Joseph von Fraunhofer (1787-1826), German optician. Fraunhofer was apprenticed as a mirror-maker and lens polisher in Munich. In his experiments on lenses, he used a prism and slit to provide a monochromatic source of light. In doing so he noticed that the Sun's spectrum was crossed by many dark lines, and he proceeded to measure the wavelengths of almost six hundred of them. These Fraunhofer lines were later shown by Kirchhoff to be due to absorption in the Sun's outer atmosphere.