Question about the ability of human visual system

Hi, all

From Digital Image Processing(Third Edition) by
Rafael C. Gonzalez and Richard E. Woods,
it reads
" human can discern thousands of color shades
and intensities, compared to about only two
dozen shades of gray. "

What does it mean? For an image with its
values in the range between 0 and 255,
does it mean that human can't discern the
difference between two grayscale values
whose difference is smaller than 10(256 / 24
is about 10)?

I do an experiment. With the background of 150,
a block is placed in the middle of the image
and its value is 151. I can't discern the difference.
If I change the value of the block from 151 to 152,
I can discern the existence of the block easily.

Can anyone provide any books or links on it?
Thanks.

Best Regards
Jogging