A device for quickly and accurately comparing the refractive index of an optical immersion liquid with the refractive index of a reference glass that the liquid is to be used with. The device comprises a cavity to at least partially contain the immersion liquid by the reference glass. The containment may be total, as in a hollow core, or may be between two windows composed of the reference glass and assembled in a spaced, facing relationship, one window having an inner face with a plano portion and a portion with a continuous slope, and a cavity intermediate the windows to contain the liquid being compared. The cavity is then filled with the liquid to be compared, transmitting laser light through the windows and the liquid to form an optical interference pattern. The method comprises filling the cavity in the device with the liquid being compared, allowing the unit to reach thermal equilibrium, inserting the device in the working path of an interferometer and transmitting laser light through the windows and liquid to create an optical interference pattern, measuring the optical path difference (OPD) shown by the pattern and determining the difference in refractive index An from the formula ##EQU1## where .DELTA.T is the depth or height of the slope in the one window.