IN Nature of February 13, p. 298, Sen and Roy report new equatorial reflexions in the X-ray diagrams of native and mercerized celluloses; the corresponding mean spacings are 13.6 A. for native, and 14.4 A. for mercerized, cellulose. Similar extra reflexions have been reported from time to time, but they have always been found to be spurious, in the sense that they were not due to diffraction of the characteristic radiation that was employed. They may even appear when supposedly monochromatic radiation, obtained by reflexion from a crystal, is used, if precautions are not taken to eliminate radiation with a wave-length half that of the selected radiation1,2. However, a commoner cause of spurious reflexions is the diffraction of certain components of the ‘white’ radiation present in the X-ray beam when unfiltered or filtered radiation is used3–6.