An extended Rayleigh formulation for the solution of transport properties of arrays of spheres of one material embedded in another is applied to the case of the optical constants of an array of spherical particles of gold in a dielectric. the formulation includes, unlike the Maxwell‐Garnett theory, the effects of proximity. The optical constants of the metal are assumed to be the bulk values. A model is presented for applying the results based on the simple cubic lattice to the case of gold cermets. New experimental results are presented for Au‐Al203 cermets. The new theory explains effects present in these and previously available data.