The curve of mean critical illumination (Im) for response to flicker as a function of flicker frequency (F) for the larvae of the dragonfly Anax junius is progressively shifted toward higher intensities the lower the temperature. The maximum flicker frequency (one half the cycle time of light and no light) and the maximum intensity with which it is associated are very little if at all affected by change of temperature.

These facts are in agreement with the requirements of the conception that recognition of critical illumination for reaction to flicker involves and depends upon a kind of intensity discrimination, namely between the effects of flashes and the after effects of these flashes during the intervals of no light. The shift of the F-Im curve with change of temperature is quite inconsistent with the stationary state conception of the determination of the shape of the curve.

The dispersion (P.E.II1) of the measurements of I1 is directly proportional to Im, but the factor of proportionality is less at high and at low temperature than at an intermediate temperature; the scatter of the values of P.E.II1 is also a function of the temperature. These facts can also be shown to be concordant with the intensity discrimination basis for marginal recognition of flicker.

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