A welder for welding a base metal. The hybrid coating is formed from a combined aqueous sol with least one hydrolyzable silane and at least one hydrolyzable metal oxide precursor where the only organic solvents present are those liberated upon hydrolysis of the silanes and metal oxide precursors. In contrast to magnetic lenses equipped with a coil, it is not easy in the case of lenses equipped with permanent-magnetic material to alter the focusing magnetic field with the aim of altering the optical power. The first mirror layer has, in a region lying opposite the contact area of the contact structure, a cutout which is larger than the contact area of the contact structure. The device may also be installed in a window opening to serve as a window screen, if desired.