A 1930 article by G. Heink of Berlin on using the microscope to analyze rayon, as a complement to chemical testing. It covers side-view examination (fiber width, swelling value, impurities such as sulphur particles) and the preparation of paraffin-embedded cross-sections cut with a microtome. It explains how cross-section shapes reveal the rayon type and the composition of the precipitation bath, and distinguishes viscose, cuprammonium and acetate rayons. Micrographs show fibers from several manufacturers.