n. An instrument used for measuring the refractive indiees of different substances.n. In the case of transparent solids which can be had in the form of a prism of known angle, the index of refraction may be determined by the deviation of a ray of light passing through the prism, and the index of refraction of transparent liquids may be determined by placing the same within a hollow prism of glass, measurements in both cases being made with an ordinary spectrometer. Where this direct method is impracticable or inconvenient, refractometers based upon other optical principles are employed. For the determination of the index of refraction of transparent solids, with plane and parallel faces, or of liquids contained in a vessel with such faces, the interferometer is sometimes employed. One of the earliest forms of interferometer, known as Jamin's refractometer, was devised by that physicist for this purpose. (See interferometer.) Other refractometers are based upon the principle that the limiting angle of total reflection at the interface between two media depends upon the ratio of their indexes. In Abbe's refractometer, an instrument of this type which is used for the determination of the index of liquids, two right-angled prisms of glass, ABC and DEF (fig. 1), are mounted with the faces AC and EF parallel. If a layer of liquid the index of refraction of which is less than that of the glass is placed between these surfaces, a beam of light from below will be totally reflected at the face EF, provided the angle i is less than the limiting angle. If i exceeds that value, the ray passes on through the liquid and the upper prism. The angle at which this change from transmission to total reflection occurs depends upon the ratio of the indexes of the glass and the liquid, and thus affords a measure of the index of refraction of the latter. Instruments similar in principle have been devised by Wiedemann, Terquem, Kohlrausch, Pulfrich, and others. For the measurement of the refracting power of crystals the method of total reflection is likewise employed. In Czapski's refractometer, a rim of glass is mounted upon the polished end of a cylinder of the same substance, and the specimen S (Fig. 2) is placed in a liquid of higher index, with its lower face parallel to the end of the cylinder. The limiting angle of total reflection at the interface between the crystal and the liquid is determined by observing the angle, i, at which a beam of light, ac, which reaches the face of the crystal at grazing incidence, emerges from the side of the cylinder below. In a similar instrument designed by Abbe for the determination of the indexes of refraction of crystals, the crystal is placed upon the horizontal plane face of a hemisphere of glass, G (Fig. 3), with an intervening layer of liquid the index of refraction of which equals or exceeds that of the glass. The beam of light a is totally reflected downward at the lower face of the crystal C, and its direction is determined by means of the position of a reading telescope which moves around a vertical circle (not shown in the diagram). By turning the specimen about a vertical axis its indexes in various azimuths may be determined.
