362 Mr. H. E. Ives on the 



of the Purkinje and similar phenomena. It shows the 

 rationale of changes which might, if obtained by another 

 method, be considered within experimental errors. 



The curves show the reversed Purkinje effect, making 

 clear that the phenomena of the method of critical frequency 

 dominate in the flicker photometer. But instead of the 

 expected straight lines the nearest approach to them lies in 

 the long, rather flat curves of the middle of the spectrum, 

 while at the red and blue ends a double curvature indicates 

 a marked deviation at high illumination from the form to be 

 expected. 



Two possible explanations of these deviations claim 

 attention. First, that the critical frequency phenomena 

 when plotted logarithmically are not really straight lines, 

 but curves, and that the flicker method actually agrees with 

 the other, but by reason of the greater accuracy of the 

 method shows the curvature clearly. Second, that while 

 the critical frequency phenomena are true straight lines as 

 plotted logarithmically, the flicker phenomena are merely 

 similar and not identical. 



The first explanation is not improbable, at least for the 

 medium and low illuminations. The deviations from straight- 

 ness are probably within the errors of measurement by the 

 critical frequency method. (The log scale of fig. 5 is many 

 times larger than that of figs. 3 and 4.) The lowest in- 

 flexion point of the red curves corresponds fairly well to 

 the illumination at which this should occur according to 

 fig. 3. At higher illuminations the flicker photometer 

 results would be explained if the separate critical frequency 

 curves showed a tendency to bend to the horizontal, the blue 

 showing the greatest tendency, the red the least, with white 

 lying between. Fig. 6 shows red and blue curves, each the 

 mean of several sets in which the eye was rested in total 

 darkness for several minutes between each set of readings. 

 Under these conditions the higher illuminations produce 

 strong complementary after-images. These curves do exhibit 

 the bending suggested and also much more for blue than for 

 red. It may probably be explained as a fatigue or contrast 

 effect. However, an extended series of such measurements 

 showed that the occurrence of the change in direction is 

 somewhat dependent on the condition of the eye, and is not 

 nearly so easy to observe with white as with coloured light. 

 It is also probably dependent on the character of the photo- 

 metric field. The small bright area of the prism face 

 surrounded by an entirely black field may be expected to 

 cause contrast and induction phenomena which would oe 



