High Temperatures by the Method of Colour Identity. 49 



E x being the radiant power of wave-length X at temperature 

 T, C^ and (J 2 being constants, n = 5 (for a black body). 



Seeing that in the experiments described in the following- 

 section (3), in which the colour of the radiation of melting 

 platinum was found to be identical with that of a black body 

 operating at the same temperature, it is reasonable to assume 

 that the power distribution, at least over the range of the 

 visible spectrum, can be expressed by a formula of the above 

 form. Lummer and Pringsheim found this condition to be 

 closely fulfilled by radiators having the characteristics of 

 platinum. 



When the visible spectrum only is under consideration, the 

 values of Ci, C 2 , and n can vary considerably without affecting 

 the shape of the curve by an amount corresponding to a 

 difference of temperature of 10° C. in the region of 2000° C. 



(c) Sensitivity of the Eye to Energy of Different Wave-lengths. 

 — By examining a large number of persons, Nutting* has 

 obtained data connecting the wave-lengths of radiant energy 

 and the luminous sensation produced per unit of power in 

 that wave-length. 



He expresses his results in the form 



= Y "'{\) e ' • • • • (3) 



where V,„ is the photometric value of a unit of power in the 

 wave-length of maximum sensitivity \„„ u is a constant, e is 

 the base of Napierian logarithms. 



For the luminous intensities ordinarily employed in photo- 

 metry he <>ives « = 181 and X„ t =0'55/x. 



Combining the expression for power distribution and the 

 sensitivity curve for the eye (equations 2 and 3), we obtain 

 for the photometric value of radiant power of wave-length 

 A,, E A V A , and for the photometric value of the whole of the 

 radiant power 



Pe a V^ (4) 



Jo 



If the power distribution can be represented by 



E, = F\- n e~^, (5) 



* Bulletin of the Bureau of Standards, vol. v. p. 2G1, and vol. vi. 

 p. 337. 



Phil. Mag. S. 6. Vol. 30. No. 175. July 1915. E 



