368 Mr. H. E. Ives on the 



peripheral vision the field flashes out brilliantly, and simul- 

 taneously a flicker of large amplitude appears. For red 

 light these conditions are reversed ; it is now the periphery 

 which is less sensitive to light and flicker. This behaviour 

 o£ red and blue light at low illuminations has been previously 

 noted by Dow *. 



Bearing upon Theories of Colour Vision. 



Koenig in his study of the visual purple found its absorp- 

 tion spectrum to correspond to the luminosity of the spectrum 

 at very low illumination, i.e., twilight or colourless vision |. 

 The product of its decomposition, the visual yellow (which is 

 in turn decomposed) has an absorption spectrum corre- 

 sponding to the distribution of the blue sensation at high 

 illuminations. Study of the red, green, and blue sensation 

 curves with continually decreasing illumination % shows that 

 the colourless sensation at low illuminations arises by gradual 

 transformation of the blue sensation — the red and green 

 retaining their distribution substantially unaltered. At high 

 illuminations, therefore, gray or white is a mixture of three 

 colour sensations (using the word sensation in Kcenig's 

 meaning), while at low illuminations the gray sensation is a 

 simple one due to the transformed blue remaining after the 

 red and green has dropped below the threshold. At high 

 illuminations Koenig considers the visual purple to be broken 

 down successively into visual yellow and the final colourless 

 product ; at low illuminations only the first transformation 

 takes place. Visual purple occurs only in the retinal rods, 

 and is absent from the cones, consequently from the fovea. 

 The fovea he finds is blue-blind. 



In the first paper of this series it was suggested that the 

 retinal cones or colour-seeing organs might be responsible 

 for the behaviour of the flicker photometer. It was assumed 

 that the peripheral cones, which are large, might be more 

 red-sensitive ; that is, more extreme in their cone character 

 than the foveal cones. Situated among the rods, their 

 exaggerated red-sensitiveness might with decreasing illumina- 

 tion tend to partly counterbalance the increasing part played 

 by the rods. In the flicker photometer these cones acting 

 alone (due in some way to the intermittent stimulus) produce 

 the reversed Purkinje effect. Could the rods be made to act 

 alone, an abnormally large Purkinje effect would result. 



* Phil. Mag. Jan. 1910, p. 58. 



t "Ueber den Menschlichen Sehpuipur," Gesammelte Abhandlungen, 

 p. 338. 



% E. Tonn, Zeit.f. Psych, u. Phys. vi. p. 279 (1894). 



