Portrait of the German physicist and optician Josef von Fraunhofer, 1787-1826. Fraunhofer was apprenticed as a mirror-maker and lens polisher in Munich. In his experiments to produce good quality achromatic 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. He used a diffraction grating to prove that the lines were not merely an effect of the glass of the prism. These Fraunhofer lines were later shown by Kirchhoff to be due to atomic absorption in the Sun's outer atmosphere.