'Refractometer' definitions:

Definition of 'refractometer'

noun

Measuring instrument for measuring the refractive index of a substance

From: GCIDE

Refractometer \Re`frac*tom"e*ter\ (r?`fr?k*t?m"?*t?r), n. [Refraction + -meter.] (Opt.) A contrivance for exhibiting and measuring the refraction of light. [1913 Webster]

From: GCIDE

Interferometer \In`ter*fe*rom"e*ter\, n. [See Interfere and -meter.] (Physics) An instrument for measuring small movements, distances, or displacements by means of the interference of two beams of light; -- formerly also called also refractometer, but that word now has a different meaning. [Webster 1913 Suppl. +PJC]