On Tue, 19 Mar 1996, Meryem Constance Ersoz wrote:
> I was giving a presentation on early film to an audience of non-academics
> last Friday and was explaining a bit about its pre-history and the
> persistence of vision when an incredibly astute first-grader--in an
> audience otherwise consisting entirely of adults--asked me if it could be
> used to explain why wheels which are going forward sometimes appear to be
> going backward or in reverse of the direction they ought to be spinning
> when they are filmed. Yow. (After my talk, the series organizer mentioned
> to me that this shrewd little guy causes his first-grade teacher a great
> deal of angst and suffering.)
> So I'm kicking the question back to the film list. Can anyone answer this
> for me? Why do wheels, when filmed, sometimes look as if they are
> spinning in a direction which appears to be the opposite of the direction
> which logic tells us they actually must be spinning? Does persistence of
> vision have anything to do with it?
The phenomenon has nothing to do with film. It's the phi-thing playing a
trick on you. You may remember the early days of hifi, when with had
these turntables we used to place vinyl-records on? Well, the fancy ones
had a little knob with witch you could adjust the speed of the turntable.
So how do you know the speed is exactly 33 rpm? Was it a cute little LED-
display? Or maybe you had to actually count? Nope. The solution was a
stroboscope. Along the edge of the turntable there was a series of small,
rectangular dots. There was a small light right by the plate, so you
could see them in the dark. The cute thing was, if you played the record
at *exactly* 33 rpm, the dots would seem to stand still! If you played it
to fast they would move clockwise, if you played it to slow they would
move counter-clockwise. It's the same thing with the wheels, it all
depends on the speed of the wheel, and the distance between the dots,
sticks or whatever that makes you see movement.
Following this, one could imagen a complete picture made in such a way
that you could only see it while rotated at a certain speed.
A couple of years back, when I had a year of psychology, our teacher
demonstrated the phi phenomenon. He had this device with two electric
lights on, lit one at a time. He could adjust the speed of the on-off
turning, as well as the distance between the lights. When everything else
in the room was completely dark, and the two variables described was
correct, it would seem as if the *same light went back and forth*, between
right and left.
The phi phenomenon is one of the triumphs of gestalt psychology, a branch
of psy. reaching it's peak in the 30's and 40's. They where particulary
interested in the phi phenomenon and other odditys that played tricks on
our eyes. For further reading: try the book "Psychology" by Gleitman.
Hope this cleared up something (and, although probably being one myself
once, I hate these little smart asses as well).
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