Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site pucc-i Path: utzoo!watmath!clyde!floyd!harpo!ihnp4!inuxc!pur-ee!CS-Mordred!Pucc-H:Pucc-I:ags From: ags@pucc-i (Seaman) Newsgroups: net.philosophy Subject: Re: Why do mirrors reverse left & right, not up & down? Message-ID: <184@pucc-i> Date: Mon, 6-Feb-84 15:13:02 EST Article-I.D.: pucc-i.184 Posted: Mon Feb 6 15:13:02 1984 Date-Received: Thu, 9-Feb-84 13:47:20 EST References: <541@bbncca.ARPA> <543@bbncca.ARPA> Organization: Purdue University Computing Center Lines: 49 > RIZZO: I appreciate your point. I do that agree your "front-to-back > mapping" accounts for why points on the reflected object correspond > to points in the image that appear to be on the "wrong side". For- > mally restated, your "front-to-back mapping" is an automorphic (maps > an object onto itself) transformation involving a 180 degree rotation ^^^^^^^^ > about a top-to-bottom axis. In effect, yours is a proposal to trans- > fer the explanatory burden from one distinction, left/right, to an- > other, front/back. Wrong. The front-to-back mapping is not a rotation at all, but a reflection. There is no top-to-bottom axis, or left-to-right axis. There is only the plane of the mirror, which represents the plane of reflection. It is exactly this tendency to replace a reflection with a rotation which confuses people into thinking there is a left-right reversal. > RIZZO: ... Why do mirrors use a vertical axis, & not, say, a horizontal one? > > (A brief pause in the conversation ensues. Then:) > > PSEUDO-TAYLOR: They don't. We do. > > RIZZO: Intriguing! Why do we? > > PSEUDO-TAYLOR: Because our vision is binocular. Binocularity creates > handedness. PSEUDO-TAYLOR was on the right track, but he came up with the wrong explanation. The real reason that WE (not mirrors) use a vertical axis rather than a horizontal one is that: (1) It is more natural. We turn around much more often than we do cartwheels. (2) Humans (and many objects) are approximately symmetric left-to-right, but not even close to symmetric top-to-bottom. If you lie down horizontally and look into a mirror, it will seem more natural to use a rotation about a horizontal axis, since this produces an image more nearly in agreement with what you see in the mirror. -- Dave Seaman ..!pur-ee!pucc-i:ags "Against people who give vent to their loquacity by extraneous bombastic circumlocution."