Photometry of Lights of Different Colours. 853 



photometry and give values for the luminosity curve of the 

 average eye. 



Attention may here be called to the fact that the solution 

 of the problem of heterochromatic photometry clears the 

 way for the establishment of a rational primary standard of 

 light flux, namely, a certain rate of flow of radiant energy 

 of maximum luminous efficiency*. It is hoped to use the 

 results of this study of coloured light photometry in a 

 determination of the luminous equivalent of the most efficient 

 radiation, a quantity which, in accordance with the above 

 suggestion, is identified with the rational primary standard 

 of light. 



Physical Laboratory, 

 National Electric Lamp Association, 

 Cleveland, Ohio. 

 June 1912. 



XCI. Studies in the Photometry of Lights of Different Colours. 

 — V. The Spectral Luminosity Curve of the Average Eye. 

 By Herbert E. Ives f. 



ACCORDING- to the results published in previous papers 

 of this series the flicker photometer, under certain 

 conditions, conforms most nearly of all photometric methods 

 to the requirements for a method of heterochromatic photo- 

 metry. Granting its adoption as the standard instrument 

 for this kind of photometry, there remain to be determined 

 the characteristics of the average human eye. Given the 

 satisfactory photometric method, and the standard eye to 

 use it, the measurement of the relative brightness of dif- 

 ferently coloured lights becomes a definite thing. 



The present paper gives the result of an experimental 

 determination of the mean luminosity curve of eighteen 

 observers, sixteen men and two women, aged from 18 to 40 

 3 r ears. Since none of these were colour-blind or possessed 

 any known abnormalities of vision, the mean of their values 

 may be considered as that of an average normal eye. The 

 number of observers is sufficiently great to warrant the 

 belief that the mean thus obtained is sufficiently near the 

 absolute mean eye for all practical purposes. 



The conditions holding during the measurements, the 

 reason for their choice, the details of the work, together 

 with the results, are discussed below under the several 

 headings. 



* il Energy Standards of Luminous Intensity," H. E . Ives, Trans. 111. 

 Eno. Soc. March 1911 ; " Luminous Efficiencv," Elec. World, June 15, 

 1911. 



-j- Communicated by the Author. 



