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Stereoscopic Vision

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Author: Marc Rosner

Have you ever wondered why humans and most other living creatures have two eyes instead of one? When you view things with both eyes, using stereoscopic vision, you have depth perception. This means that your brain takes information from both of your eyes and processes it into calculations of distance.

In certain instances, you can trick your brain into perceiving things in an unusual way—seeing depth when it is not really there, or inverting images. You can do this using a stereoscope or other optical devices, or by simply training your eyes with your fingers.

Materials

  • paper, ruler, and pen or pencil
  • stereoscope (available in science stores and museum gift shops)
  • 4 small hand mirrors
  • two 45-degree prisms
  • cardboard boxes and mailing tubes
  • sealing wax or Fun-Tak

Procedure

  1. Sketch an illustration that suggests depth to the mind: Use your ruler to draw an 8-by-5-cm rectangle. Draw a horizontal line 1 cm from the top as shown. Now draw a series of disks that diminish in size from the lower left corner to the upper right. Make an effort to have the centers and radii line up as pictured. Common sense tells you that you have made a flat drawing. Yet your brain insists that it is a three-dimensional representation—especially if you shade the disks suggestively. Your brain gives you the impression of a series of spheres that run from the foreground to a vanishing point on the horizon.
  2. Stereoscopic Vision

    The painters of the Renaissance perfected this trick of representing three-dimensional reality in two dimensions.
  3. Stereoscopic drawing is another method of representing three-dimensional reality in two dimensions, which creates an even more dramatic visual impression. Draw two rectangles, each 3.8 by 3.2 cm. Space the rectangles about 6.4 cm apart from center to center. Now draw a horizontal line through the center of each, dividing the rectangles in half. Next make a shaded, 0.6-cm disk precisely in the center of each horizontalline. Flank the disk in the rectangle at left with an identical pair of disks spaced 0.5 cm from the circumference of the middle disk. Make a similar pair on the horizontal line of the rectangle at right, but space these disks 0.8 cm from the circumference. To the casual observer, there is certainly nothing in this drawing to suggest three dimensions. But when you view the drawing through a stereoscope, which causes the pair of rectangles to blend into a single image, the disk at left is seen as a sphere floating in space above the plane of the paper. The center disk will appear as a sphere in the plane of the paper, while the right one will seem to float in space behind the paper.
  4. Stereoscopic Vision

    This is called wide-eyed stereoscopic seeing. You can also see the drawings in three dimensions without a stereoscope by the cross-eyed method. To achieve this you use only one finger. Place the drawing about 60 cm away, as before. Now put the tip of one index finger on the bridge of your nose, and while looking toward the drawing, slowly move your finger toward it, focusing your eyes on the tip. Again you will become conscious of four rectangles on the paper. Gradually, as your finger advances, the innermost pair of rectangles will fuse as in the wide-eyed method and you will see the center drawing in three dimensions. Note that the image is smaller and the two outermost spheres have inverted in their apparent orientation!
  5. See if you can train your eyes to see the stereoscopic effect even without a stereoscope. View the drawing from about 60 cm away and place the index fingers of both hands just outside your eyes. Now, while continuing to look at the drawing, move your hands slowly toward the drawing. Your left eye will see the tip of your left index finger, and your right eye the tip of your right index finger. As your hands advance, you will become conscious of four rectangles on the paper. Your brain is accepting the independent images presented by each eye. The inner pair of images will gradually overlap. Finally they will blend. When this is accomplished, transfer your full attention to the fused image. It will appear in three dimensions, just as though it were seen through a stereoscope.
  6. See if you can build these types of pseudoscopes.
  7. Pseudo- means "false." A pseudoscope is a binocular-like device that enjoyed brief popularity shortly after Sir Charles Wheatstone (1802–75) invented the stereoscope in 1838. Pseudoscopes alter the way in which the eyes normally present information to the brain. Some exchange eye positions, so your right eye sees what the left one would, and vice versa. Others exaggerate the spacing between the eyes.
    1. Hold up two hand mirrors, one somewhat to the left of the left eye, facing away from you at approximately a 45-degree angle, and the other in front of the right eye, parallel to the first mirror but facing you. The image reflected from the mirror at left should be directed into the right eye by the mirror at right. Now view an object that has writing on it. Fuse the images seen by both eyes to trick your eyes into seeing both images as if they were real.
    2. Stereoscopic Vision

    3. View the same object through two right-angle prisms as shown. Notice the inversion of the writing—you are seeing an inverted world!
    4. Stereoscopic Vision

    5. Set up a four-mirror pseudoscope as shown.
    6. To make your pseudoscopes permanent and portable, mount the mirrors and prisms in some kind of holder. You can modify cardboard boxes and mailing tubes, mounting the optic devices with sealing wax or Fun-Tak. For a real challenge, design your pseudoscope so it is adjustable for people with different eye widths.
    7. Stereoscopic Vision

References

How to Really Fool Yourself: Illusions for All Your Senses by Vicki Cobb (New York: John Wiley & Sons, 1999).

Now You See It, Now You Don't: The Amazing World of Optical Illusions, rev. ed., by Seymour Simon (Holt, Mich.: Beech Tree Books, 1998).

101 Amazing Optical Illusions: Fantastic Visual Tricks by Terry Jennings (New York: Sterling, 1998).

Optical Illusions (Scholastic Discovery Boxes series) by Kate Waters (New York: Scholastic Trade, 1996).

IlIusionWorks: www.illusionworks.com

Sandlot Science optical illusions: www.sandlotscience.com/

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