ABSTRACT
If only one electric lamp illuminated a poorly-lighted room and another lamp of the same strength was turned on, the increase of illumination would be quite striking. If, however, the illumination was furnished by 100 lamps, and another lamp were added, nobody would be able to notice the increased brightness. This fact of observation, which holds for all fields of sensation, has a fundamental significance for the development of psychology as a whole. E. H. Weber (1846) in his investigation of the skin sensations found that they followed a certain law. He found that in the case of a pressure of the strength of 100 units, a pressure of the strength of one unit must be added in order that the difference in pressure can be noticed. Likewise, in the case of a pressure of the strength of 200 units, a pressure of the strength of 2 units must be added in order for the strengthening of the pressure to be just noticeable. In other words, the ratio of the increase of the stimulus and the increase of the just noticeable sensation is constant (1/100 = 2/200). G. Theodor Fechner urged this as a useful method of measuring sensation. It was one of Fechner's favorite dreams, starting from this very promising beginning, to rule the whole psychical life with mathematical formulae. We should no longer speak of psychology, but rather of psychophysics. This wish has, of course, not been fulfilled, but psychophysics has served as the valuable source of most psychological methods of research and as the stimulus to transfer quantitative methods of work to other fields in psychology. We shall not treat of these methods in the following section as coming under psychophysics, however.
