The two-quanta explanation of the dependence of the threshold values and visual acuity on the visual angle and the time of observation.

As shown by van der Velden, for the observation of a short and small light flash it is necessary that at least two quanta of light be effectively absorbed by the visual purple within a time τ (about 0.02 sec.) and within an area corresponding to a visual angle D (about 10′): the two-quanta hypothesis, by which the laws of Ricco, Piper, and Talbot were explained. In the present paper the theoretical foundation is recapitulated and further experiments are described.The extensive experimental results in the present paper are completely covered by the two-quanta hypothesis as far as regards the flash time t<3τ or the visual angle of the light spot d<2D.It is shown, that for the simultaneous occurrence of long flashes and large visual angles, considerable deviations from the theoretical curves derived from the two-quanta hypothesis occur.From these deviations we conclude that T seconds (about 3τ) after the absorption of light quanta the condition of the retina in the neighborhood of these absorptions (within about 3D) is such as to decrease the chance of observation of a subsequent pair of absorbed quanta.Measurements of the visual acuity are found to be in agreement with the two-quanta case.The experimental results of Hecht and Pirenne and the paper by De Vries are discussed, as is also the note of C. Peyrou and H. Piatier about experiments similar to those of van der Velden.