. However a variation of 100 units in 

 C2 (e.g., C2 = 14,600) would change the mean value of Xy^ax by 

 only 0.0005 fx, v/hich is negligible. 



For computing the constant Co from an iso hromatic energy 

 curve, at any wave-length, X, Planck's equation is used in the 

 following form : 



(log^2-log^i)xrir2 _ (e-'-^A^' - e-'A^O X^i ^2 .3. 

 "^^ log e {T, - TO T, - Ti 



where Ex and E^ refer to the emissivities corresponding to the 

 temperatures Ti and Ti respectively. In this equation the terms 

 log (1— e~"Ari^ g^g^^ were expanded into a series and only the 



