From the relations between critical illumination in a flash (Im) and the flash frequency (F) for response of the sunfish to visual flicker when the proportion of light time to dark time (tL/tD) in a flicker cycle is varied at one temperature (21.5°) the following results are obtained:

At values of tL/tD between 1/9 and 9/1 the F - log Im curves are progressively shifted toward higher intensities and lower Fmax.. Fmax. is a declining rectilinear function of the percentage of the flash cycle time occupied by light.

The rod and the cone portions of the flicker curve are not shifted to the same extent.

The cone portion and the rod region of the curve are each well described by a probability integral. In terms of F as 100 F/Fmax. the standard deviation of the underlying frequency distribution of elemental contributions, summed to produce the effect proportional to F, is independent of tL/tD.

The magnitude of log Im at the inflection point (r'), however, increases rectilinearly with the percentage light time in the cycle.

The proportionality between Im and σII1 is independent of tL/tD.

These effects are interpreted as consequences of the fact that the number of elements of excitation available for discrimination of flicker is increased by increasing the dark interval in a flash cycle. Decreasing the dark interval has therefore the same kind of effect as reducing the visual area, and not that produced by decreasing the temperature.

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