On the Determination of the Cholesteric Screw Sense by the Grandjean-Cano-Method

A simple and accurate method to determine the pitch of a cholesteric substance is the observation of disclination lines in a Grandjean-Cano-wedge [1—3] . However, from the position of these lines nothing can be said about the sense of rotation of the cholesteric helix. The determination of the sense is often made by either an additional experiment [4] or by a special construction of the wedge allowing to rotate its boundaries [5]. Berthault et al. [6] give a rule which allows to determine the screw sense directly by observing a Grandjean-Cano-wedge under a polarizing microscope. Their argumentation is however very scetchy and the condition of applicability is unclear. For this reason we present here a coherent derivation of the rule.