Abstract

The simplest interpretation of the uncertainty of seeing observed at the absolute threshold is based on the theory of quantum fluctuations and assumes that in any given trial the light is seen when the retina has been acted upon by at least n quanta. When this assumption is true the frequency-of-seeing curve is a simple Poisson sum of parameter n, and the “mean threshold intensity,” that is the mean number of quanta I(55%) acting on the retina for a frequency of seeing of 55%, is very nearly equal to n. Biological variations of n would make the curve shallower than it would be if n were constant and approximately equal to the “mean threshold intensity,” I(55%). The same result is valid in the case of all other complications which might occur with regard to the physiological mechanisms involved, and which would make the simplest interpretation invalid. Thus the slope of experimental curves obtained under suitable conditions must be smaller than or equal to the slope of a simple Poisson sum the parameter of which is approximately equal to the “mean threshold intensity.” From the slope of such curves, therefore, it is always possible to derive a lower value for the “mean threshold intensity.”

© 1955 Optical Society of America

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