The diffuse spots in X-ray photographs

A crystal consisting, as it does, of some molecular combination repeated as a unit at regular intervals in three directions in space can act as a three- dimensional diffraction grating. When monochromatic rays fall upon it, the scattered rays form a diffraction pattern, which can be derived by calculation from the diffraction formula. The pattern of ‘diffuse spots’ now under discussion agrees very closely, in fact within the errors of experiment so far made, with this calculated result. Either the conditions in the crystal which give rise to the diffuse spots are those which allow the diffraction pattern to be observed or they simulate them. This diffraction pattern differs from the diffraction effects so widely used in recent years in the examination of crystal structure. The Laue pattern of spots is not a true diffraction pattern because every spot in it is due to a different wave-length. In the examination of a crystal by an ionization spectrometer or in the oscillation photograph, the different spots are not observed simultaneously. In a true diffraction pattern all the spots or lines are due to monochromatic rays and are observed at one and the same time.