Equipment for differential interference contrast in transmitted light microscopy designed, by Nomarski(1952, 1955) and manufactured under licence from the C.N.R.S. by Carl Zeiss/Oberkochen (German Federal Republic), has recently become generally available. It is one of a series of devices that rely on the interference of a pair of wavefronts (Piller, 1962) to generate contrast, and it is constructed in such a way that it can be used as an accessory on the standard Zeiss microscopes. A brief description was issued by the manufacturers in December, 1968 (Zeiss-Publication). The Zeiss equipment necessary for differential interference microscopy in transmitted light comprises the following accessories: (I) a single beam·splitting slide, consisting of a modified quartz Wollaston prism oriented at 45° to an attached analyser, and mounted in a screw-driven carriage (so that a variable amount of bias compensation can be introduced at will), which is accommodated in the normal analyser slot between the objectives and eyepiece; (II) a strain-free achromatized condenser, fitted with three auxiliary modified Wollaston prisms, in addition to two annular stops for phase-contrast microscopy (Zeiss-Publication, 1968). A detailed theoretical and practical evaluation of the prototype was undertaken by us over the last four years, at the request of Dr. Horst Piller of Carl Zeiss. The results were presented at the Centenary Conference of the Royal Microscopical Society in London, 1966 (ALLEN, DAVID, HIRSH, 1966; DAVID, ALLEN, HIRSH, WATTERS, 1966). These will be published in full later, in the Journal of the Royal Microscopical Society.