

t 845 ] 



XC. Studies in the Photometry of Lights of Different Colours. — 

 IV. The Addition of Luminosities of Different Colour, By 

 Herbert E. Iyes *. 



A WELL-NIGH indispensable requirement o£ a method 

 o£ measurement is that its renderings shall conform to 

 two geometrical axioms. The first of these is that things 

 equal to the same thing shall be equal to each other ; the 

 second, that the whole shall be equal to the sum of its parts. 



In the majority of measurements performed in the physical 

 laboratory the quantities dealt with are of such nature that 

 conformity to these axioms is unquestioned. Among these 

 measurements fall those of illuminations of identical colour 

 and quality. But when the condition of complete identity 

 of quality in the compared quantities is departed from, as is 

 the case in the photometry of lights of different colour, it is 

 unsafe to assume that these axiomatic relations hold. This 

 paper discusses the methods of heterochromatic photometry 

 from the standpoint of these axioms. It is shown that only 

 one method — the flicker method — strictly conforms to the 

 requirements in question. 



As to the first requirement, that things measuring equal 

 to the same thing shall measure equal to each other, the 

 subject matter of the previous papers of this series gives 

 complete information. By the methods of visual acuity and 

 critical frequency this is so, from the nature of the measuring- 

 process, in which no real comparison source — whose character 

 might be varied — exists. In the two remaining methods — 

 equality of brightness and flicker — the fulfilment of this 

 requirement reduces to this — that no change in the character 

 of the reference standard shall affect the relative brightness 

 of the different coloured lights under measurement. In the 

 case of spectral luminosity curves this means that the shape 

 of the curve must not be altered by a change in the reference 

 standard. In the last preceding paper of this series it was 

 shown that by the equality of brightness method various 

 distortions of the spectral luminosity curves may be expected 

 upon changing the hue of the reference standard. By the 

 flicker method such changes do not take place. Therefore 

 the first requirement mentioned above is met by the flicker 

 method, but not by the equality of brightness method. Two 

 different equality of brightness methods must, however, be 

 distinguished, the ordinary one, where large hue differences 

 are under comparison, and another, where by steps of small 



* Communicated by the Author. 

 Phil Mag. S. t>. Vol. 24. No. 144. Dec. 1912. 3 K 



