In 194zt, three independent approaches were developed, towards establishing the number of quanta effectively absorbed by the visual purple and necessary for light-perception (van der Velden, 1944. These methods were independent of the loss of light caused by reflection and absorption in the various parts of the eye, so that the absolute values of the energies employed' are of no importance. The methods are:
A. Determination of the relation between the chance of o,bservation of a flash and the average number of quanta in the flash for very small visual angles and for short flash-times.
After the war we received' the p-pers by Hecht and his co-workers (1942), who had already used this method in 1942. These results indicated that a co-operation of at least 5, 6 or 7 quanta (dependent on the observer) were necessary for the perception of light. Our results (van der Velden, 1944Velden, , 1946;; Bouman O van der Velden (1947)), on the other hand, suggested a co-operation of two quanta, independent of the observer.
B. Determination of the dependence of the average number of quanta for a fixed chance of observation on the duration of the flash for a small visual angle.
It is obvious, that when more than one quantum is necessary for light-perception, one has to ex,pect a dependence of the average number of quanta m the flash N, necessary for a certain chance of observation, on the duration of the flash t.
Assuming that tl~e retinal condition existing after the absorption of the first quantum will have a finite life-time r it is possible to calculate the number N as a function of t, at least to give the slope of this curve for large values of t T In Fig. 1 these curves are given for the cases that 1, 2 or 3 quanta are necessary for the perception of light. The experimental resuIts are in accordance with the two-quanta case, Because of