Louis Pasteur was the first person to study stereochemistry. He observed in 1849 that salts of tartaric acid collected from wine-making equipment could rotate plane polarized light, but that salts from other sources did not. This property was the only difference between the two types of salt. It is due to optical isomerism. In 1874, Jacobus Henricus van 't Hoff and Joseph Le Bel discovered the difference was caused by the way that the atoms bonded to carbon in a tetrahedral (four faced) shape.