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The Oxford Companion to the Mind


Mirrors and in-and-out reversal
Richard Gregory Scientist
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Among the most amusing, I think, phenomena of perception are not in oneself but they’re outside oneself, namely, the ordinary mirror, like a bathroom mirror, a flat mirror. I’m not talking about curved mirrors, that’s very important, flat ones are amazing enough. Now, all sorts of things about this. First of all, you look in the glass and you see yourself through the glass and you know and, indeed, you can see yourself in front of it so you’re split into two. So instead of just being you looking out at the world, here is you looking at you in a different space in front of you and that itself is pretty amazing, it really is. The fact that they reverse in and out is interesting but easily explainable with a ray-diagram bioptics, that reversal in depth is not a problem but then you get the ancient problem, it goes back 2,000 years or more, when you look in the looking glass, you hold a book, let’s say, in front of it, the letters look the wrong way round such that the rights and the lefts get reversed but the top and the bottom don’t. In other words, an E looks crazy, an M looks perfectly normal in a mirror, a capital M. Now, why is it asymmetrical that you get sideways, apparently, reversal and not up and down although the mirror itself is completely symmetrical? Now, what is fun about this is that you can’t explain that by physics in any simple way. You can’t explain it by ray-diagram because you can always turn the diagram around. It’s like a map, where is north? You can’t do it from the map itself, you have to have the map oriented. You have to know where north is from beyond the map, not within the map, then you turn the map around and there you set it to where north is, the map can’t tell you where north is. And neither can the ray-diagram tell you right and left and up and down. You can always twiddle it, and the answer is, in my opinion, that when you hold a book in front of a mirror, it’s easiest to think about with a book, you’re reading the book, looking at it, but you have to turn the book around for it to face the mirror and when you turn a book around, its vertical axis, then it gets left and right reversed because you’ve turned it so the right and left gets reversed, but if you turn a book the other way round, around its horizontal axis, what happens? It isn’t right left reversed, it’s up and down and it’s actually up and down and it looks up and down so that you see what you’ve done. And what people forget is what they’ve done to make the object face the mirror, how they’ve rotated it. Ditto with yourself. If you stand on your head in a mirror, you’re not right and left reversed, you’re simply upside down. It’s a very interesting thing but one is so used, certainly if you’re in physics, to think of a ray-diagram as giving all the answers in optics that the ray-diagram is irrelevant for this but it is relevant for in and out reversal so it’s quite a funny business. Can you go on a bit about in and our reversal? In out reversal? Yes. Well, in out reversal is simply that the length, the path length of the light changes. You walk away from a mirror, then the path length from your eye to the mirror and back again to your eye or to a book that you’re looking in the mirror, increases as you or the book get further away from the mirror and that’s as simple as that, and that produces the reversal in depth which is a change in the increase in path length, and as you move it towards you, it goes through the mirror, opposite to you, because its path length has changed, increased. And that’s straightforward physics whereas the right left reversal thing is physics but it’s not obvious physics. You’ve got this rotation thing taking place which one forgets about and, again, this is so important in thinking about what is mysterious or what is difficult to think about so it’s a lesson in thinking and I think why this is so puzzling, lots and lots of very clever people have got this really absolutely wrong this business, is because one is normally thinking in terms of things like ray-diagrams or the fact that the eyes are asymmetrical because they’re horizontally separated like that, or that the words, right and left, are ambiguous or something, all these have come up with different theories because they’re tempting alternative theories and you’ve got to latch onto the right way of thinking about it which is this rotation thing which is a very unusual way to have to think, you know. Exactly the same really with visual illusions. Where you get these alternatives as ambiguity, it’s because there are viable possibilities which rival the truth and the brain will entertain them and so here the brain will entertain these various ways of thinking about it, only one of which is right, and that one is unlikely to be the path to follow. That’s why the mirror problem is tricky, I think, until you’ve seen it, of course, then it’s obvious.

The late British psychologist and Emeritus Professor of Neuropsychology at the University of Bristol, Richard Gregory (1923-2010), is well known for his work on perception, the psychology of seeing and his love of puns. In 1978 he founded The Exploratory, an applied science centre in Bristol – the first of its kind in the UK. He also designed and directed the Special Senses Laboratory at Cambridge which worked on the perceptual problems of astronauts, and published many books including 'The Oxford Companion to the Mind', 'Eye and Brain' and 'Mind in Science'.

Listeners: Sally Duensing Adam Hart-Davis

Sally Duensing currently is involved in perception exhibition work and research on science and society dialogue programmes and is working with informal learning research graduate students and post-docs at King's College, London. In 2000 she held the Collier Chair, a one-year invited professorship in the Public Understanding of Science at the University of Bristol, England. Prior to this, for over 20 years she was at the Exploratorium, a highly interactive museum of science, art and perception in San Francisco where she directed a variety of exhibition projects primarily in fields of perception and cognition including a large exhibition on biological, cognitive and cultural aspects of human memory.

Born on 4 July 1943, Adam Hart-Davis is a freelance photographer, writer, and broadcaster. He has won various awards for both television and radio. Before presenting, Adam spent 5 years in publishing and 17 years at Yorkshire Television, as researcher and then producer of such series as Scientific Eye and Arthur C Clarke's World of Strange Powers. He has read several books, and written about 25. His latest books are Why does a ball bounce?, Taking the piss, Just another day, and The cosmos: a beginner's guide. He has written numerous newspaper and magazine articles. He is a keen supporter of the charities WaterAid, Practical Action, Sustrans, and the Joliba Trust. A Companion of the Institution of Lighting Engineers, an Honorary Member of the British Toilet Association, an Honorary Fellow of the Royal Photographic Society, the Royal Society of Chemistry, the Society of Dyers and Colourists, and Merton College Oxford, and patron of a dozen charitable organizations, Adam has collected thirteen honorary doctorates, The Horace Hockley Award from the Institute of Scientific and Technical Communicators, a Medal from the Royal Academy of Engineering, the Sir Henry Royce Memorial Medal from the Institute of Incorporated Engineers, and the 1999 Gerald Frewer memorial trophy of the Council of Engineering Designers. He has no car, but three cycles, which he rides slowly but with enthusiasm.

Duration: 5 minutes, 20 seconds

Date story recorded: June 2006

Date story went live: 02 June 2008