Sommerfeld assigned me then a thesis problem. He had liked my little paper, and so he told me 'Now you make a complete theory of electron diffraction in crystals.' And so I did and Sommerfeld still gave me advice, 'There is the theory of X-ray diffraction by Paul Ewald, you might use as a... as a basis.' And I did, and fortunately for me electron diffraction is much simpler because in X-rays you have to deal with a vector, the electromagnetic field; whereas in... in electrons you have a scalar which makes everything much easier. And without much difficulty I could produce a theory of... of electron diffraction. It wasn't a very great work, in fact Wolfgang Pauli who was always very critical told me when I first saw him, 'I have been very disappointed by your thesis. I had expected much better of you.' But von Laue who had done the first theory of X-ray diffraction back in 1913 liked my paper very well and used it in some of his later publications.