An early Bragg X-ray spectrometer. L, lead box; A, B, D, slits; C, crystal; I, ionization chamber; V, vernier of crystal table; V', vernier of ionization chamber; K, earthing key; E, electroscope; M, microscope. From W. H. Bragg and W. L. Bragg, X-rays and Crystal Structure (London: G. Bell and Sons, 1915).