The disappearing ring

by Peter W McOwan and Paul Curzon, Queen Mary University of London

You borrow a ring, put it on your finger and with the wave of your other hand the ring vanishes. Another wave and it's back.

Here at CS4FN we normally teach card tricks which rely on maths or computer science techniques to work. But in the world of magic there are a whole load of tricks that use sleight of hand, clever hidden moves that require lots of dexterity and practice to make them invisible. If you are interested in this type of magic there are plenty of great tutorials online, and it makes a great hobby.

A handy trick

The vanishing ring trick uses a very simple sleight of hand to work, so we will teach you how to do it. In particular we will look at why it works by considering the way your brain processes the information about the world you see around you.

First get a ring, borrowed if you like, or if one isn't available you can use an elastic band wrapped lightly round the finger in a ring like way. The secret of this effect is simple, you distract the viewer by moving your hand with the ring up and down quickly, and simply swap two fingers while doing it.

Put the ring on your Ring finger. That's the finger next to the pinky (ie the one next to your little finger). It's best to put the ring fairly high up on the finger so it can be seen. Extend this finger while keeping the other fingers of that hand formed into a loose fist. Make sure your viewer can see the ring. Also tell them it's there, so it gets fixed in their mind. You then simply swap fingers while doing the distraction of moving your hand up and down: you pull the ring finger (the one with the ring) down into the fist and at the same time extending your Middle finger. The viewer now sees a finger with no ring. The ring has vanished. Reinforce this quickly by saying "the ring has vanished!"

Wave your hand again and reverse the secret move: middle finger goes back down, ring finger straightens upwards ... and the ring is back. Done quickly and smoothly enough the viewer's brain won't have time to realise it's been looking at two different fingers rather than just the one.

[Note from Paul...If like me you find it really hard to hold your ring finger out on its own(!!) just do it with your index and middle fingers instead...there are lots of other ways you can vary the trick. For example, as the distraction you can instead pass your other hand in front of the hand with the ring and swap your fingers as you do it. Work out a version that with practice you can do really fluently.]

Brain blocks

Several different effects of human vision help this illusion to work, preventing the viewer from working it out easily. The ring and middle fingers look, when viewed casually about the same, just different lengths. It will help disguise the length difference if you bend both the fingers slightly when you extend them. The larger movement of the hand when it is waved or when you pass your other hand in front of it to hide the swap distracts the viewer's attention. Our brains are attracted to moving things, in fact there is a specific brain region called MT that looks for movement. If we give MT some motion stimulation (like the big hand wave or the passing of the other hand in front) then MT will register this larger motion distracting it from the smaller movement made when you swap the fingers.

Persistence

Persistence of vision also helps. It refers to the optical illusion we all see when our brains remember an image even after the light that caused it has stoped entering the eye. It is like a tiny time lag that let's us smooth the changes of information coming from the world around us. If done quickly and smoothly enough the vanish and reappear of the ring which seems to happen on a single finger is supported by the persistence of vision illusion. The speed of the trick means the brain is looking for continuity, it's easier for the ring to vanish than that you are seeing a whole different finger.

Telling Stories

The final part of brain processing that helps the trick is that of you telling the story of the ring being there, vanishing and then reappearing. Human brains are susceptible to words and stories, using them to aid in the understanding of visual information. The words or narrative used to describe a situation can change how we remember the events. A classic example experiment concerns recalling a video of a car crash. One group of viewers are shown a video of a car crash and asked "What speed was the car going when it hit the other car?" Another group are shown exactly the same video but asked instead "What speed was the car going when it smashed into the other car?". This second group that hear the word "smashed" report a significantly higher speed than those who heard the word "hit".

All these factors combine to give this very simple trick mechanic the ability to fool people. The quickness of the hand deceives the eye through distraction, continuity of vision and a well told story.

Don't fool them by accident

Does this have anything to do with Computing? Well yes. When designing software that you have to understand how easy it is for our brains to be deceived and make sure your software doesn't fool someone's brain by accident.