This effect was discovered by the German astronomer Carl Pulfrich. He was blind on one eye.
But you need two eyes for the trick... so maybe he got the idea from his assistant? We are going to try and fool our brain.
What you need:
- Two eyes (both your own)
- String
- White modelling clay
- Dark filter (cheap sunspecs split in two will do)
What to do:
With the string and a blob of clay, make a pendulum that swings in a straight arc across your line of sight. Sit about two metres away to view the swinging blob.
It should be swinging across you - left, right, left, right... Now place a dark filter over one eye, and look at the pendulum again.
Amazingly, the pendulum will seem to swing in and out as well as across in an elliptical orbit - round and round and round...
Put the dark glass on the other eye - the pendulum's orbit reverses direction.
To measure the magic:
Do the experiment over a desk. Without the dark filter on, place some sort of marker (e.g. a pencil stuck to a desk with plasticine) just under the centre of the pendulum's left-right-left-right swing.
Now put the filter on again, swing the pendulum and put a second marker where you think the centre of the round-and-round-and-round swing is.
Use a ruler to measure it - the illusion in depth can actually be greater than the physical sideways swing!
Further information:
When you cover your eye with a filter it becomes more sensitive to light. The pupil gets bigger to allow more light in, and the signals to the brain are delayed slightly, to allow more light to enter the eye (just like a camera with a slower shutter speed).
The eye with the dark glass sees the moving pendulum delayed in time and therefore in a different position from the other eye.
The brain puts the two positions together, fooling you into thinking the blob is moving in and out in an ellipse.
This is similar to when 3-D films put two images together to fool you into thinking things are coming out of the screen.
Extension ideas:
Try filters which allow differing amounts of light through (perhaps using one or more filters on the eye together).
Click to download a PDF version 