﻿Visibility of Radiation. 307 



Now the spectral energy of a normal radiator at a tempe- 

 rature T is well represented in the visible spectrum by the 

 Wien-Paschen function 



E A = CA-'^- c ^ T (2) 



Hence the light emitted by such a radiator will be given by 

 the integral of EVdA. from to x. Call this integral L, 

 then * 



L=A(J+l), (3) 



in which 



A = C 1 V m \lT{n + a-r)(a\ m y il - a+ \ and B = (U/a\ m . 



L has the maximum value 



m \n-\-a — l) 

 at a temperature 



rn v 9 



(n — l)X m 



or about 6530 if we take n=5, a = 181, and C 2 = 14500. 



The remaining visibility constant Y m must be determined 

 experimentally. It is the ratio of the candle (or lumen as 

 preferred) to the watt at the wave-length of maximum visi- 

 bility. The simpler method is to measure in metre candles 

 as light and in watts as energy some given monochromatic 

 illumination, preferably of a wave-length near that of 

 maximum visibility. The first determinations of Vm were 

 made by this method by Dr. Drysdalef and the writer f 

 seven years ago. We obtained 16*7 and 13 5 cand./watt 

 respectively, values of the right order of magnitude but 

 much too low on account of stray radiation. More recently, 

 Fabry and Buisson§ have made a determination by this 

 method, and obtained the value 55 cand./watt using the 

 green mercury line 5161 from a powerful mercury arc. 



The other method for determining Y m is indirect but less 

 subject to large systematic errors, and it gives, under certain 

 conditions, a direct relation between the international candle 

 and the watt. A source of light is used having a continuous 

 spectrum and whose spectral energy distribution is known. 

 With radiometer and photometer, the radiation at a given 



* P. G. Nutting, B. S. Bull. v. p. 305 (1908) ; vi. p. 337 (1009) : 

 'Applied Optic*,' p. 158 (1912). 



* C. V. Drysdale, Tree. Roy. Soc. lxxx. p. 10 (1907), 



t P. G. Nutting-, Elec. World, June 215. 1908. 



§ Fabry and Buisson, Compt. Rend, cliii. p. 254 (1911). 



X2 



