Take a dowel or some other stick that is approximately 1.5-2 feet in length and tie two 3.5-ft ribbons on it, some distance apart. Attach a weight to the end of each ribbon, like a tennis ball or a heavy bolt. The weight isn't important, it just needs to be enough to make the ribbons swing back and forth. Once you have gotten everything set up, have somebody stand on a chair and set the weights swinging in opposite directions so they pass each other in the middle. As anyone can plainly see, the weights pass each other and do not touch because they are going in parallel paths. Now take a pair of sunglasses or 3D Glasses and place the lens (or colored part) over one eye. Look at the weights using both eyes. Inexplicably, the weights will now be going around in a circle, and the ribbons will in fact be crossing paths, going through each other!

This amazing effect was developed by Carl Pülfrich in 1922. The reason the effect works is because when you shine a dim light, it takes the ganglion in the eye longer to send a signal to the brain. In essence, the eye sends an old image. When one eye is seeing through some sort of colored lens, that light is dimmer and therefore takes longer to register in the brain. This means that essentially one eye sees the ribbon at one position, and the other sees it at another position, the position being about 10ms apart in time. (This is a visual exaggeration, but the left eye might see the ribbon at an angle like /, and the right eye might see it like |) This creates an artificial visual disparity and registers in the brain as a stereoscopic effect, creating artificial depth. This effect is even more amazing due to the fact that Pülfrich was blind in one eye, and completely unable to see his own illusion because he had no stereoscopic vision! Score one for reasoning from first principles!

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