Reflection of light from the common surface of two media having the same refractive index for certain wave-lengths was studied, the media employed being, first, freshly split rock-salt or clean glass against a suitable mixture of carbon disulphide and benzene, and, second, glycerine against turpentine. The reflected light did not vanish completely at the point in the spectrum where the refractive indices were equal but possessed a definite intensity that was practically constant at different angles of incidence. This reflection can hardly be due to contamination of the surface but must be due to a genuine change of refractive index in the surface layer.

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