consisting of fine opaque sheets are placed against each other and held between two thick plates in order to prevent any deformation of said plates consisting of fine opaque sheets this is because one difficulty in production is the tendency to the deformation of the fine sheets which do not have the necessary rigidity and must therefore be placed between thicker supports according to one characteristic of the invention the plates are separated from each other by layers of a transparent lubricating liquid this is because in the contrary case friction between the plates make correct functioning difficult another solution is to use plates which do not touch each other but this solution is difficult since it requires excellent surface evenness of the plates sliding of one plate with respect to another can in general take place along two axes however the system is simplified according to one characteristic of the invention if this sliding takes place along only one axis in this case it is possible to use a guide rail to help maintain correct relative positioning of the plates however such a rail is expensive and poses problems of positioning in order to facilitate the relative positioning of the plates and according to one characteristic of the invention two adjacent plates sliding with respect to each other along one axis are positioned with respect to each other by microscopic guide rails a microscopic rail being fragile it is preferable according to one characteristic of the invention to use several microscopic guide rails these rails can for example be produced by lithography various solutions can be used for the arrangement of the plates and the distribution of the holes on the plates according to one.