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Fechner’s law

A pioneering though in many situations incorrect formulation of the relationship between the physical strength of a stimulus and its strength as perceived by humans, proposed by G. T. Fechner in 1860. Fechner postulated that sensation increases as the log of the stimulus. For example, by Fechner's law, if light A was twice as bright as light B (measured by an instrument), it would appear to the human eye to be log 2 (times a constant to allow for such factors as the units used) brighter than light B.

Later experiments have shown conclusively that Fechner's law doesn't generally apply. See Steven’s law.

Gustav Theodore Fechner.
Elemente der Psychophysik.
Leipzig : Breitkopf und Härte, 1860.