﻿[ 1165 ] 



CX. The Measurement of Light. By. John W. T.Walsh, 

 M.A.yM.Sc, (Department of Photometry and Illumination, 

 National Physical Laboratory) *. 



IN an article published nuclei the above title in a recent 

 number of this Magazine t, Dr. Norman Campbell and 



Mr. 13. P. Dudding have criticised the logical foundation of 

 commonly accepted systems of definitions and in particular 

 the system adopted for the definitions of the principal 

 photometric magnitudes. Their conclusions may, perhaps, 

 be briefly summarized as follows : — 



(1) The fundamental photometric magnitude is illumina- 



tion (I). 



(2) Illumination is characteristic, not of the surface 



illuminated, but of the circumstances in which that 

 surface is placed. 



(3) It can be shown experimentally that illumination is 



subject to the laws of addition if the conditions under 

 which the Purkinje effect operates he excluded. 



(4) On the a hove bases the inverse square law can be 



demonstrated by suitable experiment, and, this 

 done, the cosine law of illumination can be demon- 

 strated . 



(5) With the proven fact of the inverse square law 



the definition of luminous intensity [candle-power] 

 in a given direction follows at once from that of 

 illumination (<& = Ir 2 ) 



(6) By integrating the intensity in all directions a new 



quantity F — \<&d<0 is obtained (co is solid angle). 

 F is characteristic of the source alone, and is termed 

 the flux of light from the source. 



(7) Brightness, B, is defined as the intensity of unit 



projected area (0/Scos«)/(a), or normal brightness 



is <J>/S where S is area. 

 Thus the system proposed by the authors gives the chief 

 photometric definitions in the order (1) illumination, 

 (2) luminous intensity, (3) luminous flux, and (4) brightness. 

 This order differs from that adopted by both the American 

 and the British National Illumination Committees in their 

 systems of definitions proposed before the International 

 Commission on Illumination last year J. It is, therefore, 

 desirable to examine the reason for the difference with the 

 object of arriving, if possible, at the most natural order. 



* Communicated bv the Author. 

 t Phil. Mag. Sept. 1922, p. 577. 



X Commission Internationale de FJEclairage, 5me Session, Paris 1921. 

 Rapports (in the press). 



