higher than given by the equation above, say nl ----2.18 + 0.296 (k/103A --2.305)-1-2. No information is available on what effect surface imperfections may have on the ellipsometric method.The ellipsometric method could detect the small dependence of n, on formation conditions, such as was found to exist to at least statistical significance by the modification of the Abel,s method, only by working at the most sensitive 4o and D. Since data based on thicknesses of anodic oxide films derived from the density of Ta205 8.735g cm -~ of unknown origin given by the Handbook of Physics and Chemistry are still being published (11), it is perhaps worth noting that the thickness obtained from this density will be about 10% too small on the basis of the values of nl here confirmed. The fact that a good fit is obtained for 4o ----70 ~ and for 4o ~ 50 ~ confirms that the optical constants do not depend on the angle of incidence. The fit immersed as well as dry confirms the accuracy of nl and also shows that pores into which electrolyte may enter are absent. A further test is that when points of equal thickness are compared at different angles of incidence (Fig. 6, 7) experimental and theoretical results agree. The increases in thickness for one cycle of the ellipsometer plot ~/2nl cos 41 are consistent when the thickness is obtained from spectrophotometric data with adjustment of nl. ConclusionsThe chief value of the ellipsometric technique in the determination of the thickness of films would seem to lie in its application to films to which the more precise and speedy spectrophotometric location of wavelengths of minimum reflectivity (1, 6) cannot be applied either because the films are too thin (<150A or so for tantalum) or because they must be measured still immersed in the electrolyte or because the optical constants of oxide and substrate are poorly "matched" for sharp interference minima. The method is very sensitive to the film thickness at certain thicknesses and quite insensitive at others, as may be seen by examining the figures. The measurement of intensity of reflection of p-light ( 7) is more precise for the determination of the optical constants of the oxide and metal.

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