Measurement of fats in oil and other such products giving a reading directly in terms of the oil scale also known as butyro scale, the micrometer eye-piece allows an accuracy on this scale of 0.1 and the oil scale may also be converted into a refractive index scale using conversion tables. This allows measurements on materials with index of refraction in the range of 1.42 to 1.49 with an accuracy of 0.0002 as tables are supplied for converting oil scale reading into index of refraction, relative to mean sodium lines.The prism box is water jacketed for temperature control and a sample achromatizing device enables the removal of all colours from the border lines thus enabling the instrument to be used with whitelight.