In 1979, three British-American astronomers, Walsh, Carswell and Weymann discovered by chance in the sky the first example of a gravitational lens mirage, an illusion at the cosmic scale, which consists of two images of a bright background quasar (an active galactic nucleus), formed by a gravitational lens in the foreground. The formation of such cosmic mirages had been predicted within the general relativity developed in 1917. According to this theory, Einstein predicts that light rays emitted by a distant background source are deflected near a heavy and compact foreground massive object (the lens) located along the line of sight, like a star, a galaxy or even a cluster of galaxies. Therefore, a large concentration of mass may act as a kind of lens, called a gravitational lens.