part c:

stroke varies from 0 ->N.
N needs to be around 80 in order to perceive a
gradient variation.

gradient variation is very perceptable when N=100.

part d:

stroke varies from 255 ->N
(from white to black)

gradient variation is perceptable when N = 237.

part e:

since there is a larger perceptional gap from 0 up
(from black to a perceptable gray variation) than from 255
down (from white to a gray).. indicates that the perceptable
grey-scale is really scewed towards the higher range of the
gray scale. a more Perceptable grey scale (with a larger
range of contrast) would be from 80 or 100 to 255.