Photometry of Lights of Different Colours. 153 



The Requirements of a Good Method of Measurement. 



As a preliminary to an investigation of these methods of 

 measurement, the requirements necessary to a satisfactory 

 method should be kept clearly in mind. These are, first, that 

 the range of possible settings of the measuring instrument shall 

 be small ; that is, the method shall be as definite as possible. 

 In the photometry of lights of the same colour, for instance, 

 the range with a Lummer-Brodhun contrast photometer is 

 of the order of one per cent. Second, that the quantities to 

 which values are given by the measurements shall behave 

 as arithmetical or physical quantities. This involves that 

 quantities measured as equal to the same thing shall measure 

 equal to each other, and that the. sum of the parts shall be 

 equal to the whole. This second requirement of a method 

 of measurement is not one often dwelt upon in the measurement 

 of purely physical quantities, because such quantities naturally 

 fulfil the requirement. In dealing with physiological quan- 

 tities, however, it cannot be assumed without proof that this 

 is true. As a third requirement in the present case may be 

 added that the values given to different colours shall represent 

 as nearly as possible their useful value as light. In explanation 

 of this last requirement it will suffice to point out that the 

 first two requirements, which pertain to any method of mea- 

 surement, would be met by simple measurement of the energy 

 radiated. Light, however, considered as a sensation, is repre- 

 sented by very different quantities of energy at different 

 wave-lengths in the visible and invisible spectrum, and such 

 a measuremeut would not give a number to the light value. 

 Or, again, if there exists a fundamental quantity of brightness, 

 which can only be separated and measured under very special 

 conditions, this measurement, while perhaps satisfactory to 

 the psychologist or physiologist, would not satisfy the photo- 

 metrist, who is interested in light as used for illumination, 

 even if the underlying real brightness quality is masked by 

 other qualities such as hue. What is therefore required is 

 a method of measuring lights of different colours with the 

 same order of definiteness which pertains to strictly physical 

 measurements, using units which may be dealt with as 

 physical quantities, but yet yielding values closely repre- 

 senting the values of the lights for illumination, for good 

 seeing, or as producers of the sensation of brightness. 



Degree to which these Requirements are met by Existing 



Methods. 

 Comparison of the brightness of two fields of different 

 colour is a somewhat uncertain and unsatisfactory process. 



