Foveal increment thresholds, with particular reference to the blue, were investigated when the retina was in the process of recovery from preadaptation to high and medium luminance fields of long wavelengths. While confirming Wald and Auerbach’s finding of an inflection in the blue recovery from adaptation to very high luminance, and Stiles’s observation of an initial rise of threshold, over the light-adapted value, the results also indicated (1) that the inflection could not readily be related to any of the π1, π2 and π3 receptors, and (2) that the maximum of the π2 sensitivity in the blue region could not lie at wavelengths longer than 470 mµ.
S. R. DAS, "Foveal Increment Thresholds in Dark Adaptation," J. Opt. Soc. Am. 54, 541-545 (1964)