A refractometer is disclosed having a light source and an optical window to be positioned in a process liquid. A beam of rays is directed from the light source to an interface (RP) between the process liquid and the optical window and leading back part of the beam of rays refracted from the interface to the inside of the optical window. An image formed in the above manner is observed. The beam of rays from the light source is directed to the interface (RP) between the process liquid and the optical window from the side of the process liquid and in the direction of the interface.