X-ray crystallography. An early design of X-ray spectroscope made in the 1910s. A beam of X-rays passes through a collimator and slit (left), hits the sample (centre) and is diffracted towards a detector (right). The detector contains a gas with a voltage applied across it - the passage of X-rays ionises atoms in the gas allowing the passage of a current proportional to the X-ray intensity. The angle of diffraction and the intensity of the X-rays is related to the structure of the crystal being studied. This apparatus was used by W.H. and W.L. Bragg during their pioneering research on crystallography and is preserved at the Royal Institution, London.