A Nichols radiometer is the apparatus used by Nichols and Hull in 1901 for the measurement of radiation pressure. It consisted of a pair of small silvered glass mirrors suspended in the manner of a torsion balance by a fine quartz fibre within an enclosure in which the air pressure could be regulated. The torsion head to which the fiber was attached could be turned from the outside by means of a magnet. A beam of light was directed first on one mirror and then on the other, and the opposite deflections observed with mirror and scale. By turning the mirror system around so as to receive the light on the unsilvered side, the influence of the air in the enclosure could be ascertained. This influence was found to be a minimum, and to have an almost negligible value, at an air pressure of about 16 mm of mercury. The radiant energy of the incident beam was deduced from its heating effect upon a small blackened silver disk, which was found to be more reliable than the bolometer when it was first used. With this apparatus the experimenters were able to obtain an agreement between observed and computed radiation pressures within about 0.6 of 1%.