WEBER-FECHNER LAW. See FECHNER’S LAW.
WEBER’S LAW. = Weber’s fraction = Weber’s function = Weber’s ratio. The German physiologist/psychophysicist Ernst Heinrich Weber (1795–1878) formulated this psychophysical generalization, which states that the justnoticeable differences (or JNDs), that is, the differences between two stimuli that are detected as often as they are undetected, in stimuli are proportional to the magnitude of the original stimulus (Weber, 1834). Weber described the relationship between existing stimulation and changes in that stimulation in what historians of psychology have called the first quantitative law of psychology (Schultz & Schultz, 1987; cf: the quotient hypothesis, which is an interpretation of Weber’s law according to which the quotients/ratios of any two successive JNDs in a given sensory series are always equal; and Breton’s law, which is a formula proposed by P. Breton as a substitute for Weber’s law that states a parabolic relation between stimulus and JND; Warren, 1934). In formal terms, Weber’s law states that delta I/I = k, where I is the intensity of the comparison stimulus, delta I is the increment in intensity just detectable, and k is a constant. The law holds reasonably well for the midrange of most stimulus dimensions but tends to break down when very low- or very high-intensity stimuli are used. For instance, for very low-intensity tones the Weber fraction is somewhat larger than it is for moderately loud tones (Reber, 1995). Representative values of the Weber ratio for the intermediate range of some sensory dimensions are brightness, .02 to .05; visual wavelength, .002 to .006; loudness, .1 to .2; auditory frequency, .002 to .035; taste (salt), .15 to .25; smell, .2 to .4; cutaneous pressure, .14 to .16; and deep pressure, .01 to .03. The law of progression (Warren, 1934) refers to a formulation devised by the Belgian psychophysicist J.L.R. Delboeuf (1831–1896) as a partial substitute for Weber’s law and states that successive sensation increments increase by arithmetical progression when the