Staring at an object causes it to disappear?

This phenomenon is called Troxler effect. The Troxler effect is an optical illusion that occurs when a person stares at a stationary object for a long period of time. The object will appear to fade or disappear, and then reappear when the person looks away. The effect is thought to be caused by the adaptation of the retinal cells in the eye. When the eye is fixated on a stationary object, the retinal cells responsible for detecting that object become fatigued and stop sending signals to the brain. This causes the object to fade or disappear from view.