G3 Program 14: Optical Illusions

How many legs does the elephant have?

Are the horizontal lines straight or wavy?

Before movies (motion pictures) came into existence, did people entertain themselves with moving pictures in any way? Absolutely! For centuries, men and women have been fascinated with optical illusions. This fascination was particularly obvious in the 1800’s when a string of optical illusion inventions had enormous popularity. In fact, it was these inventions that eventually led to the successful creation of modern movies. But first, to get ourselves in the right frame of mind, I showed our G3 scientists a series of optical illusion images and asked them to share what they saw. Some of the images could be interpreted in 2 ways; some of the images played with perspective and made objects of a similar size seem larger or smaller than each other; and some images just seemed impossible (like the elephant with an endless number of legs!). What do you see in the images to the right?

After we finished playing with the optical illusion images, we turned our attention to the popular optical illusion toys that led to the creation of modern movies…and our G3 scientists were tasked with recreating several of them. The basic timeline for the toys/devices is as follows:

  1. Thaumatrope (1824)
  2. Phenakistoscope (1833)
  3. Zoetrope (1834)
  4. Praxinoscope (1877)
  5. “Modern” movies!


Thaumatrope in action!

The Thaumatrope (“Turning Marvel” or “Wonder Turner”) gained in popularity in Victorian London when it was presented at the Royal College of Physicians to represent the “persistence of vision.” The persistence of vision is a scientific principle which notes that the human eye retains memory of an image for 1/20 of a second past when an image has already left our field of vision.  This persistence of vision helps make many optical illusions possible since the human eye actually imperceptibly blurs together a series of still images and tricks the human brain into thinking separate images are actually part of the same larger image. Even though the Thaumatrope had a surge of popularity in the 1800’s, there was a reported discovery of a prehistoric thaumatrope carved on a bone disc and found in 1868 in the Dordogne, France! [Note: the linked web site is in French.]  You can read more about the discovery of several presumed prehistoric thaumatropes here. The images of the prehistoric thaumatrope from Dordogne are below.

Prehistoric Thaumatrope in action!

Prehistoric Thaumatrope Image #2

Prehistoric Thaumatrope Image #1

Our G3 scientists were able to create their own Thaumatrope. One of the most classic pair of images used is a bird and a cage…so that when the thaumatrope starts to spin it looks like the bird is actually inside the cage. For our thaumatropes, we used the image of a bird and an image of a branch. We also used a drinking straw to spin between the two images versus using a string or rubber band attached to both ends of the images. The results were amazing and our scientists were enthralled by the optical illusion created.


Using a mirror to see the phenakistoscope in motion…

The thaumatrope eventually evolved into the phenakistoscope. The phenakistoscope is a disc mounted on an axis that when spun gives the impression of a scene in motion. In order for the illusion to work, the viewer needs to face the image towards a mirror. When it is spun, the viewer looks through the slits along the edges toward the images presented in the mirror. Discs/wheels with different images could be placed on the axis to give the viewer a chance to see different types of action. The phenakistoscope was a step up from the thaumatrope because of the number of images that could be used together to create the impression of motion. The thaumatrope can only use two images, which extremely limits the optical illusion motion; the phenakistoscope can make use of a series of images to present a larger range of motion. Check out the youtube video below…

Our G3 scientists were also able to create their very own phenakistoscope. The images represented a stick figure jumping rope. The disc was attached to the eraser of a pencil using a pushpin, and a floor length mirror was handy for viewing the resulting illusion/animation. There is a trick to viewing through the phenakistoscope, so it took some of our scientists a few tries to get the knack of viewing. But once they did, it was a pretty cool thing to behold!


The zoetrope’s great improvement over the phenakistoscope was that multiple people could view the illusion of animation at the same time (the phenakistoscope was limited to one person because of the way it needed to be viewed through slits in a mirror). The Zoetrope made use of a core of mirrors to reflect the images from a disc of spinning images. Disney Pixar has taken the idea of the zoetrope to new levels by creating a 3D model of a zoetrope using popular characters from their movies, like Toy Story. The youtube video below is a really amazing look at how many animated movies are made!

For our final project, our G3 scientists were given the tools necessary to create their own zoetrope. A circle of images was mounted on a wooden spool, which could then be spun be inserting a pencil through the bottom hole of the spool. This project proved the most challenging for our scientists. While they could clearly see the illusion of animation through my sample model, they had difficulty seeing the same results with their own zoetropes. Part of the problem may have been the use of too much tape to link the tabs together, which then made it difficult to view the images through the slots. But all went home with the tools to play around with the zoetrope on their own.


Though we did not have the materials necessary to recreate a praxinoscope during the program, I brought in my personal praxinoscope to show all of our scientists exactly how one works. The praxinoscope is the pre-cursor to modern movies! With this device, for every image on the wheel/disc, there needs to be a matching mirror in the center. When the device is spun, thanks to the persistence of vision and how our brain interprets the spinning images, it appears that an object is in motion. My praxinoscope had a disk of a man on a horse. When spun, it looked like the horse was galloping. If it was spun in the opposite direction, it looked like the horse was galloping backwards! The praxinoscope was without a doubt a favorite among the scientists.

Modern Movies

Where modern movies improve leaps and bounds above the other optical illusion devices is the number of images that can be used. Even the praxinoscope is limited in the number of images it is possible to string together to create motion. With modern movies, still images are captured with special cameras onto film reels. When they are played back, it appears that there is continuous action from people, items, and events. In the past few decades, there has been more frequent use of digital filming (which operates in an entirely different way than traditional movies in how it captures motion). But even today, most movies you see in the theater are made up of a series of still images that are rapidly played back to give viewers the illusion of motion!

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