In an earlier paper on “Optical Rotatory Dispersion” (‘Phil. Trans.,’ 1912, A, vol. 212, pp. 261-297) a description was given of the measurement of the rotatory power of quartz for 24 wave-lengths in the visible region of the spectrum from Li 6708 to Hg 4358. Two important features of this research were:— (1) The discovery, after several years of work on inferior material, of a crystal of quartz of extraordinary optical purity, in which none but mechanical flaws could be detected in a plate 58 mm. in thickness and 150 mm. in diameter. (2) The use of long columns of quartz, made up of cylinders drilled from this crystal, giving even in the visible region rotations of the order of 10,000°, which could be read with an average error amounting only to a few parts per million.