Troxler’s Fading

https://commons.wikimedia.org/wiki/File:Troxler-Effekt.jpg

Stare at the cross from a short distance away without moving your eyes. After a few seconds, the colors will fade away.

The effect was discovered by Swiss physician Ignaz Paul Vital Troxler in 1804. The reasons for it aren’t clear — possibly neurons in the visual system adapt to unchanging stimuli and they drop out of our awareness.