[zeroca]

O.k. Then you remember my posts about central and peripheral visions (peripheral, i.e. field of vision). Narrow your eyes (up to very small vision slit) and look at grid only with your central vision. You will notice that effect of illusion eliminates. Even you can move (turn your head) left and right in turn with narrowed eyes and you will notice that illusion doesn’t exist.

Another way: you should fix your binocular vision on one point of grid without any little shifting of them (trying your best not to move them), but it's hard and not everybody is able to conduct.
You will notice that illusion eliminates. What’s the reason?