A mirror of known type comprises a stack of pairs of dielectric material layers of alternating high and low refractive indices and a metal layer, preferably gold, capping one end of the stack. For improvising the adherence of the metal layer to the stack, the metal is directly contiguous with a layer of tin oxide. In different mirror structures, the tin oxide can be an extra layer added between the end-most pair of layers and the metal layer; or the tin oxide layer can comprise one layer of only the end-most pair of dielectric layers in the stack; or all of the pairs of dielectric layers in the stack can comprise a layer of tin oxide as one of the pair layers.