The Shadow of Mona

Here is a classic work of art! Keep your gaze fixed on the dot in the left frame for about 30 seconds. Then look at the white frame next to it, and a famous painting will appear.

This effect is usually called an "afterimage" and in this case black is exchanged for white, similar to the way in which a film negative is exchanged to a photograph when using a traditional camera (not one based on digital technology).