Whirling Watcher
When you view short bursts of moving images, you see some interesting effects.
A series of slits moving rapidly past your eye allows you to see images in short bursts. Such rapid but fragmented views of moving objects can make the objects appear to jerk along, change speed, or even move backward.

(30 minutes or less)

Print out the stroboscope pattern provided on a separate screen here. Enlarge it if you wish. Cut out the pattern and glue it to the posterboard.

Cut the posterboard to the same shape as the stroboscope, including the slits. You can cut with a good pair of scissors alone, or use scissors in combination with a utility knife.

Mount the stroboscope disk on the rotator. You can use adhesive Velcro™ to mount the disk to the electric drill or other device. If you use a drill with a chuck, you can use a bolt as a shaft, with two nuts to hold the disk. (You can also make a simple manual rotator by simply sticking the pushpin through the center of the disk and into the end of a pencil or wooden dowel.)

(15 minutes or more)

Close one eye. Hold the stroboscope so that the side with the horses is facing away from you, and so that you can see through a slit with your open eye. Spin the disk and look through the slits at your surroundings. Notice that you can see the entire scene on the other side of the disk, not just one small strip of it.

Try spinning the disk faster, then slower, and compare the results.

Have a friend hold a hand so that you can see it through the spinning disk. Ask your friend to move his or her hand from side to side. Notice that the movement you see is jerky rather than smooth. Have your friend move his or her hand rapidly, and then slowly. Notice that the amount of jerkiness changes as the speed of the hand movement changes.

Stand facing a mirror, and hold the disk and rotator in front of you. Be sure the disk is mounted on the rotator so that the horses are facing the mirror. Spin the disk and watch its reflection in the mirror through one of the slits. Concentrate your attention on one of the horses, and you will see it gallop!

Let water run slowly enough to produce a stream that breaks up into separate droplets as it falls. Place a black background behind the well-illuminated drops of water. Look through the spinning stroboscope and watch the water-droplets fall in slow motion. Vary the stroboscope's speed and see if you can make the water-droplets stand still or even look as if they are moving upward.

As the strobe disk rotates, a series of open slits moves rapidly past your eye. Each time a slit passes your eye, you see a glimpse of the scene on the far side of the disk. Each open-slit image lingers in your eye and brain long enough to merge with the next image. This phenomenon, called persistence of vision, can combine the glimpses in such a way that your brain sees continuous motion.

If an object in the scene moves, your eye and brain can draw incorrect conclusions about that object's motion. When you look at the stream of water, for example, one slit allows you to view a droplet in a particular position. Depending on how fast your strobe is turning, the next slit might let you see a different droplet just slightly below the position of the one previously viewed. Your eye-brain system interprets the combined views as the slow motion of a single droplet. If the second view catches the droplet in a position just above that of the previous view, the droplet will seem to rise.

Place a bicycle upside down and spin a wheel. Look at the spinning spokes through the slits in the Whirling Watcher. You can see the spokes stop, or move slowly forward and backward, like the wheels on a moving stagecoach in an old Western. In modern Westerns, special wheels with unevenly spaced spokes are put on the stagecoaches to avoid the strange appearance of backward rotation when the moving wheels are filmed. A regular set of wheels with evenly spaced spokes is used for scenes in which the stagecoach is not moving.

You can exercise your creativity by making your own moving pictures. On the opposite side of the Whirling Watcher disk from the horses, in the space between each pair of slots, draw images, each of which is slightly different from its neighbors. (A running stick figure is an easy set of images to start with. ) Look through one of the slots at a mirror, just as you did with the horses, and spin the disk.

For more information about devices like this (called zoetropes, fantascopes, or phenakistoscopes), see the book Seeing the Light, by David Falk, Dieter Brill, and David Stork (Harper & Row, 1986, p. 195). A related snack is Persistence of Vision.