Oscillation photographs from crystals of the complexes of 4:4'-dinitrodiphenyl with 4-bromo- and 4-iododiphenyl show a mixture of sharp and diffuse spots. When the oscillation is about the
c
-axis the spots on every seventh layer-line are observed to be sharp, those on all the other lines being diffuse. The diffuseness varies from line to line, but in all cases corresponds to an elongation of the maxima of the interference function in the reciprocal-lattice space in the b*-direction. The spacing of the diffuse layer-lines in the
c
-axis photographs from the complex with 4-bromodiphenyl is anomalous, and the positions of the lines correspond to those of optical ghosts accompanying the sharp layer-lines, such as would be expected if a fundamental spacing of 3.69 A were periodically distorted, the period of the distortion being rather less than 3 1/2 times that of the fundamental spacing. The diffuseness of the spots suggests that the structure also contains faults in the
b
-direction. A suggested explanation of the effects in terms of the structure is put forward. In the second part of the paper, a mathematical discussion of diffraction by a simplified model, consisting of a lattice with a periodic error in spacing in the
c
-direction, and faults consisting of sudden random changes in the phase of a periodic error in lattice planes perpendicular to the
b
*-axis, is discussed, and it is shown that the diffraction pattern given by such a structure exhibits features similar to those observed.
Kronig–Penney model in reciprocal lattice space