This third edition of a classic text which was first published in 1976 is the only comprehensive, up-to-date presentation of psychophysics currently available. It has been used by undergraduate and graduate students, and scholars throughout the world and is consistently thought of as the best single source for learning the basic principles of psychophysics. The coverage of the field is comprehensive, including topics ranging from the classical methods of threshold measurement, to the modern methods of detection theory, to psychophysical scaling of sensation magnitude. The approach is one in which methods, theories, and applications are described for each experimental procedure. New features found in this third edition include: * methodological and theoretical contributions made in the field during this time period, * descriptions of adaptive procedures for measuring thresholds, context effects in scaling, theory of quantal fluctuations, multidimensional scaling, nonmetric scaling of sensory differences, and the relationship between the size of the DL and the slope of the sensation magnitude function, * new methods for measuring the observer's sensitivity of criterion and an expanded discussion of category scaling including the range frequency model and verbally labeled categories, and * methods used to control the observer's nonlinear use of numbers in magnitude estimation such as line-length scaling, magnitude matching, master scaling, and category-ratio scaling.