This method is for determining the refractive index, accurate to one unit in the fourth decimal place, of transparent and light-colored hydrocarbon liquids in the range of 1.33 to 1.60 at a temeperature of 20 C. This method is not applicable to liquids having colors darker than ASTM Color No. 4 (see ASTM Method D 1500) or to liquids having bubble points so near the test temperature that a reading cannot be obtained before substantial weathering takes place.