An instrument for measuring the index of refraction of a liquid, gas, or solid. Refractometers in general use in meteorology operate in the microwave region and are based on the principle that the resonant frequency of a cavity depends on the dielectric constant of its contents. In the Crain refractometer, a microwave oscillator is stabilized by the cavity at its resonant frequency. Two such stabilized systems are used, one with a sealed cavity and one with a perforated cavity open to the atmosphere. These are spaced in frequency by 30–50 MHz, and the difference frequencies are measured. The change in difference frequency is almost linearly related to the change in refractive index of the perforated cavity's contents. The Birnbaum refractometer is similar in its operating principles.