A method has been developed by which the optical constants of an absorbing layer on an absorbing base can be determined. The amplitude and phase of the light reflected by such a system is a known function of the parameters - complex refractive indices of film and substrate, film thickness and angle of incidence. But the equations are intractable and cannot be solved for the refractive index of the film. However, the use of a search technique for an electronic computer makes the reverse calculation possible. Given suitable starting values, one can determine fairly quickly the film index giving rise to measured values of the intensity. and phase difference of the reflected light. Application of this method to thin layers of "nickel black" shows that the layer is inhomogeneous, i. e. the complex refractive index varies with distance from the substrate. The consequences of in homogeneity are discussed. It is pointed out that many of the values for the optical constants of thin films which have been reported by other workers are in fact "average" values for inhomogeneous layers. Such values have only limited significance in predicting the optical behavior of inhomogeneous layers.
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