2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/208401We compare the option pricing formulas of Louis Bachelier and Black-Merton-Scholes and observe -- theoretically as well as for Bachelier's original data -- that the prices coincide very well. We illustrate Louis Bachelier's efforts to obtain applicable formulas for option pricing in pre-computer time. Furthermore we explain -- by simple methods from chaos expansion -- why Bachelier's model yields good short-time approximations of prices and volatilities.to appear in Mathematical FinancePricing of SecuritiesHistory and OverviewProbabilitytext