An interferometric method is described for determining the absorption coefficients of metals from the change in phase accompanying the reflection of light at normal incidence. The reflecting layers in an interference filter are made of the metal being studied. From the optical thickness of the dielectric between the reflecting layers and the wavelength of the light transmitted, the phase change can be calculated. A monocromatic light source is not required, and with photographic methods it is possible to extend the measurements beyond the visible region. Results are given for silver and aluminum in the wavelength range of 3600A to 8100A. For aluminum the index of refraction is also given for this range. It was found that the dispersion of evaporated films of MgF₂ increased with thickness. An interferometric method is described for studying the rate of growth of corrosion layers on films which have been exposed to air. Silver films could be exposed without any apparent corrosion. The oxide layer on aluminum films reached a maximum thickness of about 45A.

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