A quantitative approach to an understanding of fluctuating phenomena encountered near the threshold of vision has involved an analysis of frequency of seeing curves. The results of various investigators are discussed with reference to the possibility of an explanation based exclusively on the statistical fluctuations in the stimulating energy. Psychological variability in the conditioning of the subject can account for controversial results. The integrative actions in the visual system measured with the aid of flashes containing two separate small and short stimuli—with variable time and distance apart from each other—were discussed. Various merits of such experiments are put forward. The absolute threshold behavior with respect to dependences of threshold energy on duration, velocity of moving sources, length and width of line-shaped targets, diameter and size of circular and square targets, was confronted and found in agreement with the two quanta explanation. Objections and modifying suggestions of other investigators on the two quanta concept are discussed. It is concluded that experimental results deny the correctness of such objections or modifications. The brightness impression of the perceived flash under the various conditions decreases with increasing target-size or test area. Suggestions are made as to the mechanisms which accounts for this effect. Such mechanisms account for deviations from theory at the simultaneous occurrence of large and long test flashes.
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