The Collaborative International Dictionary of English v.0.48:

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]

The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

refractometer
    n 1: measuring instrument for measuring the refractive index of
         a substance