With the discovery of the directional dependence of elastic and optical phenomena in the early 19th century, the special nature of the physical behavior of crystalline bodies entered the consciousness of the natural scientist. The beauty and elegance, especially of the crystal-optical laws, fascinated all outstanding physicists for over a century. For the founders of theoretical physics, such as, for example, Franz Neumann (1798-1895), the observations on crystals opened the door to a hidden world of multifaceted phenomena. F. Pockels (1906) and W. Voigt (1910) created, with their works Lehrbuch der Kristalloptik (Textbook of Crystal Optics) and Lehrbuch der Kristallphysik (Textbook of Crystal Physics), respectively, the foundation for theoretical and experimental crystal physics.