A form of reflectometer for measuring the "show through" tendencies of printing papers is described. Its grading of a series of nineteen papers covering a wide range of opacity is in good agreement with the judgment of 173 trade observers. The instrument can also be used for measuring reflexion factors. "Diffuse illumination, normal viewing" and "normal illumination, diffuse viewing" can both be used and are shown experimentally to be practically equivalent, but the latter is more convenient. The opacity figures can be converted to the U.S. "TAPPI Opacities" by means of an equation due to Judd: the results obtained are in good agreement with measurements made on the American Bausch and Lomb Opacimeter.