Wiktionary
abbe refractometer
n. (context physics English) An instrument used to measure the refractive index of a liquid, by utilizing the critical angle for total reflection.(R:MW3 1976: page=2)(R:SOED5: page=3)
Wikipedia
Abbe refractometer
An Abbe refractometer is a bench-top device for the high-precision measurement of an index of refraction.