The instrument is primarily intended for the determination of dry weight of biological objects through measurement of the optical path difference (o.p.d.). This is accomplished with the aid of a shearing interference microscope and a servo driven analyzer. The specimen is placed on a precision scanning stage capable of moving in two perpendicular directions. The scanning is performed in a pattern consisting of equidistant parallel lines covering the whole object. During scanning a recorder pen draws momentary o.p.d. values on a chart that moves synchronously with the specimen, thus giving “cross sections” of the object. The area below this curve is continuously measured and recorded by an integrator, thereby giving data that are easily calculated into total dry weight values.