De Tactu

De Tactu is a work by Ernst Heinrich Weber, a German anatomist and physiologist. Published in 1834, this work, meaning "Concerning Touch," details his fundamental studies on the sense of touch. Weber introduced the concept of the "just-noticeable difference," which is the smallest perceivable difference between two similar stimuli. This concept is crucial in psychology and sensory physiology.1

Weber's research established that a threshold of sensation must be met before an increase in stimulus intensity can be detected. He observed that this necessary increase is a ratio of the total stimulus intensity, not an absolute amount. For example, a greater weight must be added to a 100-pound load than to a 10-pound load for the change to be noticed. These findings were later elaborated upon and formed the basis for Weber's law, which quantifies the perception of change in a stimulus.12