Thursday 27 March 2008

Pulfrich Pendulum Effect

I was reading through the very first version of ITLAD today when I came across this little gem that for some reason did not make it into the heavily edited,and less scientifically-based, second version. (indeed the version that was eventually published was the third). In the chapter on time I suggested that the reader should:

"Make a pendulum out of approximately one metre of string and a bob (any reasonably heavy object will do). Find an old pair of sunglasses and take out one of the lenses. Put the sunglasses on then swing the pendulum in a straight arc at right angles to your line of sight. Now look at the bob. You will be surprised to see that the bob does not appear to swing in a straight line but describes an ellipse. What is happening is that the dark eyeglass is delaying the signal from that eye because it is reducing the amount of light being received. The receptors in your eye are therefore taking longer to respond and as such your brain is receiving the signal from this eye later than that from the uncovered eye. As the bob speeds up so the delay becomes greater and the eye with the filter lens ‘sees’ the bob further and further behind that seen by the unoccluded eye – in effect the past being perceived as the present. To accommodate this, and to continue with stereo vision, the brain mixes the two signals and in doing so you causes you to perceive an effective horizontal shift of the moving object. By perceiving two differing sections of movement-time you ‘see’ a totally inwardly created adaptation of ‘reality’.

This is known as the Pulfrich Pendulum Effect and was first described by Carl Pulfrich (1858-1927) in 1922.

In the mid 1970’s it was discovered that this Pulfrich Effect can be observed with objects lit by a stroboscope and even when viewing an ‘electric snowstorm’ on a de-tuned television set.
In one of those peculiar ironies Pulfrich himself was incapable of seeing the effect named after him because he was blind in his left eye. (See page 664 of The Oxford Companion to the Mind (1998) and M J Morgan & P Thompson “Apparent motion and the Pulfrich Effect – Perception, 4, 3-18 (1975)."

1 comment:

Karl Le Marcs said...

Regarding many of the Phenomenology of Motion effects including Pulfrich, take a look at some of the similar weirdness on the Center For Imaging Science website:

It's all fabulous fun.