The Absolute Threshold of Vision

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Abstract
The number of quanta that must be absorbed by the dark adapted human retina to produce a sensation was estimated at 1-10. Higher values were obtained by a method assessed by frog visual purple at a time when it was assumed to be identical with the human variety. When estimated with appropriate correction, results approximate the lower figures. The value of 1 seems inconsistent with the demonstrated spatial summation at threshold. Two quanta would seem the most probable figure, though fluctuation of threshold occurs, and a single quantum, which decomposes a single molecule of visual purple, may trigger an excited retina.
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