356 



Mr. H. E. Ives on the 



two different logarithmic rates at which critical frequency 

 varies with illumination. Porter * has found for white 

 light that the relationship n — k log. I+p represents the 

 critical frequency phenomena, where n= critical frequency 

 in cycles per second, 1= illumination, k andjo are constants. 

 The constant k has, however, two different values, one below 

 *25 M.C., another above that illumination. When plotted 

 on semilogarithmic paper this relation is shown by two 

 straight lines of different inclination meeting at *25 M.C. 

 It has been surmised that this illumination represents the 

 .point of change from cone to rod vision. 



Fig. 3 gives the results of a determination of these 



FU 



30 



■*s . 



Artificial Pop. I (isq rnm] 

 Natural • ^ ' 



z.oy. 



Critical Frequency-Tlluminatiou Relations for " White " Light. 



straight lines with the author's apparatus for the " white " 

 light of the standard carbon lamp. The abscissse are 

 logarithms of illumination units (metre candles on magnesium 

 oxide surface, viewed through artificial pupil of 1 sq. mm.)f ; 



* Proc. Boy. Soc. lxxix. p. 313 (1902). 



f The illumination is that given by the intensity of the light source 

 divided by the square of the distance. The illumination upon the 

 retina, when the disk is in motion, corresponds to a steady illumination 

 of half the plotted values. 



