Thus, as has been suggested 
by Duncker and by the 
psychologist Gunnar Johansson 
at the University of Uppsala, 
under some conditions a path of 
motion will yield two 
components of perceived 
motion, as if the actual path 
were split into two vectors, as 
shown in the illustration at 
left. Some insight into the basis 
of this kind of effect can be 
gained by considering an 
example from daily life. 
Suppose you watch a friend 
leaving on a train. Your friend 
waves at you. You perceive the 
waving hand moving up and 
down, although, in fact, as the 
train moves forward, the hand 
is moving along a path 
resembling a sine curve. In this 
example, perceiving the hand 
as moving vertically is not an 
illusion because, relative to 
the train, it is moving 
vertically.