In 1948 Merton made an important basic advance in the art of ruling diffraction gratings. Since 1880 these had been ruled groove by groove by the method used by Rowlands. In place of this, Merton ruled a very fine helix continuously on a steel cylinder which he then opened out upon a plane gelatine-coated surface by his copying method. No lathe could, however, rule a helix free from errors of pitch and these Merton eliminated by an ingenious device. It consisted of a ‘chasing lathe’ by which he cut a secondary helix on the same cylinder with a tool mounted on a ‘Merton nut’ lined with strips of cork pressed upon the primary lathe-cut helix. Periodic errors were thus averaged and eliminated by the elasticity of the cork.