Photometry of Lights of Different Colours. 359 



the straight lines of fig. 4 and plotting them in such units 

 that at certain critic \1 frequencies the values of Log. I are 

 the logarithms of the experimentally determined slit-widths. 

 The slit-widths for any other critical frequency (and hence 

 illumination, by reference to the line given by the standard 

 light source) may be read off the plot by laying a straight 

 edge parallel to the axis of abscissae at the desired critical 

 frequency. 



From the standpoint of the present investigation the chief 

 interest in the study of the method of critical frequencies 

 lies in the light it throws on the flicker photometer. For 

 the sensibility of the method is too low for it to claim a 

 place in accurate photometry, a matter which is discussed 

 later. Does any simple relation similar to that just found, 

 exist between the flicker photometer luminosity curves at 

 different illuminations ? A number of similarities between 

 the critical frequency and the flicker photometer curves point 

 to their close relationship. They both show the reversed 

 Purkinje effect. The luminosity curves are at least approxi- 

 mately alike. Moreover, a peculiarity of the low illumination 

 flicker curves given in the last paper, which apparently was 

 an anomaly, is in accordance with the critical frequency 

 phenomena. The point referred to is this : in the curve 

 for the lowest illumination at which it was found practicable 

 to work with the flicker photometer, the red side starts to 

 decrease in brightness — that is, to give a Purkinje shift — 

 while the blue side is still decreasing, thus resulting in a 

 narrowing of the curve. Examination of the critical 

 frequency plots of figs 3 and 4 shows that the bend in the 

 " white " curve occurs at a higher illumination (or critical 

 frequency) than for blue, and at a lower illumination than for 

 orange and red. Consequently, if the white light curve be 

 taken as a reference standard, the change of relative direction, 

 constituting the shift from the reversed to the true Purkinje 

 effect, occurs sooner for red than for green or blue, precisely 

 what was observed. 



Can the flicker photometer be considered merely as a 

 device for equating two flicker sensations which are dove- 

 tailed one into the other ? The similarities just considered 

 would suggest this possibility. We may consider two ways 

 by which this equating could take place. Either the illumi- 

 nation determines the relative quantities of light which 

 measure equal ; or else the speed at which the flicker 

 photometer is run in order to make the colour alternation 

 just disappear determines these quantities. Both factors 

 may play a part, and others may enter as well. 



2B2 



