SOME years ago, one of us1 used commercial ‘Cellophane’ in conjunction with a polarizer and an analyser to produce colours in unstained microscopic sections of decalcified teeth. Subsequent investigation has shown that a sheet of this kind of ‘Cellophane’ exhibits the curious property of behaving like a half-wave plate. We do not think that this property can be explained in the usual way, that is, by ‘Cellophane’ thickness in conjunction with ordinary and extraordinary ray velocities, because it is improbable that the manufacture of commercial ‘Cellophane’ involves such precise control of thickness as the usual half-wave plate theory demands. We are therefore led to conclude that its surfaces are instrumental in giving ‘Cellophane’ this property.