The author first presents a summary of previous methods of measuring the absolute reflecting powers of surfaces. Nutting's apparatus depended on the principle of infinite parallel planes, and was subject to various inaccuracies. Most of the other devices utilise the integrating sphere. Apparatus. - The new reflectometer devised by the author utilises a sphere with whitened interior, a portion of which can be removed and the surface to be examined substituted. A narrow beam of light can be directed on to this area through an aperture in the sphere immediately opposite. The brightness photometer is directed towards the wall of the sphere through an aperture such that the line of view is at right angles to the beam of light. The positions of photometer and tu

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