Interpretation of light diffraction by cross‐striated muscle as Bragg reflexion of light by the lattice of contractile proteins.

1. The diffraction spectrum obtained by illuminating a cross-striated muscle fibre with a narrow beam of monochromatic light generally shows asymmetry of light intensity between left and right spectral lines. 2. The asymmetry of light intensity in left and right spectral lines is reversed when the fibre is rotated by 180 degrees. This indicates that intensities of the spectral lines are determined more by Bragg reflexion than by simple diffraction as in a plane grating. Populations of myofibrils with differing tilt of lattice planes must exist in each illuminated fibre segment to account for simultaneous Bragg reflexion to the left- and right-hand sides. 3. The light intensity of spectral lines shows a complicated dependence on the angle at which the incident beam is directed against the fibre. This 'intensity distribution' seems to reflect the specific myofibrillar arrangement at the site of illumination. 4. The intensity distributions of the left and right first order lines show close correlation if the deflexion of the incident beam differs by twice the Bragg angle. 5. The deflexion angle of the incident beam at which maximum intensity in spectral lines is obtained depends on sarcomere length as predicted by Bragg's formula. 6. Intensities of the first and second order lines are little correlated when recorded at the same beam deflexion, but are well correlated when recorded at beam deflexions calculated from Bragg's formula. 7. When a diffraction line is scanned perpendicularly to the meridional plane, the light intensity shows fluctuations resembling those obtained by beam deflexion within the meridional plane.