A refractometer has a housing (1), a measurement cell (8) arranged in the housing (1), and a lid unit (2). The lid unit has a base plate (3) with a cutout (7) allowing access to the measurement cell, and a lid (4) for covering the measurement cell. The lid is connected to the base plate by way of a hinge. The lid unit also has a lid insert (11, 17, 18, 28, 31, 35, 39) that is arranged replaceably in the lid. The lid unit (2) is detachably connected to the housing by means of a connecting element that is itself connected to the base plate.