The Troxler Effect is a phenomenon in which a stationary visual stimulus, such as a dot or a circle, gradually fades away and disappears from view when stared at for a prolonged period of time. It was first described by Swiss physician and philosopher Ignaz Paul Vital Troxler in
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.