 This in turn allowed the condenser to be focused on the specimen using a field diaphragm and condenser focus control.This superior illumination scheme is still widely used in modern microscopes and forms the basis for phase contrast, differential interference contrast, epifluorescence, and confocal microscopy.Köhler's groundbreaking work on microscope illumination was published in the "Zeitschrift für wissenschaftliche Mikroskopie" in 1893 in Germany, followed by an English summary of his work in the "Journal of the Royal Microscopical Society" one year later.