ABSTRACT Various measures to increase the sensitivity of the polarizing microscope for visual observation are discussed. These include choice of polarizing apparatus, reducing condenser and objective aperture, reducing lamp iris, blooming lenses, use of mica plate compensators and use of bright light sources. By such means as these it is possible to detect retardations down to 0.28 A., or 1/20,000th of a wave-length. A photographic method for measuring retardations down to the same limit in small biological objects, is also described. We are deeply indebted to Mr E. W. Taylor of Messrs Cooke, Troughton and Simms Ltd., for his great help in fitting our microscope with double Polaroids and a graduated compensator mount, as well as for blooming our lenses and making special low retardation mica plate compensators. We are also indebted to Dr L. E. R. Picken and Dr A. F. Hallimond for helpful suggestions at various stages of the work.