In 1941 IIecht, Shiner, and Pirenne (7) reported energy measurements of a visual stimulus sufficient to provide a specified threshold effect in the darkadapted eye of a human observer. The threshold effect was defined in terms of the percentage of positive responses occurring in a situation where two respunses were possible, one being indicative of "seeing," the other, of "not seeing." Energy measurements at the position of.the cornea, coupled with corrections for energy loss between the cornea and the point of absorption of the energy, led the authors to the conclusion that the amount of absorbed energy necessary for detection by the human observer 60 per cent of the time was of the order of 8 to 14 quanta. A second part of the paper demonstrated that the relationship between percentage of positive responses and the logarithm of stimulus intensity could be represented by a cumulative Poisson curve whose slope had a value compatible with an interpretation that 6 to 10 quanta were required to provide a threshold effect. The two lines of evidence, direct and statistical, point to the importance of quantal considerations in a theoretical treatment of the absolute threshold.Since the pubfication of the Hecht, Shlaer, and Pirenne paper, many frequency of seeing functions have been determined under a variety of experimental conditions, and many visual phenomena have been treated in quantum terms. For example, Bouman and van der Velden (3) present an account of visual acuity and intensity-time functions in terms of quantum concepts; Baumgardt (2) employs similar principles in his discussion of area-intensity and intensity-time relations; and de Vries (5), Rose (12), and Hendley (8) consider the differential threshold within a framework of quantum theory.The literature on vision contains many data on the relation of the just detectable difference in intensity, M, to the intensity level, I, but no studies show *

8 The literature on relativistic wave equations is very extensive. Besides the papers quoted in reference 11, we only mention the book by de Broglie, L., Theorie gene'rale des particides a' spin (Paris, 1943), and the following articles which give a systematic account of the subject:

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