Jamin refractometer

Jamin refractometer

[jə′mēn ‚rē‚frak′täm·əd·ər]
(optics)
An instrument for measuring the index of refraction of a gas in which two light beams from a common source are each passed through an evacuated tube and recombined, and the displacement of interference fringes is noted as gas is slowly admitted into one of the tubes.