SENSATION. 



is sometimes spoken of as the threshold value of the stimulus. 

 This threshold value is a variable quantity, varying for different 

 individuals and for the same individual at different times. It de- 

 pends partly upon the condition of the end-organ and the over- 

 lying integument, and partly upon the receptivity of the sensory 

 cerebral area involved. 



If the intensity of the stimulus is increased progressively above 

 the threshold value, the intensity of the sensation increases also, 

 up to a certain maximum, beyond which an increase in the strength 

 of the stimulus produces no further increase in the intensity of 

 the sensation. This maximum occurs with comparatively weak 

 stimuli. The range of sensory variation is, therefore, not large. 

 Between the maximum and minimum a variation in stimulus is 

 accompanied by a variation in sensation. This variation cannot 

 be measured by the subject of the sensation. He can tell that one 

 stimulus is stronger or weaker than another, but not how much 

 stronger or how much weaker. 



An increase of the stimulus above the maximum of sensory 

 interpretation very rapidly fatigues the sense organ. Even with 

 weak stimuli, the sensory apparatus rapidly tires. 



Weber's Law. E. H. Weber, the first to make systematic obser- 

 vations along these lines (1831), formulated the following conclu- 

 sion, which has since been known as Weber's law: "An increase in 

 a stimulus sufficient to call forth a conscious increase in the sensa- 

 tion must always bear the same ratio to the original strength of 

 stimulus to which it is added." 



For example, if to a weight of i it is necessary to add a weight 

 J in order that the subject of the experiment may detect a differ- 

 ence, then, if a weight of 10 is used, the added increment necessary 

 to produce an increase of sensation will be 10 divided by 3. 



I. CUTANEOUS SENSATION. 



1. Tactile Sense. To map out the touch-spots in a certain 

 region of skin, some form of instrument, known as an aesthesiom- 

 eter, is used. A simple form of lesthesiometer is made by fasten- 



