36 Messrs. Paterson and Dudding on Estimation of 



apparently selective in favour of the visible spectrum as a 

 ivhole, emit light throughout that spectrum without any 

 appreciable deviation from the distribution to be found in 

 the visible spectrum of a black body*. For instance, con- 

 sider a tungsten filament adjusted to a suitable temperature, 

 and compared spectrophotometrically against a carbon fila- 

 ment. The one is mainly selective in favour of the visible 

 spectrum as a whole, and the other acts in this respect as a 

 black body. The spectrophotometer, dealing only with the 

 visible spectrum, cannot detect any relative difference between 

 the two at different wave-lengths throughout the portion of 

 the spectrum with which it deals, and thus a comparison 

 of the total visible radiation is possible with an ordinary 

 photometer, exact identity of colour being obtainable. That 

 is to say, these substances virtually radiate as " grey" bodies, 

 so far at least as the visible spectrum is concerned, and it is 

 this close approximation to grey body radiation in the visible 

 spectrum which lies at the root of the method discussed in 

 this paper. Hence, identity of colour can be obtained not 

 only when comparing one tungsten lamp against another, 

 but also when comparing a tungsten lamp against a black 

 body. If the temperature of these bodies is pushed to an 

 extreme value a very slight difference of colour is perceptible 

 at the point where the colour balance is closest, but such 

 differences are too small to prevent an observer obtaining* 

 consistent results in judging the colour balance between two 

 radiations. 



A comparison of colour is made similarly to photometric 

 comparisons of intensity. The current in the comparison 

 lamp is varied so that the colour of the light fluctuates on 

 both sides of the mean, first inclining to be redder and then 

 to be bluer than the light from the test source. The current 

 in the lamp is then readily determined at which the observer 

 judges the colour balance to occur. It must be remembered 

 that in these comparisons it is the hue of so-called white 

 light which is under consideration, and not that of spectral 

 or other colours. 



The colour-identity method depends on the combined 

 effects of the light emitted in all wave-lengths in the visible 

 region. If the intensity is relatively greater at the red end 

 than at the blue end, the hue of the resulting radiation will 

 tend to be red, and vice versa. The radiation from a black 

 body at 1750° C. has a definite hue depending on the relative 



* Coblentz, "Kadiation Constants of Metals," Bull. B. S. vol. v. p. 359. 

 Hyde, " Selective Emission of Incandescent Lamps," Trans. Ill Eno- 

 Soc. 1909. 



