Fig. 15. Equivalent background experiment of Blakemore and Rushton.45 Similar experiments were performed earlier by Crawford.5 To avoid contamination from the cone system, the subject has achromatopsia. The change in sensitivity during dark adaptation (left panel) and during increment thresholds with small (5 minutes of arc) or large (6 degrees). This type of plot permits estimation of dark adapted sensitivity in background light levels of determined in the increment threshold experiment (right panel). (Blakemore CB, Rushton WAH: Dark adaptation and increment threshold in a rod monochromat. J Physiol [Lond] 181:612, 1965.)