the intensity of a sensation varies by a series of equal increments (arithmetically) as the strength of the stimulus is increased geometrically; if a series of stimuli are applied and so adjusted in strength that each stimulus causes a perceptible change in intensity of the sensation, then the strength of each stimulus differs from the preceding one by a constant fraction. Thus, if a perceptible change in a visual sensation is produced by the addition of 1 candle to an illumination with an original strength of 100 candles, then 10 candles will be required to produce a perceptible change in sensation in an illumination with an original strength of 1000 candles. SYN: Fechner-Weber law, Weber law.