Arthur Gugick built this version of Hermann's Grid. Notice the pale gray dots that appear at the intersections, but disappear when you look directly at the intersection. This is a physical illusion, based on the excitation and inhibition of receptors in the eye. When a receptor is stimulated by a bright region, it is excited. But, if the receptors surround it are also stimulated by bright regions, this serves to inhibit the central receptor. I believe that this is done to enhance contrast, to help you see patterns in areas of similar colors - this will help you see the tiger camouflaged by the tall grass, which will give you some survival advantage. The points at the intersection are surrounded by more bright regions than other portions of the white lines, and so therefore these are somewhat dimmed by inhibition of the receptors. This effect is exaggerated in the peripheral vision, and so the gray dots disappear when you look directly at them.