§ I. The Just Noticeable Sensation. § II. Upper and Lower Limit of Weber's Law. § III. Psychological Interpretation of the Law.

FOR the solution of all the problems which may arise in any definite sense-department, there are required, as we have seen, two kinds of measurement. First, we must know the constant relation in which alteration of sensation-intensity stands to alteration of the intensity of stimulus; and, secondly, the magnitude of the just noticeable sensation must be determined. The first of these measurements we have carried out; the second now remains to be performed.