From Dr. Michael Bach’s Visual Phenomena & Optical Illusions page:
The picture below looks a little strange, some jumbled monkeys there. It will be worth your time to fixate it for at least 10 seconds. Let your gaze “hang” on the tiny red target at the center. After fixation, close your eyes. After a few seconds a face will appear inside your head. Alternatively, instead of closing your eyes you can shift your gaze to a white wall or a blank sheet of paper.
The longer and the more steady you fixate, the longer lasts the afterimage. When it fades, blink and it will be prolonged.
Hint: Here’s what you should see…
Dr. Michael Bach of Universitäts-Augenklinik in Freiburg, Germany has collected 93 Visual Phenomena & Optical Illusions for your amazement. All of the illusions are explained and many of them are interactive. His site will test your perceptions and keep the little monkeys busy for a long time. Enjoy!