Stare at the bluish dot
for awhile without moving your eyes or your head. The dot will
gradually fade into the field of green. Some people find that
the dot fades more rapidly if they stare blankly at the edge
of the square. As soon as you move your head or eyes, notice
that the blue reappears.
Even though you're not aware of it, your eye
is always making tiny jittery movements. Each time your eye
moves it receives new information and sends it to your brain.
You need this constant new information to see images.
You may have noticed that although the dot
fades just about everything else in your field of vision remains
clear. That's because everything else you see has distinct edges.