The Instructional Phenakistoscope

 Purpose:  To see how a succession of still images can be interpreted by the eyes as a constantly moving figure. This phenakistoscope disc has 8 circles, inside of which are 8 small balls.  Observe the rotating disc above.  Now, click STOP to pause the animation.  This will allow you to proceed through the rest of the exercise. When looking outward from the center of the disc at each of the circles, you might notice that the position of each ball changes depending on what circle you're looking at.  If you don't see it right away, don't worry.  First, look at the topmost circle on the disc, and note the position of the ball within this circle.  Is it at the top, right, or bottom left?  Now, look at the circle just to the left of the topmost one.  Imagine that this circle has now become the topmost circle, that the entire disc has been rotated 1/8 of a revolution to the right.  Think about how the ball inside this circle would change position once the disc has been rotated.  Can you imagine where its new position would be? The ball should have moved 1/8 of a revolution in a clockwise direction around its circle.  One can see that everytime the disc turns 1/8 of a revolution, the ball in the topmost circle appears to have moved clockwise by a slight amount (that also happens to be 1/8 of a revolution).  Now the key word here is "appear," because the ball in the topmost circle hasn't really moved.  It only appears to have moved.  The 8 circles on the disc all have balls that have been displaced by a certain amount.  If you start with the topmost circle, the ball in the circle to the left has been displaced by 1/8 revolution.  The ball in the next circle to the left has been displaced by 2/8 revolution.  The ball in the circle to the left of that has been displaced by 3/8 revolution, and so on.  When the disc is rotated and one looks at the top of the disc (or at any point around the disc), these circles pass by rapidly, creating an illusion of stationary circles with balls rotating around them.