On the assumptions (1) that the state of the photoreceptor is completely determined by its previous history in respect of exposure to light, (2) that photoreception involves two opposed processes, one of which is light-sensitive, and (3) that sensation is determined by the rate of the light-sensitive process integrated over a short period, it is shown that Talbot's law has as a necessary consequence that the velocity of the light-sensitive process must be directly proportional to the intensity of the stimulating light.

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