The 1932 report of this Commission, and the report of the ensuing discussion, revealed an unsatisfactory position with regard to sources of the red line of cadmium which could be relied upon to give the adopted standard value for the wave-length 6438.4696 x 10−10 metre.

Dr Meggers has summarised the position very clearly in a recent paper on “ Interference Measurements in the Spectra of Noble Gases” (B.S.J. Research, 13, 293, 1934) and has expressed a strong preference for the specification of the Michelson lamp adopted in 1927 by the International Conference of Weights and Measures (see Trans I.A.U. 4, 58, 1932). He points out that the I.A.U. specification of 1925 is less restricted, inasmuch as it does not exclude high-frequency excitation and makes no mention of the volume or capillary bore of the tube, but requires that it must give interferences with differences of path of at least 200,000 waves. The last condition is considered objectionable on the ground that this is less than half of the theoretical or actual limit of the Michelson tube, and it is further considered that cadmium sources in which any such reduction in interference order occurs will certainly yield a different value for the primary standard.