Abstract: An apparatus for measuring the fat content of milk includes an optical guide having two light passageways for guiding light delivered from a light source. One of said light passageways is provided with a cuvette through which the fat of milk flows. Disposed adjacent to the cuvette is an optical correction device, which includes an open window and at least one optical correction window. The optical correction window comprises a transparent glass plate over which there are uniformly formed a number of scratches each having such a dimension as is capable of giving an optical result corresponding to the milk fat particles.