One of the earliest important consequences of Max von Laue’s great discovery of X-ray diffraction was its use in verifying the theory of the atomic numbers of the elements. This was done by H.G.J. Moseley in Manchester towards the end of 1913, when he measured the frequencies of the K-spectra of the elements from calcium to zinc. It may be interesting to tell how this came about, and in describing it I propose not to adhere to the rule that no discovery can be claimed until it has been published. It seems to me that a juster picture will be given by a record of the opinions prevailing in the laboratory, and the gradual growth of the conviction with which they had been held often months before any publication.