Correlation between the light diffraction pattern and the structure of a muscle fibre realized with Ewald's construction. 
Ewald's construction of reciprocal space is used to explain the diffraction pattern obtained by illumination of a skeletal muscle fibre with laser light.  The question of whether the illuminated fibre segment acts as a monocrystal or as a conglomerate of crystallites is answered by determining the longitudinal extension of the layer lines; neither the whole muscle fibre nor the single myofibril acts as the diffracting unit.  The characteristic fine structure within a layer line ('streaks') is explained by Ewald's formalism as being caused by diffraction from partial volumes of the illuminated fibre segment.  These partial volumes constitute the main diffracting units.  Any cross-section of a 100 micron diameter fibre contains 3-6 such units.  They are delineated from each other by the skew of the diffracting planes with respect to the fibre axis, and possibly by small differences in sarcomere length.  Within a unit, skew angle and sarcomere length are nearly constant.  Light intensity measurements along the length of a layer line at various tilt angles (omega) between fibre and beam axis provide an overview of the size and the skew angle of the various diffracting units within the illuminated fibre segment.  The cross-sections of the diffracting units were measured in 'mini'-omega-scans of streaks created by them and from the maximal streak length.  In general, the units had noncircular cross-section, the length of the main axis ranging between 10 micron and the fibre diameter.  The axial extensions of the diffracting units determined in fibre length scans was 100-300 micron.  The results of diffraction experiments reported in the literature as well as the data from light and electron microscopy are in agreement with the fibre structure suggested above.  Sarcomere length determination by light diffractometry is discussed within the framework of the Ewald formalism, and recommendations for an experimental procedure are given.