or Weber-Fechner law
In psychophysics , a historically important law quantifying the perception of change in a given stimulus.
Originated by the German physiologist Ernst Heinrich Weber (1795–1878) in 1834 and elaborated by his student Gustav Theodor Fechner , the law states that the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. It was later shown not to hold for extremes of stimulation.