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Weber's Law

A principle of human perception put forth by E. H. Weber in the early 1800's: basically that the difference between the intensities of two stimuli causing two just barely distinguishable sensations (a “JND”, just noticeable difference), is proportional to the physical intensity of the stimuli.

For example, imagine two lights that differ in physical intensity (the kind that can be measured by instruments) just enough that an observer can tell they differ. (The observer's judgment is a measure of perceived intensity.) Weber's Law says that the difference in the physical intensities of two just distinguishable bright lights will be greater than the difference between two just distinguishable dim lights. Or that the difference between the physical intensities of two very loud sounds that are barely distinguishable in loudness, will be greater than the difference between two barely distinguishable faint sounds.

Later workers have refined Weber's insight. For an incorrect but well-known 19ᵗʰ century generalization see Fechner's Law. For a modern generalization of the relationship between stimuli and perception, see Steven's Law.