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It has b een found from theoretical considerations that the use of a cover glass may result in an error of as much as 10 percent when two diffusing surfaces, one light and one dark, are compared for apparent reflectance. The error arises from multiple reflections between sample and cover glass, the lighter sample obtaining much more added illumination from this cause than the darker. The error de-pends on the angular distribution of the illuminant, the angle of view, and the diffusing characteristics of the samples as well as their reflectances.