An instrument used to determine refractive index. Several types are available. (a) The Herbert Smith refractometer measures the critical angles by total internal reflection, using a glass hemisphere through which light is directed upwards and reflected from the mineral surface. The reflected beam produces an image in the observing telescope, where a graduated scale measures the critical angle. The refractive index is then calculated by Snell’s law. (b) The Abbé refractometer is used primarily for determining the refractive index of liquids. It consists of a pair of glass prisms with a film of liquid between them. The line of the critical angle is measured through a fixed telescope and the refractive index is read off a graduated scale calibrated against a glass plate of known refractive index. (c) The Leitz–Jelley refractometer is used to determine the refractive index of small amounts of liquid. It consists of a glass prism cemented to a glass slide which can hold the liquid. A beam of light is directed at right angles to the glass and is refracted by the liquid on to a graduated scale from which the refractive index can be read.