The Afterimage

   Color is the easiest channel to use to notice afterimages. There are two kinds of afterimages: negative afterimages, and positive afterimages. Negative afterimages have been discussed in a previous section where you can make a plain white screen appear colored.

   Positive afterimages occur because of the property of persistence. Because chemical reactions are not instantaneous, the visual stimulation remains in the brain for a certain amount of time. Normally this amount is a small fraction of a second, so that when something moves we don't even notice the positive afterimage. However it is possible to force a signal to travel to the brain for a longer period of time.

	Stare at this picture for 30-60 seconds. This will cause your eyes to get used to sending a particular signal. Quickly close your eyes and cover them to block out any light that may be entering through your eyelids. In the darkness there is no visual stimulus to replace that of the image you have been staring at. You will very briefly see the positive afterimage of PeeZee as the brain continues to process the old signal. By staring at the picture for a longer amount of time you can cause the positive afterimage to last longer, but it will still fade relatively quickly to darkness.

Latency

   For the same reasons that visual stimulations aren't instantaneously processed in the brain, they also do not instantaneously reach the brain. This concept, called latency, is a source of two other very interesting illusions.

   The length of time between light first striking the eye and the brain responding to the light varies greatly upon the amount of light and the color of the light. The more light there is to process, the faster the brain will respond to it. Each of the three color channels has different chemical reactions so has different latency times.

	This experiment requires low level light, so cannot be viewed from a light-emitting computer monitor. Click on the image above which will open it in a new window, and print it in color. Bring the picture into a very dimly lit room so that your latency time will be increased. Shake the picture back and forth. Because each color stimulates a different color channel, your brain has a different latency time for each square. The squares appear to be disconnected because your brain is noticing their movements at different rates.

   The brain has compensated slightly for latency, which ironically produces another very interesting illusion. As the diagram above shows, the brain can react to an alternating pattern of on-and-off stimulations and, believing that they are caused by latency and the high frequency of light, translate them into a wave pattern. This wave is then processed to be a wave of light, and you see the black and white as color!

   This idea was first noticed by a toymaker in the nineteenth century who created tops that took advantage of this trick. Benham's disks, named after the toymaker, are still popular at toy and novelty shops because of their fascinating properties.

	Make your Own Benham's Disk Click on the above image to open it in a new window, then print it and cut it out. Take a small object such as a pen and puncture a small hole in the center of the disk. (For even better results, use Benham's idea and place the disk on a top!) Rapidly spin the disk (either by hand or on a turntable). Notice that as you reach a high enough speed, such as 30 rotations a minute, the lines appear colorful!

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Move on to Light Rays