Abstract: A refractometer with a measuring prism on whose measuring surface a sample to be tested can be mounted, which sample can be illuminated by a light beam emitted by a source of light under a range of angles that includes a critical angle for total reflection, and with a reciever for recieving the reflected light. An optical device that decomposes the reflected light into a color spectrum that is mounted in the path of the reflected light between the measuring surface and the reciever.