Geometric representations of crystalline substances. As early as 1665, the British physicist, Robert Hooke (1635-1702), suggested that various forms of crystals might result from different arrangements of spherical particles. Over a century later, the physicist, William Hyde Wollaston (1766-1828,) invented a reflecting ‘gionometer' for measuring the angle of crystals. The German chemist, Eilhard Mitscherlich (1794-1863), discovered ‘isomorphism' whereby substances similar in chemical composition crystallise in identical or similar forms. Experiments by the French bacteriologist, Louis Pasteur (1822-1895), showed that crystal formation was related to chemical structure. This prepared the way for theories of the arrangement of atoms in compounds.Engraving by John Pass after H Lascelles. Published by J Wilkes, London, 1802.