Concentrating sunlight. Illustration showing various methods of concentrating sunlight in order to ignite some material. The main illustration depicts a lens mounted on a castle turret being used to focus sunlight onto an attacking enemy ship. At the top is a diagram showing parallel light beams being focused by a mirror. In the centre are ray diagrams of a parabolic mirror (left), spherical mirror (right) and the two foci of an ellipse (centre). At the bottom right a light source at the focus of a parabolic mirror is used to project a parallel light beam. This illustration is taken from Kircher's Ars magna lucis et umbrae printed at Rome in 1646. .