 When Köhler joined Zeiss in 1900, Ernst Abbe and glass specialist Otto Schott had already paved the way for microscope improvements through their contributions to precise optical theory and the development of appropriate glass formulas.Köhler's expertise and his illumination technique helped to improve the microscope optics to achieve optimum resolution, using the entire resolving power of Abbe's objectives.Köhler remained an active staff member of Zeiss for 45 years, contributing numerous innovations during this time.