714 



LIGHT AND LIFE 



(b) Relation oj Increment Threshold to Visual Pigment 



Fig. 3 shows some measurements upon my own cones. The circles 

 show the percentage of total pigment present at various times from 

 the moment of shining upon the fovea a bright light of 60,000 

 trolands (= 6,000 mL through a 2 mm diameter pupil) . The upper 

 branch starts from the dark-adapted state, the lower from the com- 

 pletely bleached state produced by an intensely bright light kept 

 shining until the moment of transition to 60,000 Td at zero time. 

 The change in pigment level is what would be expected from the 

 fundamental kinetic equation (Rushton, 8) 



dx 

 dt 



= (\ - x)I - X 



where / is intensity in units of 40,000 Td, x is the fraction of pigment 

 bleached, and t is time in units of 2 minutes. 



The experiment was repeated, but now instead of measuring the 



Fig. 3. White circles, right scale, percentage of cone pigment on human fovea 

 when exposed to a light of 60,000 trolands, starting either from darkness or 

 from full bleaching. Tracings, left scale, log increment threshold superimposed 

 u|)()n (iO.OOO 1(1 background for ().,') min followed by absolute thresholds (zero 

 background). Upper tracings start from dark, lower tracings from full bleach. 



