This refractometer is used in conjunction with a light scattering apparatus to determine high molecular weights. The self‐balancing feature relieves the operator of the tedium of making delicate visual observations while retaining high sensitivity and accuracy. Mounted on a shop surface plate it measures 15 in. wide by 18 in. long by 7 in. high exclusive of the light source transformer. A Brown Electronik amplifier and servo motor are used to position the photocells on the light beam with a repeatability of about 30 microinches. The beam deflection is read on a counter and vernier dial. The range of the instrument is 0.0175 RI unit and the stability and accuracy are within 5×10−7RI units.Differential refractometers are also useful in following fractionation by distillation or adsorption, in determining concentrations of colorless solutes in dilute solutions and in studying sedimentation equilibrium.