As you move your eyes around the image, dark dots quickly appear and disappear at the intersections.
However, whenever you look directly on any intersection, the dark dots vanish

The original version of this illusion was first reported by German physiologist Ludimar Hermann in 1870,
and simply involves a white grid on a black background.   As you move your eyes around the image,
dark dots quickly appear and disappear at the intersections.   However, whenever you look directly on any
intersection, the dark dots vanish.   For years it was widely believed that the illusion worked because of
'lateral inhibition' - the term used to describe the complex way in which the cells on the back of the retina
respond to areas of black and white.   There is, however, little point in explaining the theory.   Why?
Because a few years ago it was shown to be completely untrue, and thus the explanation for the illusion remains a mystery...

Explanation by RICHARD WISEMAN