A type of optical observation for the study of very thin metal films is reported. The shape or intensity distribution of reflected interference fringes from an interferometer which involves the thin film as its front or incident surface is observed to undergo a cyclic change as the metal film increases in mass per unit area. The effect is studied throughout the visible spectrum for the metals Ag, Au, Al, Cu, and Cr, evaporated in such a manner as to form films of gradually increasing mass per unit area up to an equivalent thickness of 400 angstroms. The effect is sensitive to the rate of formation of the films, to effects of increased temperature of the substrate, and correlates with recent electron microscope observations on the stable form of thin films of these metals. Brief mention is made of the practical applications of the several types of reflection fringe intensity distributions which are possible, to current investigations of the mechanisms of slip and twinning in metal crystals.
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