High Temperatures by the Method of Colour Identity. 37 



proportions of the energy in the red, green, and blue regions, 

 and any other radiator emitting light in the same relative 

 proportions will have the same hue of radiation, no matter 

 what the absolute intensity of the radiations. Thus it is that 

 tho radiation from a grey body will be identical in hue with 

 that of a black body, and compared on the colour-identity 

 basis the grey body will be given its true temperature. The 

 optical pyrometer, on the other hand, only takes account of 

 the relative intensities of the light from the black and grey 

 bodies, and therefore estimates the temperature of the grey 

 body at a value far below its true temperature. It follows, 

 therefore, that the measure of the accuracy of the colour- 

 identity method is the extent to which bodies radiate as grey 

 (or black) bodies throughout the visible spectrum. 



Throughout this paper the usual conception of a grey body 

 is adopted — i. p., one which, at any temperature, does not 

 radiate as much energy in the various wave-lengths as a 

 black body at the same temperature, but in any wave-length 

 the intensity per unit area of the surface is a constant fraction 

 of that of the black body in the same wave-length. 



By a selective body is meant one in which the amounts of 

 energy radiated in the various wave-lengths throughout the 

 whole spectrum do not bear a constant proportion to those 

 in the same wave-lengths for a black body at the same 

 temperature. 



Section 1 of this note deals with the establishment of 

 electric sub-standards of colour, which are intended to serve 

 for defining the colour of the radiation from any incandescent 

 bodies compared against them, and so to fix the temperature 

 of such bodies in terms of the temperature of a black body 

 whose radiation is identical with theirs in colour. 



Section 2 gives the determination of " colour identity " 

 temperatures of carbon and tungsten glow-lamps when 

 burning at different efficiencies, and contains expressions for 

 such efficiencies in terms of temperature based on Wien's 

 equation for intensity of energy distribution and Nutting's 

 equation for the sensitivity of the human eye. 



Section 3 discusses the accuracy of such determinations, 

 and deals with the " colour identity " temperature of platinum 

 at the melting-point, showing that even for a selective 

 radiator such as platinum this temperature is a measure of 

 the true temperature of the platinum filament, although it is 

 glowing under open radiation conditions. Filament tempe- 

 ratures for carbon and tungsten (vacuum and gas-filled) are 

 also discussed. 



