Subject: How many shades of gray can the average human eye see?

I understand that what we perceive as color is the reflection or absoprtion 
of light. I also understand that some people have a greater ability to 
distinguish between varying wavelengths or reflections of color, and that 
this may vary (i.e. colorblind may be able to see a greater number of 
grays.)
What I would like to know is the average number of grays or colors an eye 
with normal vision can see.
Thanks.

Re: How many shades of gray can the average human eye see?