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Seeing Double
Cheshire Cat
Usually both of our eyes receive almost the same views which are fused together in the brain to form a single three-dimensional view of the world around us. At this exhibit visitors explore the phenomenon of eye rivalry. One person acts as the subject and another as the observer. The subject sits against a white background. Using an arrangement of hinged mirrors that can be moved by the visitor, the observer views the subject with one eye and a blank white surface with the other. The observer then sweeps a hand across the blank field. Movement attracts the brain's attention. When the observer's hand sweeps across the screen, the brain pays more attention to the movement than it does to the stationary subject - and part of the subject appears to be erased by the moving hand.
Professor Pulfrich's Universe
When two eyes see slightly different images, the brain combines these views to create a three dimensional picture. In this art work by Gerald Marks, objects such as a spinning globe, and a star are intensely illuminated creating shadows against a back wall. While keeping both eyes open and placing a dark filter over one eye, visitors view the shadows and observe which way the shadows appear to turn. Visitors also observe what happens without the use of the filters or with only one eye open. Visitors discover that by switching the filter from one eye to the other, the shadows appear to turn in the opposite direction. The filter cuts down the amount of light that one eye receives. The eye that is looking through the plastic sees a darker image than the other eye. Because the image is darker, the eye and brain need more time to process it. You could say that one eye is seeing slightly in the past. Because the image is darker, the eye and brain need more time to process it.
Reverse Distance
This exhibit gives the visitor a chance to see a visual field reversed, left for right by looking through a viewer containing a pair of prisms. When left and right are reversed, the world is seen not only backwards, but also inside out. Visitors experiment with reverse distance by manipulating and observing two vertical rods of identical thickness, one black and one yellow, mounted next to each other on a rotating wheel. While looking through the viewer the visitor turns the pulley on the table until the yellow rod is farther away than the black rod. Looking at the rods without the viewer, visitors notice that the yellow rod is actually closer than the black rod. One of the clues that your brain uses to calculate distance is the very slight differences between what your left eye sees and what your right eye sees. Trading left for right reverses your depth perception so that nearby things look far away and distant things look near. Additionally, visitors can test their eye-hand coordination. While looking through the viewer, visitors hold a rod in each hand and try to touch the tips together. The eyes and the hands give the brain conflicting information about the location of the rods. The brain pays more attention to the incorrect information from the eyes than it does to the information from the hands, making touching the rods very difficult.
Stereo Rule
The combined views from two eyes provide a three-dimensional pictureof the world, this is called stereo rule. A long panel containing pairs of eyeholes at various heights hangs in front of two metal rods. Positioned behind the eyeholes are two wheels that when turned by the visitor will block either one or both of the eyeholes. When looking through the holes at the metal rods with only one eye visitors notice that it is difficult to perceive depth - the two rods appear to be side by side. When looking with both eyes visitors notice one rod is actually in front of the other. Each eye sees a slightly different view of the rod. The brain automatically combines these two views to produce a single view of the rods. This combined view has depth.
Two Eyes
This exhibit illustrates how our two eyes cooperate to give us an accurate comparison of objects at different distances. It consists of a pair of swiveling model eyeballs, 10 times the actual size, with translucent frosted backs. The retinal images inverted as in real eyes can be seen directly on the back surfaces. There is a pair of lighted targets, a red letter "P" and a green letter "F" whose relative distance from the eyeballs can be changed by the visitor. By manipulating the targets and looking through the giant eyeballs visitors explore how and why images formed on the retina of the right eye differ from images formed on the left eye. When the letter "F" on the left, is closer to the eyeballs, the images of the "F" and "P" are closer together in the left eye than they are in the right eye. The brain judges the distance of nearby objects in part by comparing the different images in your two eyes. Another clue that helps the brain judge distance is the size of the image an object makes on your retina. As a letter moves farther from the eyeballs, it makes a smaller image.