Is anyone able to give an answer to the question :

         How many colors are the color-sensitive cells in the
         human eye able to distiguish ?

         Please EMail !

The normal human eye has 4 types of receptors: rods, and red, green, and
blue cones.  There is some controversy about whether the rods can
participate in color vision (they are actually purple), but in bright
light, they probably can't, so there are 3 primary colors.  This means that
the space of colors the eye can distinguish is 3-dimensional.  In
principle, the *number* of colors is infinite.  However, it is limited to
merely a huge number by the fact that light is quantized, and by the fact
that the brain uses pulses (although the early layers of the retina use
"graded", i.e. continuous, potentials).  Without recalling the reference,
it has been shown that a single quantum of light can cause a change in a
receptor, and 3 quanta are perceivable (this probably was done with rods).

--peter