Necker's 1832 paper reveals the famous optical illusion now known as the Necker cube
Necker, 1832: "An optical phænomenon on viewing a figure of a geometrical solid"
In 1832, L. A. Necker published observations on optical phenomena seen in Switzerland and on a phenomenon occurring when viewing a crystal or geometrical solid. This paper introduced the Necker cube, a wireframe cube that spontaneously flips orientation in perception. The discovery became a classic example of how the brain interprets ambiguous visual information.
I observed that the lines of the crystal sometimes appeared to change their relative position, so that what was before the front seemed now to be the back.