THE CHARACTERISTICS OF STIMULI 47 



of physiology with a very detailed and comprehensive treatment. 

 I allude to the teaching concerning sensation. Ernst Heinrich 

 Weber^ first called attention to the relation between increase in 

 sensation and that of the stimulus in the case of the sense of 

 touch. His observations, which have been formulated into 

 "Weber's law," have been the object of animated discussion. A 

 presentation of this law is the following: "The amount of pres- 

 sure necessary to produce a perceptible increase of sensation 

 always bears the same ratio to the amount of the stimulus already 

 applied." 



If in accordance with Ziehen 2 we designate the relative 

 increase in pressure to that already applied, which is necessary 

 to produce a perceptible increase in sensation, as the threshold of 

 relative differentiation, we can formulate the law in the simplest 

 way thus : The relative threshold of differentiation is constant. 

 Fechner? who indeed attempted to apply this law, applicable to 

 the sense of pressure, to all the other special senses, has given 

 us a mathematical formula, based on the assumption that the just 

 perceptible increase of sensation has the same value at all levels. 

 By this assumption he was able to establish for the first time a 

 relation between the intensity of sensation and that of stimulus, 

 for it follows that "the sensation increases in intensity in arith- 

 metical progression, whereas the intensity of the stimulus in- 

 creases in geometrical progression" From this Fechner has 

 worked out a psychophysical formula, which today is generally 

 termed the Fechner law. This is the law : The intensity of sensa- 

 tion varies with the logarithm of the intensity of the stimulus. 



Soon the Weber as well as the Fechner law had been extended 

 over the w r hole field of sensation and stimulation. In this con- 

 nection Preyer* has formulated his "myophysical law," which 

 states that there is the same relation between strength of stimulus 

 and the intensity of response of the muscle as is laid down by the 



1 Weber: "Annotationes anatomicse et physiologies." Lips. 1851. The same: "Der 

 Tastsinn und das Gemeingefiihl," in Wagner's Handworterbuch d. Physiologic Bd. III. 

 2. Braunschweig 1846. 



2 Ziehen: "Leitfaden der physiologischen Psychologic in 15 Vorlesungen." VI 

 Auflage. Jena 1902. 



3 Fechner: "Elemente der Psychophysik." Leipzig 1860. 2 Auflage 1889. 

 4Preyer: "Das myophysische Gesetz." Jena 1874. 



