It is pointed out that the extinction theorem of dispersion theory, for which an elementary derivation is given, shows that an incident light wave is extinguished at the surface of a dielectric. This may mean that information about the velocity of light from a moving source would be lost if the light passed through intervening transparent, stationary material before it was measured. All past laboratory measurements to verify the constancy of the velocity of light from moving light sources and mirrors and from extraterrestrial sources were made only after the light had passed through stationary material. Double stars, especially close binary pairs, are surrounded by a common envelope of gas which may contain enough matter to extinguish the direct light from the stars. Thus de Sitter's proof of the constancy of the velocity of light may not be conclusive. It is concluded that there may not exist any sure experimental evidence for the second postulate of special relativity.