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The Cheshire Cat Illusion is one of the all time favourites
in my whole long life of searching for effects.
It is an optical illusion invented by Sally Duensing
and an associate about 30 or 35 years ago,
although I only came across it 20 years ago, and I have used it many times.
What I like about this illusion is it is very strong and very remarkable,
and you only need a minimum of props,
a small hand mirror, that is all. So the set-up is I'll put it over my nose here, so that
my left eye is looking directly at the camera.
The right eye, because the mirror is inclined,
is looking at the right hand wall, which is completely blank.
I bring my hand up into the middle of the mirror,
so I can now see the hand pretty well disappears,
because the camera lens is more interesting actually.
Now here is where the fun begins.
As I move my right hand, wiggle wiggle wiggle... the camera lens completely disappears.
Gosh. Absolutely completely. It comes back again, after a while,
but then instantly it disappears again, as I get motion there.
So what is going on?
At a very deep level in the brain there are two channels of vision,
one is looking at texture, size, contrast, light and these things;
and the other more important channel in the brain is looking at motion.
Motion is utterly important. I suppose when we were primitive creatures,
millions of years ago,whatever was moving was something you were
going to eat, or it was coming to eat you.
So motion takes the priority.
And this extraordinary, simple test shows it.
Now a better thing to look at than a camera is at person's face.
And for this you need to get a friendly face, a person to sit opposite you,
and hold their face still, like this, with their teeth showing,
and their eyes wide open. And they need to freeze like that for about 5 seconds.
And in that position you perform the same mirror trick like this,
hold it like that, get their face to stay still,
and as soon as you start moving your fingers
- that camera lens has disappeared again! -
the person's face disappears. But in a rather interesting way.
Because there is another factor going on here. The person's face -
what actually happens is that their eyes persist
and their mouth persists, for a little longer. The rest of the face just melts away.
Which is why she [Sally Duensing] called it The Cheshire Cat illusion.
The reason for that is that the eyes and the mouth are the biggest ways
of detecting what a person's mood is,
when you meet them in the jungle. Are they friendly, or are they fierce?
And the best way to do that is to look at the eyes and the mouth,
and that is what your brain does automatically.
So it is playing on two things; this one of motion, and looking at a person's face,
which is the strongest thing to look at, and you get this extraordinary effect.
If you have not got a friendly face to look at, I have sometimes looked at a portrait,
a big portrait, in front of me. And again the portrait disappears
while the eyes and the mouth persist.
It is really a most extraordinary illusion. It does not work for everybody,
and it does not work all the time.
So many people have a kind of sensor in the brain which says I refuse to see this nonsense,
and you don't, if you are not in the right mood.
But if you are relaxed and enjoying optical illusions, this one is the one to try,
because it works so powerfully, and it is really probably my all time favourite illusion.
The Cheshire Cat Illusion. Find a mirror!