﻿the Measurement of Light. 57 ( J 



appear to us exceptionally unfortunate. Thus the inverse- 

 square law, upon which most practical photometry depends, 

 is treated very inadequately. The units cannot possibly be 

 defined without reference to the law. It is usually implied, 

 and not openly stated, as we hold it should be: and state- 

 ment by implication is peculiarly objectionable here, because 

 the law is only true in very special conditions which cannot 

 be sufficiently described. But it is more serious that it is 

 treated as a primary and necessary law for the measurement 

 of all photometric magnitudes, so that the only form in 

 which it can be stated with full precision is that photometric 

 measurement is possible> This view seems to us utterly 

 mistaken ; one photometric magnitude at least can be defined 

 without reference to it; and when that magnitude is de- 

 fined the proposition can be stated as an independent law 

 concerning it, and can be subjected to direct experimental 

 proof. Closely connected with this defect is the total 

 absence of any clear statement of the meaning of the addition 

 of illumination, although in the practical use of light that 

 law is much more important than the inverse-square law. It 

 is more important to know that we can double generally the 

 illumination by doubling the number of lamps than that we 

 can multiply it four times by halving their distance. Lastly, 

 the significance of the magnitudes is made to depend wholly 

 upon a theory of illumination — the theory which may be 

 baldly stated in the terms that illumination is due to the 

 incidence of something called light. That theory is wholly 

 unnecessary ; and since, like all theories, it might be doubted, 

 it should be studiously avoided and all definitions framed in 

 terms of experimental concepts only. 



With these preliminary remarks we will proceed to busi- 

 ness. Perhaps the reader should be warned that what follows 

 is not put forward as a model of an actual formulation, but 

 merely as a sketch of the principles to be followed. The 

 actual formulation would require much greater verbal pre- 

 cision and much less explanation of reasons for the scheme 

 adopted. 



The nomenclature of concepts concerned with measure- 

 ment adopted here is that employed by one of us in 'Physics/ 

 Part II. 



3. Light-measurements are based on judgments of equality 

 of brightness of photometric surfaces. These judgments 

 are made by the direct perception of normal persons — 

 normality in this matter meaning simply agreement with 

 the great majority of mankind. 



2 P 2 



