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Some Historical Extracts Relevant to the Discovery Crystallography Reviews ISSN: 0889-311X (Print) 1476-3508 (Online) Journal homepage: http://www.tandfonline.com/loi/gcry20 Some historical extracts relevant to the discovery and application of the diffraction of X-rays by crystals to contribute to the Centennial celebration and the International Year of Crystallography John R. Helliwell , Alexander J. Blake , John Blunden-Ellis , Moreton Moore & Carl H. Schwalbe To cite this article: John R. Helliwell , Alexander J. Blake , John Blunden-Ellis , Moreton Moore & Carl H. Schwalbe (2012) Some historical extracts relevant to the discovery and application of the diffraction of X-rays by crystals to contribute to the Centennial celebration and the International Year of Crystallography, Crystallography Reviews, 18:1, 3-19, DOI: 10.1080/0889311X.2011.641958 To link to this article: http://dx.doi.org/10.1080/0889311X.2011.641958 Published online: 16 Dec 2011. Submit your article to this journal Article views: 788 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=gcry20 Download by: [T&F Internal Users], [Joshua Bayliss] Date: 10 March 2017, At: 01:22 Crystallography Reviews Vol. 18, No. 1, January 2012, 3–19 Some historical extracts relevant to the discovery and application of the diffraction of X-rays by crystals to contribute to the Centennial celebration and the International Year of Crystallography John R. Helliwella*, Alexander J. Blakeb, John Blunden-Ellisc, Moreton Moored and Carl H. Schwalbee aSchool of Chemistry, University of Manchester, Manchester, M13 9PL, UK; bSchool of Chemistry, The University of Nottingham, Nottingham NG7 2RD, UK; cFaculty Team Manager, Engineering and Physical Sciences, John Rylands University Library, University of Manchester, Manchester M13 9PL, UK; dDepartment of Physics, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK; eSchool of Life and Health Sciences, Aston University, Birmingham B4 7ET, UK (Received 8 November 2011; final version received 15 November 2011) Illustrative extracts from the writings of Paul P. Ewald and of Max von Laue are presented. The latter in turn contains extensive text contributions from William Lawrence Bragg. These selections we have chosen so as to indicate the nature of the discovery of X-ray diffraction from crystals (experiments undertaken by Friedrich, Knipping and von Laue) and its early and prompt application in crystal structure analyses (by William Henry Bragg and William Lawrence Bragg). The platform for these discoveries was provided by a macroscopic physics problem dealt with by Ewald in his doctoral thesis with Arnold Sommerfeld in the Munich Physics Department, which is also where von Laue was based. W.L. Bragg was a student in Cambridge who used Trinity College Cambridge as his address on his early papers; experimental work was done by him in the Cavendish Laboratory, Cambridge, and also with his father, W.H. Bragg, in the Leeds University Physics Department. Of further historical interest is the award of an Honorary DSc (Doctor of Science) degree in 1936 to Max von Laue by the University of Manchester, UK, while William Lawrence Bragg was Langworthy Professor of Physics there. Keywords: Centennial celebration; discovery of X-ray diffraction; Paul Ewald writings; Max von Laue writings; William Lawrence Bragg writings; honorary DSc for von Laue Contents page 1. Introduction 4 2. Extracts from Paul Ewald’s writings [1] 4 3. Extract from Max von Laue’s writings [2] 7 *Corresponding author. Email: [email protected] ISSN 0889–311X print/ISSN 1476–3508 online ß 2012 Taylor & Francis http://dx.doi.org/10.1080/0889311X.2011.641958 http://www.tandfonline.com 4 J.R. Helliwell et al. 4. Short postscripts 13 5. Concluding remarks; glimpses of the interactions between Max von Laue and William Lawrence Bragg 14 6. Honouring the Centennial 16 Authors’ Biographical sketches 17 Acknowledgements 18 References 19 1. Introduction Although X-rays had already been discovered by Wilhelm Conrad Ro¨ntgen in 1895 and were immediately used for imaging, their nature was not properly understood for well over a decade. The platform for understanding and applying X-ray diffraction was provided by a macroscopic physics problem dealt with by Paul Peter Ewald in his doctoral thesis supervised by Arnold Sommerfeld in the Physics Department of the University of Munich. Ewald’s conversation about his thesis topic with Max von Laue, who was Sommerfeld’s deputy, prompted von Laue to speculate whether X-rays could interact with crystals, and to persuade Paul Knipping and Walter Friedrich to join in carrying out experiments. Their results were soon communicated to William Henry Bragg and his son William Lawrence Bragg, who developed the science of crystal structure determination. W.L. Bragg was a student in Cambridge who used Trinity College Cambridge as his address on his early papers; experimental work was done by him in the Cavendish Laboratory, Cambridge, and also with his father, W.H. Bragg, in the Physics Department of the University of Leeds. We present extracts from the writings of Ewald and von Laue that provide a comprehensive description of the background to and development of X-ray crystallogra- phy. An additional question concerns the personal relationship between von Laue and the Braggs. One might expect that a certain amount of scientific rivalry could have existed, exacerbated by the fact that during World War I von Laue worked on military communications for the German army, while W.H. Bragg worked on submarine detection for the British Admiralty and W.L. Bragg developed sound ranging techniques for the location of guns. However, the evidence is that these great men maintained the highest regard for each other, and von Laue’s text includes important contributions from W.L. Bragg. 2. Extracts from Paul Ewald’s writings (1) Reproduced with the permission of the International Union of Crystallography (IUCr): Paul Ewald Chapter 4 ‘Laue’s Discovery of X-ray diffraction by Crystals’ in Fifty Years of X-ray Diffraction P.P. Ewald Editor, Published for the IUCr by N.V.A. Oosthoek, Utrecht, The Netherlands. Extracted from Section 4.2 Ewald’s Thesis: Towards the end of the summer semester of 1910 the present author, Paul Ewald, had belonged to the group of students centering about Sommerfeld [in the Institute for Crystallography Reviews 5 Theoretical Physics of the University of Munich] for about two years, and he felt that he could venture to ask his teacher to accept him as a doctorand. ...At the end of [Sommerfeld’s] list stood the problem: ‘To find the optical properties of an anisotropic arrangement of isotropic resonators’. Sommerfeld presented this last topic with the excuse that he should perhaps not have added it to the others [there being ten or twelve other topics suitable for doctoral theses] since he had no definite idea of how to tackle it, whereas the other problems were solved by standard methods of which he had experience. In spite of the warning, Ewald was immediately struck by the last topic on the list, and even if he politely postponed the decision to the next appointment a few days later, he went home determined that it would be this topic or none. When this was agreed to, at the second interview, Sommerfeld gave Ewald a reprint of Planck’s paper on the Theory of Dispersion (Berlin Academy 1902), and recommended him to study H. A. Lorentz’s corresponding paper .... It should not be assumed that the division of the problem into that of dispersion and that of refraction was understood at the beginning of Ewald’s investigation – it developed clearly only in the course of the work. What Sommerfeld had in mind was this: in Planck’s and also in Lorentz’s then known work, an amorphous medium had been assumed, characterized by a random distribution of the resonators in space. This led, naturally, to a single value of the refractive index, valid for all directions of the light ray travelling through the medium. If the same type of resonators were placed in a lattice array, with perfect regularity but different distances along the three coordinate axes – would the dispersive and refractive properties of this medium be those of a crystal? Would there result, for a general direction of propagation, two refractive indices whose magnitude depends on the direction and the polarization of the wave? In other words, would it be unnecessary to assume an inherent anisotropy of the resonators themselves for the explanation of crystal optics? These were the questions which preoccupied the author in the next two years. Heavy mathematics was involved in finding a general answer, and again in transforming this answer to a form where the magnitude of the effect could be calculated. All this mathematical technique was, much later, recognized as Fourier transformation – a concept which had not yet been formed at the time – with the result that nowadays the mathematical derivations can be presented to a class of graduates in a two-hour session without undue strain. The model used for the theory was a simple orthorhombic lattice of isotropic resonators (or dipoles as they are also called); the positions of the resonators along the x, y, z Cartesian coordinate axes are X, Y, Z ¼ la, mb, nc, where l, m and n are integers ranging independently from 1 to þ1 and a, b, c are the axes or transformations of the lattice. Ewald showed that the model fulfilled the general laws of crystal optics. In order to check on the magnitude of the effect, he took, on the advice of Groth, the axial ratios of anhydrite (CaSO4), a:b:c ¼ 0.8932 : 1 : 1.0008. The result of the calculation was that in two directions, the double refraction of the model was 3–4 times the observed one, and in the third direction, it was six times smaller.
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