Theory of the Flicker Photometer. 



713 



value than this cannot be appreciated by the ultimate 

 receiving apparatus. Each different critical frequency 

 value then means the same contour (relative amplitude or 

 percentage range) of the sensation, but this same contour is 

 produced by a progressively lower speed of alternation as 

 the illumination is decreased. It is easily to be seen that 

 the colour-receiving elements of the eye, whatever they 

 may be, may be likened to lamp filaments of different 



Fig. 5. 



Result of alternating two colours of unequal brightness, but each 

 showing the same relative amplitude of fluctuation. This shows the 

 effect of alternating the two lights a and b of fig. 1, which have the same 

 critical frequency. 



diameters whose heat capacities increase as the illumi- 

 nation decreases, and at different rates for the different 

 elements. 



Consider two coloured stimuli, 180° out of phase, falling 

 upon their two appropriate elements, which have different 

 " diffusivities." If they are of equal brightness, and if either 



Phil. Mag. S. 6. Vol. 28. No. 167. Nov. 1914. 3 A 



