Unit of Candle-power in White Light. 65 



terms of which differences in the hue of light may be 

 expressed. In this paper spectral colours are not being- 

 dealt with, but only so-called white light — that is to say, 

 light whose spectral distribution is continuous and not very 

 different from that radiated by a black or grey body. The 

 most common way of defining the hue of white Kght is to 

 state the specific consumption in watts per candle of an 

 incandescent lamp, the hue of whose radiation is identical 

 with it. This value of specific consumption, however, will 

 not be the same for a carbon as for a tungsten lamp, both of 

 whose radiations have the same hue. A reason for the 

 difference is the tendency for the tungsten lamp to radiate 

 selectively in favour of those wave-lengths lying within the 

 visible spectrum. It is necessary, therefore, if this basis of 

 comparison is used, to say if the watts per candle are on the 

 carbon or tungsten basis, and hence it will be seen that 

 although the values of watts per candle of incandescent 

 lamps are convenient as a general guide to colour, this basis 

 of measurement is arbitrary, and not very satisfactory for 

 precise definition. 



A series of comparisons was therefore carried out in order 

 to express the hue of radiation from electric glow-lamps in 

 terms of the temperature of a black body whoso radiation 

 was identical in colour with that of the glow-lamps in 

 question. It has long been recognized that such colour 

 comparisons can be made very accurately with a Lummer- 

 Brodhun photometer *. The methods used are those de- 

 scribed in another paper by the authors t, which should be 

 referred to for further details. 



In fig. 1 are reproduced curves from this paper in which 

 the ordinates are temperatures and lumens per watt abscissae. 

 Curve A shows the lumens per watt of a carbon lamp corre- 

 sponding with the temperature of a black body whose radiation 

 is identical in colour with that from the lamp. Curve B gives 

 the same relationship for tungsten extrapolated according to 

 the equation found to express this relationship, the extra- 

 polated portion of the curve being shown by a dotted line. 

 Temperatures are on the optical scale, using a dominant 

 wave-length of \ = 0'650/a giving the melting-point of 

 platinum as about 1750° C. 



From this diagram (line 1) it will be seen that a carbon 

 lamp, the hue of whose radiation matches that from the 



* Hyde, Cady, and Middlekauft', " Selective Emission of Incandescent 

 Lamps," 111. Eng. Soc, New York, vol. iv. (1909). 



t Paterson and Dudding, " The Estimation of High Temperatures by 

 the Method of Colour Identitv,"sw/)r« p. 34. 



Phil. Mag. S. 6. Vol. h. No. 175. July 1915. F 



