Re the moving mirror -- it's dawning on me that I've not been clear on the meaning of a mirror "moving parallel to itself". If this means moving so it *remains* parallel to itself. then there are many kinds of motion that fit that description. If it means moving so that it remains in the same plane, ditto. But the paper mentioned by Censor uses this phrase. Can someone explain to me what it means? Thanks, Dan _____________________________________________________________________ "It don't mean a thing if it ain't got that certain je ne sais quoi." --Peter Schickele