  We have performed comparative measurements of the Casimir force between a
metallic plate and a transparent sphere coated with metallic films of different
thicknesses. We have observed that, if the thickness of the coating is less
than the skin-depth of the electromagnetic modes that mostly contribute to the
interaction, the force is significantly smaller than that measured with a thick
bulk-like film. Our results provide the first direct evidence of the skin-depth
effect on the Casimir force between metallic surfaces.
