We have seen that, in the dark, the sensitivity of the human eye to light increases until a maximum is reached after about 30 minutes; this gives the dark-adaptation curve showing the progressive fall in threshold. We have already concluded that the curve represents the course of recovery of function of the rods. This curve, however, is not smooth, but shows a ‘kink’ so that adaptation consists in an initial rapid phase of recovery of sensitivity, which is finished in about 5 to 10 minutes, and a slower phase.
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