Photometry of Lights of Different Colours. 155 



visual acuity would not appear to deserve much attention. 

 To the writer's knowledge no measurements are on record 

 which were made to determine whether the arithmetic sum 

 of two differently coloured illuminations measures equal to 

 the two illuminations together. One advantage the method 

 of visual acuity has over that of equality of brightness, is that 

 by the nature of the criterion two illuminations equal to the 

 same are assuredly equal to each other. This fact depends 

 of course on the illuminations not being compared one with 

 another, but with, as it were, an outside standard of different 

 character. The importance of this will depend on whether in 

 other methods any error is shown due to characteristics of the 

 compared lights, other than their luminosity. 



Measurement of illumination is possible by determining the 

 critical frequency of flicker. The method is subject to very 

 much the same criticisms as apply to measurement by visual 

 acuity. Taking as illustration the investigation of Kennelly 

 and "Whiting, we find that " every time the illumination on 

 the target is doubled, the mean vanishing flicker frequency 

 iucreases by 3*3 cycles per second approximately/' The 

 observations of each observer used to obtain the mean of three 

 show sometimes values 1J or 2 cycles per second above the 

 mean adopted, sometimes as much below, indicating a possible 

 range in settings corresponding to about 100 per cent, in 

 illumination. In short, the method appears to be not so much 

 for accurate light measurement as for orders of illumination. 

 It possesses the characteristic noted in connexion with visual 

 acuity methods, namely, that it is not necessary to view the two 

 differently coloured fields simultaneously in order to make a 

 measurement; two illuminations equal to a third are, therefore, 

 of necessity equal to each other. Tests of the results of 

 adding illuminations of different colours are lacking. 



A word may be said in passing in regard to the common 

 characteristic of the last two methods, that by neither is it 

 necessaiy to juxtapose the observed illuminated surfaces. In 

 one way this is an advantage. The problem of judging 

 between two hues is eliminated. Judgment is made on a 

 quality common to both illuminations (their detail or flicker 

 revealing power) which is not confused by hue, as is the 

 sensation of brightness. On the other hand, both visual acuity 

 and ability to perceive flicker are largely influenced by physio- 

 logical conditions such as adaptation and fatigue, and are apt 

 to vary from time to time. The uncertainties introduced by 

 these latter are probably as great as the uncertainties of indi- 

 vidual settings during one measurement. At any rate it is 

 significant that none who have worked -with. cither visual acuitv 



