The red radiation, 6438.4696 A., emitted by a cadmium lamp of Michelson type was first chosen in 1907 by the International Union for Co-operation in Solar Research (Trans. I.U.S.R. 2, 109, 1907) as a definition of the unit of wave-length. This primary standard was subsequently adopted by the International Astronomical Union (Trans. I.A.U. 1, 35, 1922) and by the International Committee on Weights and Measures (Procès-Verbaux Comité Int. Poids et Mesures (2), 12,67,1927). Specifications for the production of this primary standard were adopted provisionally by the I.A.U. in 1925 (Trans. I.A.U. 2, 47, 232, 1925), and by the I.C.W.M. in 1927 (Procès-Verbaux Comité Int. Poids et Mesures (2), 12, 67, 1927). Three reports of this Commission (Trans. I.A.U. 3, 77, 236, 1928; ibid. 4, 58, 233,1932; ibid. 5, 81, 299, 1935) have discussed the divergences in these specifications and pointed out the unsatisfactory features of each. This discussion culminated in a revised specification (Trans. I.A.U. 5, 303, 1935) which was adopted unanimously by the I.C.W.M. in 1935 (Procès-Verbaux Comité Int. (2), 17, 91,1935).