﻿588 Dr. N. Campbell and Mr. B. P. Dudding on 



in some direction, specified b} r its relation to lines charac- 

 teristic of that system, is 1. {Intensity of standard candle, 

 or 1 candle-potver.) 



We choose the centimetre as unit of distance, square 

 centimetre of area, and steradian of solid angle. We have 

 now to choose the formal constants a, ft, y in the laws 



\$>d<0. 



F=/3$c 



B = 



y& COS OL 



We have hitherto treated these constants as all being 

 unity, but there is no reason, if we see fit, why we should 

 not assign to them other values; and the choice of unity is as 

 arbitrary as any other. As a matter of fact, a, ft, <y are all 

 usually chosen to be 1, and there is no objection to that 

 practice. The unit of illumination (phot) is then that of a 

 P.S. distant 1 cm, from the (point) standard candle in the 

 prescribed direction ; the unit of flux (lumen) that emitted 

 by the standard candle within unit solid angle surrounding 

 the prescribed direction, if the condition is fulfilled that the 

 intensity is the same within all parts of that solid angle. 

 The unit of brightness [lamhert] is that of a perfectly matt 

 surface which emits light of unit intensity when its area 

 projected on the plane normal to the direction of viewing is 

 one square centimetre. It is to be noted that these derived 

 units (unlike the fundamental unit, which is not that of the 

 fundamental magnitude) cannot all be actually realized. 

 But such a feature of derived units is quite usual ; e. g., unit 

 Young's modulus cannot be realized. 



12. It may possibly be useful to compare our nomencla- 

 ture and notation with that adopted by some official body. 

 We choose for this purpose that of the Standards Committee 

 of the Optical Society of America, described by its Chair- 

 man, P. E,. Nutting * : — 



Our Notation. American. 



Illumination ... I Flux density ... D 



Intensity <I> Intensity C 



Flux F Flux F 



Brightness B Brightness B 



Of course, notation is a matter of mere convenience ; but 

 we have departed from the American scheme because we 



* Journ. of the Opt. Soc. of America, iv. p. 230, July 1920. 



