 There he made accurate measurements of the thermal expansion of aluminium and lead and gained a second PhD in 1942, acquiring a lifelong interest in X-ray crystallography in the process.His interest was particularly stimulated by a research paper he was asked to review on deriving absolute from relative intensities of X-ray diffraction data which made him aware of the analytic power of crystallographic statistics.His book "X-ray Optics", first published in 1949, is still a definitive work on the subject.