A chart for interpreting the Weissenberg photographs of pseudo-merohedral twins

A chart is presented on which the appropriate zero-level Weissenberg photograph of a pseudo-merohedral twin may be superimposed. This enables the separation of twin-pairs of reflexions at any point on the film to be related to the twin obliquity angle. A separate chart which may be used with the zero-level Weissenberg photograph of any crystal, twinned or untwinned, gives the film-to-reciprocal-lattice magnification ratio corresponding to any short interval measured along a festoon.