Grab some sunglasses and watch this video to experience the
Pulfrich illusionS  Having a good weekend? Got some sun? Great.
Grab some sunglasses and stare at the computer screen. We’ll
show you an example of the Pulfrich illusion.  The Pulfrich
illusion, or Pulfrich effect, was first noticed by Carl
Pulfrich, who described it in 1922. All you need is something
that acts like a pendulum swinging in front of you. Grab a pair
of sunglasses, and put one lens in front of one eye. Look at the
pendulum again. It should suddenly look like it’s moving in an
elliptical path, instead of just swinging from side to side. The
same thing can be done with film of objects moving in a circle
on a plain. If you don’t happen to have a pendulum handy, just
click the video link and watch through your single lens. It
should look suddenly three dimensional.  Pulfrich theorized that
this effect as due to the brightness reduction in one eye, which
the eye responds to with a tiny delay in its optical
information. This gets your brain t