By the use of two-color threshold technique, the critical duration was measured at 10 retinal-illuminance levels of the adapting field. The critical duration does not always decrease monotonically with increase of the adapting retinal illuminance; it sometimes decreases at first and then increases again before it finally decreases. All these results can be explained very clearly by the hypothesis that the critical duration is determined by the state of adaptation of the cone system concerned. If it is well adapted, the critical duration is short, in general. The experimentally obtained critical-duration-vs-adapting-illuminance curve is composed of component tc–E curves of the underlying cone systems, the π1, π2, π4, and π5 mechanisms. The differences among these mechanisms, as to their critical-duration characteristics; were found; the first two, or blue systems, had long critical durations and less decrease with adaptation but the last two had short critical durations and considerable decrease with adaptation.
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