This part of GB/T 13531 specifies the method for measuring the refractive index of liquid and semi-solid cosmetics, which is divided into the first method, the Abbe refractometer method, and the second method, which are suitable for the determination of the refractive index of transparent liquid cosmetics and liquid or semi-solid cosmetics, respectively. The Abbe refractometer method is based on the law of refraction to determine the critical angle, which requires the sample to remain isotropic and transparent. The instrument is calibrated with analytical pure reagents and reference materials to ensure accuracy and Precision.