BRIGHTNESS; THE PURKINJE PHENOMENON 



87 



tion of two 'pure' hues are compared, we are really comparing their 

 intrinsic subjective white-sensation-arousing power, their white valences, 

 or their nearness to whiteness. Yellow is not white, but it is more like 

 white than red is, because yellow stimuli more effectively stimulate the 

 whole white-seeing mechanism of the cones and their central connections. 



Brightness and the Purkinje Phenomenon — Brightness has the 

 same meaning in cone-mediated sensations that it has in achromatic rod 

 sensations, and is just as independent of actual physical intensity. But 



THE PURKINJE PHENOMENON 

 Dark Adaptation Produces 



© 

 fo shift in the position of 



red orange yellow green blue violet 



Wavelength 



Fig. 30 — Graphic depiction of the changes which comprise the Purkinje phenomenon. 



the relation of brightness to intensity is different for the cone- and rod- 

 mechanisms. While the brightest part of an equal-energy spectrum is the 

 yellow-green for the cones, it is in the green for the dark-adapted, func- 

 tionally pure-rod eye. This shift results in a change in the relative bright- 

 nesses of colored objects as intensity drops below the cone threshold or 

 rises above it. This change is the 'Purkinje phenomenon', which is simply 

 a betrayal of the change-over from predominantly cone vision to rod 

 vision (Fig. 30; and see Fig, 35, p. 102). It naturally occurs only in 

 duplex retinae, or duplex retinal areas, and its occurrence is a part of the 

 great mass of evidence for the Duplicity Theory. 



