Named after the German physicist and psychologist Gustav Fechner. Large increases in the intensity of a stimulus produce smaller, proportional increases in the intensity perceived. It is expressed mathematically as S=k log R, where S is the intensity experienced, R is the actual physical intensity and k is a constant. This is one of the first attempts at a mathematical statement of a psychological PHENOMENON. It grew out of the studies in PSYCHOPHYSICS with which PSYCHOLOGY became an experimental science in the 1870s.