The level of dark adaptation of the whirligig beetle can be measured in terms of the threshold intensity calling forth a response.

The course of dark adaptation was determined at levels of light adaptation of 6.5, 91.6, and 6100 foot-candles. All data can be fitted by the same curve. This indicates that dark adaptation follows parts of the same course irrespective of the level of light adaptation. The intensity of the adapting light determines the level at which dark adaptation will begin.

The relation between log aI0 (instantaneous threshold) and log of adapting light intensity is linear over the range studied.

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