hat is the smallest instrument in the history of psychology? A strong candidate is a pair of compass points. Ernst Heinrich Weber touched the skin with both points at once and asked whether the person felt one touch or two.

what it measures

The two-point threshold: how far apart the points must be before they feel like two. Weber found it varies enormously over the body, small on the fingertips, tongue and lips and much larger on the back and thigh (Weber, 1834). Later versions with a scale and sliding point were called aesthesiometers.

why it matters

The compass gave Weber a way to ask a precise question of a private sensation, by turning it into a yes-or-no answer about a measurable distance. The same move, with weights instead of points, gave him the constant fraction that became Weber's law, and from there Fechner's psychophysics. Today a two-point test is still part of some clinical examinations of touch.

Try the weight version, with lines: the Weber line. [playful] Or borrow two pencils and a patient friend.

in the tour: the psychophysics hall has a placard called "Weber's fraction" ···

Sources

1. Weber (1834). De pulsu, resorptione, auditu et tactu: annotationes anatomicae et physiologicae. Leipzig: Koehler.

Try it yourself. [playful] Small toys on the toy shelf that show this in action:   • the line judge — find your Weber fraction for line length

see also