 Sometimes the direct view of the original object itself is dimmed or absent.The illusion is caused by a gravitational lens, in space between the object and the observer's telescope, which bends light as it travels.The effect is analogous to the atmospheric mirage, which has been observed since antiquity, in circumstances where the air temperature varies strongly with height over the ground or sea; the rapidly changing refractive index bends light, producing inverted and/or multiple images "floating" in the air.