Angewandte Chemie International Edition
© WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Cover Picture (Angew. Chem. Int. Ed. Engl. 1/1992)
The cover picture is not “abstract art”, but a crystal plate observed through crossed polarizers (45°) along  of 1,8-dihydroxyanthraquinone. Its optically anomalous behavior (birefringence) was reported long ago, but recently such compounds have also been of interest as potential materials for nonlinear optics. The most famous optically anomalous compound is probably the silicate, apophyllite, which already at the beginning of the 19th century was Brewster's main interest. A complete explanation for the cause of birefringence in apophyllite is still not known today. McBride and Kahr report on the historical argument surrounding the attempts to explain the optically anomalous behavior of particular crystalline compounds and on the current level of knowledge on p. 1ff.