This chapter describes a new psychophysical law which may be called the law of comparative judgment and to show some of its special applications in the measurement of psychological values. The law of comparative judgment is applicable not only to the comparison of physical stimulus intensities but also to qualitative comparative judgments such as those of excellence of specimens in an educational scale. The scale difference between the discriminal processes of two specimens which are involved in the same judgment will be called the discriminal difference on that occasion. The law of comparative judgment is basic for all experimental work on Weber's law, Fechner's law, and for all educational and psychological scales in which comparative judgments are involved. The formulation of the law of comparative judgment involves the use of a new psychophysical concept, namely, the discriminal dispersion.