An improvement in analytic instruments which measure liquid properties by optical techniques, the improvement providing an optical path through the liquid under test as low as about 0.1 mm by presence of a flat transparent surface across the optical beam and by flowing a film of the liquid under test down the surface, the depth of the film being the optical path through the liquid. Desirably the surface is normal to the beam, but at an acute angle to horizontal. A matted surface is desirable for spreading high viscosity solvents across the surface.
