Stratton’s inverting lenses 
yield illusory motion of a 
scene.  When Stratton turned 
his head from the position 
shown on the left to the 
position shown on the right, 
the retinal images of stationary 
objects shifted in a direction 
opposite to their normal 
direction.

Around the turn of the century, 
George Stratton, a psychologist 
at the University of California 
at Berkeley, performed an 
experiment that is still being 
discussed and disputed. For 
eight days in succession, 
Stratton wore lenses mounted 
in a tube in front of one of his 
eyes that inverted and reversed 
the images that reached his 
retina. He was interested in 
discovering whether or not the 
scene that appeared upside 
down would eventually appear 
right side up if he continued to 
wear the lenses, an issue we 
will take up in Chapter 8. What 
is of interest here is Stratton’s 
observation that objects viewed 
through the lenses appeared to 
shift in direction in an 
abnormal way when he moved. 
At first, a stationary scene 
appeared to move in the 
direction of his own movement 
and at a faster rate. We can 
conclude from this fact that, 
when there is an abnormal 
change in direction during an 
observer’s movement, things 
will appear to move. Position 
constancy is lost. More recent 
experiments indicate that the 
same is true if the rate at 
which things change their 
direction is altered during the 
observer’s movement, even if 
the direction of movement 
itself is not altered.