The refractive index is an optical material characteristic, whereupon it is a dimensionless physical quantity. It indicates for which coefficient the wavelength and the phase velocity of the light is lower in the analysed fluid than in vacuum. For the determination of the refractive index we use a so-called Abbe-Refractometer. The measuring pricinple is the fact, that the critical angle of the received total reflection at the interface is depending on the used optical materials. The refractometer, which is developed by E. Abbe, enables us to determine the critical angle of a thin liquid layer, which is filled through the glass prisms, very precisely. The refractive index can depend according to the determined material strongly from the temperature. Therefore the measuring system is thermostated during the measurement.