Log S’o
Log S
Membrane
Response V
Fechner’s law: 
               I = A  log (S/So)  for S > So
                       I = 0                    for S < So
S:  Physical stimulus intensity
So: Threshold stimulus intensity
A:  Constant
Log So
Log S
Subjective
Intensity I
V = A’  log (S/S’o)  for the linear range
S:    Physical stimulus intensity
S’o: Threshold stimulus intensity
A’:  Constant
Fechner’s Law