We have realized total electromagnetic mode confinement in a microscopic optical, Casimir-type, cavity and detected the resonant change of the molecular fluorescence time under short-pulse excitation due to a spontaneous-atomic-decay enhancement-inhibition process (Purcell effect). This corresponds to the first realization in optics of the resonant coupling of atoms with a single mode of the radiation field. © 1987 The American Physical Society.592629552958