1. Rhodopsin has been measured by Rushton's method of reflexion densitometry in a retinal region 18° temporal to the fovea, using a wavelength of measuring light (555 nm) so far into the long wave part of the spectrum that possible blue absorbing intermediates (e.g. transient orange) do not interfere.2. Rhodopsin was bleached by a strong light for 10 sec and then held steady by a weaker light. During a 10 sec bleach, no regeneration occurs and the rate of bleaching is proportional to the quantum catch. The proportionality constant is about 10-7 (td sec)-1.3. From 2, the rate of photolysis at equilibrium produced by the steady light was calculated. Since conditions were at equilibrium, photolysis matched regeneration. It was found that the rate of generation was proportional to the amount of pigment still bleached. The proportionality constant was about 0·0025 sec-1.4. It was found by several different methods that the constant in 3 is the same in the light or dark and hence regeneration occurs independently of bleaching.5. Therefore, the results from bleaching and regeneration experiments can be combined to give the general equation [Formula: see text], where p is the fraction of rhodopsin, t is time in sec and I is the retinal illuminance.6. This equation describes the results of partial bleaching and regeneration experiments under a variety of different exposure intensities of moderately long (at least 10 min) exposure durations.7. The dark adaptation curve in a peripheral region of the rod monochromat's retina where there are few cones follows a simple exponential course over nearly 7 log10 units. Rhodopsin regeneration and log threshold for this region are described by the same curve with a time constant of about 400 sec. Each log unit fal in threshold is accompanied by 0·835% increase in rhodopsin. This time constant is in agreement with Rushton's (1961) finding, but appreciably longer than that reported by Ripps & Weale (1969a).8. The Ripps & Weale result was, however, obtained by bleaching with a very short bright xenon flash (as they did). Under these conditions, blue absorbing intermediate(s) is (are) formed, the time constant of regeneration of rhodopsin is much faster than after long tungsten bleaches, and the kinetic equation is not valid.9. The general equation, together with the relation found in 7, successfully accounts for results previously published by others of the effect of duration and intensity of bleaching on the recovery of rod threshold in the dark, provided only that more than 5% of the rhodopsin was bleached at the beginning of dark adaptation.
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