Model. This apparatus was used to prove that crystals are composed of regular arrays of atoms. Friedrich and Knipping allowed the rays from an x-ray tube to pas through a fine hole onto a crystal of zinc sulphide set up on a goniometer (an instrument to measure crystal angles). Opposite they placed a photographic plate, which on developing showed a regular pattern of spots. This could only have been produced by the combination of a wave system and something analogous to a diffraction grating.
© Science Museum / Science & Society Picture Library