Variations on the Hermann grid: an extinction illusion

When the white disks in a scintillating grid are reduced in size, and outli ned in black, they tend to disappear. One sees only a few of them at a time , in clusters which move erratically on the page. Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are n ot actually present. Some black sparkling can be seen at those crossings wh ere no disk is seen. The illusion also works in reverse contrast.