to adjust the sensitivity of the ear to different ambient levels.

A human can see objects in starlight or in bright sunlight, even though on a moonless night objects receive one billionth (10^-9) of the illumination they would on a bright sunny day; a dynamic range of 90 dB. Change of sensitivity is achieved in part through adjustments of the iris and slow chemical changes, which take some time.

In practice, it is difficult for humans to achieve

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