The Bragg initial experiments

As mentioned in the Introduction, two papers (1) were presented to the Bavarian Academy of Sciences; the first on June 8th 1912 communicated by Sommerfeld described the experimental results and with the names of all three authors (Friedrich, Knipping and Laue). The second paper, from July 6th 1912, was authored by Laue alone. In the paper he attempted to interpret the zinc blende diffraction patterns.

These two papers were published in late August, but even before then, word got around - even to Cloughton! The way it did so is of some interest. Laue discussed his work at a meeting of the Berlin Physical Society also on June 8th and again at the University of Wurtzburg. The Norwegian, Lars Vegard, who had worked with WHB at Leeds, attended this talk and on 26th June wrote to WHB about 'new curious properties of X-rays' enclosing a photograph which he had obtained from Laue.

Laue's photograph clearly showed the wave nature of X-rays but both father and son were reluctant to abandon the corpuscular hypothesis and on their return to Leeds carried out experiments and attempted to interpret the patterns in terms of corpuscles travelling along 'avenues' between the atoms. This work resulted in a letter to Nature(3) (communicated on October 18th and published on October 24th), authored solely by WHB. It represents the last attempt to 'save' the corpuscular hypothesis.

The first 'breakthrough' was made by WLB following his return to Cambridge in October. He noticed that the spots were elliptical in shape, the long axes of the ellipses being transverse to the radial direction on the film - and that this was just the sort of shape one would expect if the X-rays were reflected from sheets of atoms in the crystal (the sheets of atoms corresponding to the lines of a two-dimensional light diffraction grating).

Moreover, he had been introduced to the very simple models of crystal structures then being advanced by William Pope, Professor of Chemistry at Cambridge and William Barlow, the science-amateur who had some twenty years previously had described the 230 crystallographic space groups. But WLB knew no crystallography, not even Miller indices, which even first-year students learn about today! He did however realise, following the ideas of Pope, that if the atoms in zinc blende were packed in the form of a face-centred cubic, rather than a simple cubic lattice, he was able to explain the positions of all the spots in the Laue photograph.