X-ray focusing by bent crystals: focal positions as predicted by the crystal lens equation and the dynamical diffraction theory

The location of the beam focus when monochromatic X-ray radiation is diffracted by a thin bent crystal is predicted by the `crystal lens equation'. This equation is derived in a general form valid for Bragg and Laue geometries. It has little utility for diffraction in Laue geometry. The focusing effect in the Laue symmetrical case is discussed using concepts of dynamical theory and an extension of the lens equation is proposed. The existence of polychromatic focusing is considered and the feasibility of matching the polychromatic and monochromatic focal positions is discussed.

A Cycle Of Enzymatic Reactions With Some Properties Of Neuronal Circuits

A cycle of four methylation and four demethylation reactions with repression or allosteric inhibition of enzymes is considered. The corresponding dynamical system is characterised by two parameters: the sum of reagent concentrations (C) and the ratio of rate constants of forward and backward reactions (k). In a symmetrical case (k=1) the system has a unique equilibrium. At C>4 the equilibrium is unstable and the system has oscillatory solutions. At k essentially different from 1, the system becomes excitable or behaves as a bistable trigger.

Editorial

Editorial to the issue on "Convergent Television(s)". The history of media convergence, especially of convergent television, is a field that needs to be further investigated. Media convergence is often considered a taken-for-granted phenomenon, a kind of ‘irresistible’ force that has changed and is continuously changing media ecosystems. Furthermore, it seems to be mainly an American phenomenon because it has involved US politics and companies and because the most relevant reflections and publications on this topic come from American scholars. This issue of VIEW tries to deal with this complex and polysemic concept from different points of view, adopting several theoretical and methodological frameworks. It attempts to counteract some of the aforementioned taken-for-granted ideas, analyzing TV convergence from a historical and long-term perspective, considering symmetrical case studies of success and failures, concentrating on the European dimension through the lens of transnational, comparative, and national contributions.