T. P. MURRAY1 described the measurement by ellipsometry of the thickness of oil films present on commercial tinplate. In his arrangement the elliptically polarized light reflected from the specimen surface was brought into a plane by means of a quarter-wave plate and the extinction position of the analyser found. After de-oiling the specimen a new extinction position was found by adjustment of the analyser only, the difference being proportional to oil film thickness. Angular shifts of a few tenths of a degree are common whereas the light intensity varies only slowly for several degrees about the minimum. Murray overcame this difficulty by reading the analyser for equal photoelectric responses either side of the minimum, but it occurred to me that a more elegant technique, leading to an easier to construct and possibly more accurate instrument, would be to use Faraday cells as described by E. J. Gillham2 for an automatic polarimeter.