 These include the development of a microscope operating with ultraviolet light (together with his colleague Moritz von Rohr), pioneering what would become the starting point for fluorescence microscopy, and the discovery of grid illumination, a method that would later be used in the treatment of tumors.A suggestion by Köhler led to the development of parfocal lenses which allow the specimen to remain in focus when changing objectives on a microscope.As a member of Zeiss, August Köhler filed at least 25 patent applications in Europe as well as at least ten patents in the USA.