THE large lattice spacings occurring in such organic matter as fibres or crystalline viruses give rise to X-ray diffraction patterns with Bragg angles of minutes of arc rather than of degrees. The neighbourhood of the direct beam is, however, for a variety of reasons, easily obscured by scattered radiation and insufficient collimation, so that special measures such as very long film-specimen distances1 or focusing crystal monochromators2 have been used for obtaining satisfactory photographs. In particular, Furnas3 has recently made an important contribution to the latter method.