In the Drude approximation, and for long wavelengths, the real part n of the complex index ñ=n-ik of island films of metals is shown to be independent of the electronic properties of the parent metal, and of the wavelength of radiation. The value of the index is found to depend on the Maxwell Garnett density parameter q only, and is always greater than one, in agreement with experimental results. © 1970 The American Institute of Physics.