Ernst Weber (1795-1878) 
          Weber  presented  an  extensive  experimental 
              exploration of the sensory phenomenology of 
              tactile experience 
          Coining  the  phrase,  just  noticeable  difference 
              (JND)  to  refer  to  the  smallest  perceptible 
              difference  between  two  sensations  that  is 
              detectable by a human being or other animal.
          Weber  provided  an  existence  proof  for  the 
              possibility    of    establishing      quantitative 
              relationships  between  variations  in  physical 
              and mental events 
     Gustav Fechner (1801-1887). 
     A  pioneer  in  experimental  psychology  and 
      founder of psychophysics, the study of the 
      relationship between stimulus intensity and 
      subjective  experience  (detection)  of  the 
      stimulus
     Psychophysicists usually employ experimental 
      stimuli  that  can  be  objectively  measured, 
      such as pure tones varying in intensity, or 
      lights  varying  in  luminance. All  the  senses 
      have  been  studied:  vision,  hearing,  touch, 
      taste, smell, and the sense of time.
    The  most  common  use  of  psychophysics  is  in 
     producing  scales  of  human  experience  of  various 
     aspects of physical stimuli. For example the physical 
     stimulus of frequency of sound. 
    Frequency of a sound is measured in hertz, cycles per 
     second. But human experience of the frequencies of 
     sound is not the same as the frequencies in hertz. 
    Doubling the frequency of a sound (e.g., from 100 Hz to 
     200 Hz) does not lead to a doubling of experience. 
     The perceptual experience of the frequency of sound 
     is called pitch
        Hermann von Helmholtz  (1821-1894) 
        He viewed perception as requiring an 
           active,   unconscious,      automatic, 
           logical  process  on  the  part  of  the 
           perceiver     which      utilizes    the 
           information  provided  by  sensation 
           to  infer  the  properties  of  external 
           objects and events. 
        Helmholtz  anticipated  much  of  later 
           top-down cognitive psychology.
    Helmholtz had also made another major contribution to 
     physiology. Stimulating nerves at various distances 
     from  a  muscle  and  measuring  the  time  it  took  for 
     muscular contraction, he estimated the rate of travel 
     of  the  nervous  impulse,  and  in  the  process 
     incidentally introduced the technique of reaction-time 
     into physiology and psychology