The Troxler's Fading Effect is a phenomenon in which a stationary visual stimulus gradually disappears from an observer's vision, even when the observer is fixated on the stimulus. This effect was first described by Swiss philosopher and physician Ignaz Paul Vital Troxler in 1804.
From Wikipedia
Troxler's fading, also called Troxler fading or the Troxler effect, is an optical illusion affecting visual perception. When one fixates on a particular point for even a short period of time, an unchanging stimulus away from the fixation point will fade away and disappear. Research suggests that at least some portion of the perceptual phenomena associated with Troxler's fading occurs in the brain.