An instrument devised by Nichols and Hull for use in the measurement of radiation pressure. The device has two small silvered, glass mirrors that are suspended, in the way of a torsion balance, by a fine fiber of quartz that is surrounded by an enclosure where the air pressure is controlled. The fiber is connected to a torsion head that can be turned from outside the enclosure with a magnet. A light beam strikes first one mirror, then another, and its deflections are examined with mirror and scale. The effects of the air in the enclosure are deducted by turning the mirror system so that light strikes the unsilvered side. The effect is minimal and has an almost negligible value at a pressure of approximately 16 mm of mercury. Observation of the heating effect on a small, blackened, silver disc reveals the radiant energy of the incident beam.