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From the data of experiments with bees in which threshold response is employed as a means of recognizing visual discrimination between stripes of equal width alternately illuminated by intensities I(1) and I(2), it is shown that the detectable increment of intensity DeltaI, where DeltaI = I(2) - I(1), is directly proportional to sigma(I2) (I(1) being fixed). From tests of visual acuity, where I(1) = 0 and the width of the stripes is varied, sigma(I2) = kI(2) + const.; here I(2) = DeltaI, and DeltaI/I(2) = 1. When the visual excitability of the bee is changed by dark adaptation, lambdaI identical with kDeltaI (= k' sigma(DeltaI)) = k'' I + const. For the measurements of critical illumination at threshold response to flicker, sigma(I2) (= sigma(DeltaI)) = k I(2) = k' DeltaI + const. The data for critical illumination producing threshold response to flicker in the sun-fish Lepomis show for the rods sigma(I2) = K I(2) for the cones sigma(I2) = K'(I(2) + const.). The data thus indicate that in all these experiments essentially the same visual function is being examined, and that the recognition of the production of a difference in effect by alternately illuminated stripes takes place in such a way that d (DeltaI)/d (sigma(I2)) = const., and that DeltaI is directly proportional to I (or "I(2)," depending on the nature of the experiment). It is pointed out that the curve for each of the cases considered can be gotten equally well if mean I or sigma(I) is plotted as a function of the independent variable involved in the experiment. Certain consequences of these and related facts are important for the treatment of the general problem of intensity discrimination.

ON THE VARIABILITY OF CRITICAL ILLUMINATION FOR FLICKER FUSION AND INTENSITY DISCRIMINATION