If denotes the reaction of the body to some stimulus of strength
, the \ sensitivity is defined to be the rate of
change of the reaction with respect to . A particular example
is that when the brightness of a light source is increased, the
eye reacts by decreasing the area of the pupil. The experimental
can be used to model the dependence of on when
is measured in square millimeters and is measured in
appropriate units of brightness.
Find the sensitivity corresponding to .