Diffuse reflexions of X-rays by crystals

In 1937 Dr H. Wilman and I (Finch and Wilman 1937) first drew attention to a new effect consisting in the occurrence of broad bands or areas of blackening in electron diffraction transmission patterns obtained from crystals of organic compounds such as long-chain hydrocarbons and poly- cyclic aromatic compounds. We found that the position of these diffuse areas corresponded to the arrangement and separation of the carbon atoms in the molecules. Subsequently, in collaboration with Dr A. Charlesby (1939), we were able to show that this diffuse-area pattern appeared to be due to molecules which, though orientated with respect to the beam in accordance with the crystal orientation, nevertheless bore no definite phase relationship to each other, and this pattern thus resembled that of a gaseous stream of orientated molecules. We found that this lack of phase relationship could be explained on the assumption that, due to thermal agitation, the individual molecules were displaced from their mean position in a random fashion, there being no connexion between the displacement of adjacent molecules.