﻿1168 The Measurement of Light. 



The function of a system of definitions is, in the writer's 

 opinion, so to describe a number of different quantities and 

 their relations to one another that a previous understanding 

 of any one of these quantities (regarded as the fundamental), 

 together with the definitions, enables all the remaining 

 quantities to be understood also. If this be granted, the 

 fundamental quantity should be that most generally 

 understood. The sequence of the remaining quantities is then 

 a matter of convenience. 



In conclusion, it may not be out of place to point out 

 that Dr. .Campbell and Mr. Dudding appear not to have 

 understood the "lambert" as a unit of brightness. It is 

 unfortunate that they were apparently only aware of the 

 brief statement in the Keport of the Standards Committee of 

 the Optical Society of America* that "a Lambert is the 

 brightness of a surface emitting one lumen per square 

 centimetre of projected area in the direction considered." 

 They naturally remark that " To speak of a lumen emitted 

 in a direction is to talk nonsense." 



Although, of course, this description of the lambert is quite 

 unsound, it is unfortunate, to say the least, that the authors 

 did not refer to the official definition of the lambert (by no 

 means a new unit). They would then have realized that 

 their difficulty arose, not from a miscalculation on the part 

 of the American Committee as they seem to infer, but from 

 an inaccuracy in the wording. " A lambert is the brightness 

 of a perfectly diffusing surface (i.e. one obeying the cosine 

 law of emission) emitting or reflecting one lumen per square 

 centimetre 5 ' f. 



Summary. 



(1) The fundamental photometric magnitude from the 

 point of view of visual measurement is brightness and not 

 illumination. 



(2) The photometric unit is one of luminous intensity 

 (candle-power) or luminous flux. 



(3) Of the two possible systems of definitions based on 

 these respective magnitudes, that based on the magnitudes 

 in which the unit is maintained seems preferable because 

 it follows the natural order of mental conception. 



(4) The relation between the flux unit of brightness (the 

 lambert) and the intensity unit (the candle per square 

 centimetre) is pointed out. 



* Opt. Soc. Am. J. iv. p. 230 (1920). 



t Report of Standards Committee of the Ilium. Eng. Soc. N, Y. 

 Trans, xiii. (1918). 





