Gives two methods for determining the refractive index of transparent plastics, namely the refractometric method for measuring the refractive index of transparent cast, moulded or sheet materials by means of a refractometer. Recommended when great accuracy is required. Secondly, the immersion method for determining the refractive index of powdered or granulated transparent materials by means of a microscope and making use of the Becke line phenomenon. Monochromatic light should be used to avoid dispersion effects. The accuracy of this method is about the same as that of method A.