The rectilinear propagation of light rays was known to the ancient Greeks (400-300 bc), and so were the basic principles of mirrors. They also knew that a lens could serve not only as a magnifying glass but also to light a fire. The law of refraction was more difficult to discover. Euclid had a qualitative impression of it from viewing objects immersed in water. The first common use of the pinhole camera was described by the Italian Giambattista della Porta (1536-1615) in his book Magia Naturalis from 1558. In 1550 Cardani replaced the pinhole by a convex lens, thereby making the image much brighter. One of the earliest theoretical diffraction studies of imaging is due to Airy (1835). The general problem of diffraction of aberrated wavefronts through circular pupils was re-examined by Nijboer in the 1940s. Nienhuis (1948) applied Young's idea to discuss the structure of diffraction images in the presence of aberrations.