Photometry of Lights of Different Colours. 151 



of lights of different hue or saturation. This difficulty has 

 long been recognized, and various opinions as to the solva- 

 bility of the problem have been expressed. Various plans to 

 accomplish its solution have been proposed. The belief has 

 been expressed by some that comparison of the luminosities 

 of two differently coloured lights is no more possible of exact 

 accomplishment than the exact comparison of a sound with 

 an odour. Helmholtz declared his lack of confidence in his 

 own ability to make such comparisons. On the other hand, 

 some observers have bv practice acquired skill and apparent 

 certainty in heterochromatic photometry. Again, methods 

 of photometry have been discovered and developed which, in 

 the process of measurement, evade and perhaps eliminate the 

 differences in hue and saturation. 



By these different methods different criteria for equality of 

 luminosity are obtained. What are the relations between the 

 criteria ? Are two coloured lights wdiich are equally bright 

 as measured by one method, of equal brightness as measured 

 by another ? 



The relative brightness of differently coloured lights is not 

 constant under all conditions. Physiological factors enter to 

 a large degree. The " Purkinje effect " is the name given 

 to the increase of blue sensitiveness at low illumination. The 

 " yellow spot effect " is the name given to the change in 

 the relative brightnesses of different colours occurring when 

 the size of the field of view is changed. To what degree do 

 these enter in the various methods of colour photometry? 



The advent of high efficiency incandescent lamps and of 

 new illuminants with ^ide range of colour, the prospect 

 of an even greater variety of illuminants in the future, 

 diverging still further in colour from the present flame and 

 carbon lamp standards, all render the problem of measuring 

 the luminosity of lights of various hues one of the most 

 important in photometry. A complete solution of the problem 

 can come only through knowledge of the relative behaviour of 

 each method of comparison under the various conditions known 

 to affect the relative brightness of different colours. That 

 knowledge acquired, we may proceed to a process of test and 

 selection, and so hope to determine that method which meets 

 to the fullest degree the requirements of a method of 

 measurement. 



The chief obstacle in the way of giving preference to one 

 method of colour photometry over another is at present our 

 lack of data. Some forms of flicker photometers have been 

 compared with some forms of direct comparison photometer*. 

 Spectral luminosity curves for the eye have been derived by 

 tests of visual acuity, by the flicker photometer, by equality 



