When we were very young we were shown the ‘two noses’ illusion.
If you cross your fingers and touch a small object (such as the tip of your nose), there will seem to be two of whatever it is. Not being able to see the object strengthens the illusion, and because you can’t see the end of your nose very well it is a suitable tactile target. Besides, using your nose is amusing.
This illusion has been known for at least two thousand years. Aristotle wrote (Metaphysics Book 4):
It’s an example of a tactile illusion.
Lots of optical (alias visual) illusions are known. They startle and intrigue: some seem almost incredible. Some have been discovered or invented in the last decade, others go back centuries. They all cast light on visual perception, and are all, even the most well known, still the subject of research and often controversy as well.
Tactile illusions are less well known and most people are only aware of the crossed fingers one. But many have been discovered and written about, and new ones emerge quite regularly. There is an excellent survey here.
One I particularly like is the ‘salad bowl after effect’. Take a smoothly concave bowl (like a salad bowl) and press three fingers – there is no need for it to be hard – against the inside curve of the bowl for a few seconds – perhaps 10 – and then touch them on a flat surface. If you are like me it will feel convex, as though a bump has risen up! This strange (and to my mind eerie) feeling only last a short time but for me at least its is quite strong. I’ve even found I can make it work with the inside of my glasses case.
I wondered if there is a reverse effect – if I held my fingers against the outside of the bowl, which is convex, would a flat surface then feel concave? I tied it and there may be an effect but it seems far weaker.
At this point you may be wondering where the experiment in vision is, since everything seems to be about touch. So here is the experiment. It mixes touch, vision and awareness in a surprising way.
You and the edge of a table.
Place an index finger on the edge of the table. Allow your eyes to cross, so you see two fingers. Now think which of the two fingers you see is the one that feels the table edge. You will find that with only a small effort of will you can switch the sensation from one apparent finger to the other!
This remarkable illusion was discovered ralatively recently by Edmond Wright, see this letter to the New Scientist.