Photometry of Lights of Different Colours. 353 



hold for all illuminations. Allen * has obtained " persistence 

 of vision curves " showing a greater sensibility to pure 

 flicker in the peripheral retina as compared with the fovea 

 or central retina. On the other hand, the nicker photometer 

 has been found by the writer to be less sensitive when the 

 peripheral retina is used. 



The present investigation for the most part clears up these 

 seeming discrepancies. Most of the results of other observers 

 have been confirmed. Their divergence is shown to be due 

 to different experimental conditions. 



The procedure for obtaining the luminosity curves was 

 similar to that described in the previous paper. The largest 

 field (5°*2 x 8°*6) was used most of the time, because with 

 it the Purkinje and similar effects are most marked. First 

 a set of flicker and of i equality of brightness measurements 

 was made as in the previous work. Then the spectral light 

 was extinguished and the speed was determined at which 

 flicker vanished when the rapidly rotating white disk was 

 illuminated by the comparison lamp alone. The various 

 spectral regions were then substituted for the comparison 

 lamp, and the slit widths were determined which would 

 •cause flicker to disappear at the speed found for the com- 

 parison lamp. With critical frequency settings the mean of 

 appearing and disappearing flicker was taken. Ten readings 

 for each illumination were usually made, and between each 

 set the eye was rested about a minute in the faint light of 

 the photometer room. In this manner three kinds of 

 spectral luminosity curves were found at each illumination. 

 The first measurements were made at two illuminations, a 

 medium and a low, namely 50 I. U. and 5 I. U. The results 

 are given in fig. 1, curves B and C. (Later a variation in 

 the procedure was found preferable as will be noted and the 

 •curves A, B, and C (fig. 1) were made by this more satis- 

 factory method.) 



It appears from these curves that the flicker and the 

 critical frequency method both show the reversed Purkinje 

 effect, in contrast to the regular Purkinje shift with the 

 equality of brightness method. It appears too that the 

 areas of the three kinds of curves differ. 



The occurrence of the reversed Purkinje shift by the 

 critical frequency method is in apparent contradiction to 

 the work of Hay craft. It claimed first attention. For 

 purposes of comparison Haycraft's results are reproduced in 



* Phys. "Rev. vol. xi. p. 257 ; vol. xxviii. p. 45. 



