48 IRRITABILITY 



Fechner law for stimulation and sensation. Pfeffer^ has found 

 that Weber's law applied also to the relations of the chemotaxis 

 of bacteria, to the intensity of the chemical stimulus, and likewise 

 the attempt has been made to show that all living substances 

 respond in the manner laid down by the Weber-Fechner law. 

 Unfortunately the innumerable investigations in this field have 

 shown more and more clearly that it is not possible to formulate 

 a general mathematical law, which strictly fixes the relations of 

 the intensity of the stimulus and the intensity of response. Even 

 in the field of the physiology of the special senses many voices 

 have opposed the general application of the Weber and the 

 Fechner law. Lotze, G. Meissner, Dohrn, Hering, Biedermann 

 and Lowitt, Funke and numerous other investigators have already 

 demonstrated for some decades, partly by means of critical 

 inquiry, partly by experimentation, that these laws are not strictly 

 valid. Above all these experiments have shown that logarithmic 

 relations are not tenable and likewise are not applicable to very 

 strong stimuli. The assumption made by Fechner, that is, the 

 acceptance that all barely perceptible increases of sensation have 

 an equal value, has been set aside as incorrect, and with this his 

 mathematical formulation within those boundaries of intensity 

 of the stimulus, in which the Weber law has proven itself valid, 

 must also be abandoned. That which we can say today with cer- 

 tainty concerning the relation between the intensity of stimulus 

 and the amount of response is as follows : A law generally appli- 

 cable to the relation between the strength of the stimulus and 

 the amount of response cannot be mathematically formulated. 

 For a great number of living systems the rule which holds for 

 the intensity of stimulation within certain boundaries is the fol- 

 lowing : With increase of the intensity of stimulation the response 

 at first increases rapidly and later more and more slowly. 



This rule of course only applies within the boundaries of the 

 intensity between the threshold of stimulation and maximal 

 stimulus. The interval, however, between these intensities varies 



1 Pfeffer : "Ueber cbemotaktische Bewegungen von Bacterien, Flagellaten und 

 Volvocineen." Untersuchungen aus dem botanischen Institut zu Tubingen. Bd. II, 

 1888. 



