\▐rē-▐frak-▐ta-mә-tәr\ n [ISV] (ca. 1859) An instrument used to measure the index of refraction of transparent materials, both solid and liquid. Refractive indices are often taken as a guide to the purity of raw materials, such as drying oils. The Abbé design is convenient and is used worldwide. See Index of Refraction, Differential Refractometer, and Abbé Re-Fractometer.