One afternoon in 1808 a French scientist called Etienne-Louis Malus discovered something remarkable about light reflected from transparent materials. From his home in the rue d’Enfer in Paris, Malus examined the sunlight reflected from a window in the Palace of Luxembourg, just over 1 km away to the north-northeast. Looking through a birefringent crystal, he expected to see two images of equal brightness (see chapter 3) but instead he found that if he rotated the crystal around the line of sight, each image from the window was dimmed in turn every 90 ◦. As night was drawing on, he continued his observations with candlelight reflected by glass (as in figure 7.1) and also when reflected in the surface of a bowl of water. He found that the effect was most marked when the angles of incidence and reflection were around 55◦, though slightly more for glass than for water.