Mathematician/author Cliff Pickover posted the mind-melting optical illusion below. It's an example of the "Hermann grid illusion," first described by Ludimar Hermann in 1870 when he was reading a book about sound and noticed gray spots appearing at the intersections of lines in a diagram on the page. As explained by SharpBrains, "the illusion results from retinal cells adjusting the brightness of an image by adjusting the intensity of the light signal in many small sections, which allows you to see a wide range of both bright and dark details in the same image."
