Negative Afterimage
and Other Visual Experiments
Negative Afterimage Review
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Who knows what form the forward momentum of life will take in the time ahead or what use it will make of our anguished searching? The most that any one of us can seem to do is to fashion something -- an object or ourselves -- and drop it into the confusion, make an offering of it, so to speak, to the life force. --Ernest Becker, The Denial of Death |
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In order to see the larger picture, let's first turn our attention to science and the evolutionary study of the eye.
Among the first scientists considering the anatomy and physiology of the human eye was Pythagoras in the 6th Century BC. He believed objects were seen by rays emerging from the eye striking the objects. Two thousand years ago, the Greek philosopher Democritus proposed that matter consists of atoms and believed that objects gave off atoms that struck the eye and caused it to see. Another Greek philosopher, Empedocles, believed that an invisible substance radiating from the eyes caused sight. Euclid also believed the eyes projected a series of individual rays. Aristotle discarded the idea of rays radiating from the eye and taught that the lens itself did not exist during life but was only a coagulation formed after death. Plato, a generation later, believed that rays from the eye mingled with light rays emitted by objects and caused the eye to see. Ya' qub ibn-Ishaq al-Kindi, an Arab philosopher, defended Euclid's ray theory and attempted to prove it by eliminating those of Plato, Aristotle, and Democritus. Al-Kindi did not agree with Euclid entirely, proposing that rays flowed in one continuous stream. Greek physician Galen revived the Pythagorean belief, but believed the optic nerves were channels carrying visual essences from the brain which irritated the air outside the eye. Then the excited air reached the objects, causing the eye to see them. Avincenna, an Islamic philosopher, defended Aristotle's ideas that rays from the eyes could not possibly reach the stars and rejected Galen's theory. Second Century BC eye operations in the Egyptian city of Alexandria gradually advanced knowledge of the human eye. The muscles, iris, lens, and retina were discovered during operations. Then 11th Century Islamic scholoar Alhazen attacked the ray theory, demonstrating that light affected the eye and anyone staring at the sun felt pain. He integrated anatomical, mathematical, and physical ideas, theorizing the eye worked like a primitive camera. Alhazen believed that "light" shone through the pupil, through a lens, and produced an image on the retina. His was the first idea that "light" created sight. Eyeglasses were in use by the early 14th Century, although no one knew how or why they helped vision. People merely looked through various eyeglasses until a pair was found that improved vision. 17th Century German astronomer Johannes Kepler discovered the eyeglass lenses and the eye's lens correct vision by bending light rays. The degree of bend depends on the lens shape and thickness. Then Isaac Newton theorized that light consisted of vibrating particles of varied sizes flowing in straight rays from illuminated objects. He believed that the prism divided light resulting in bands of different colors. The next major advance in understanding the eye came in 1851, when German scientist Hermann von Helmholtz invented the ophthalmoscope, the eye-examination tool used today. Theories then surfaced that chemicals in the retina were sensitive to red, green, and violet, and that color vision depended upon these substances. In 1877, Wilhelm Kuhne discovered that the retina contained light-absorbing chemicals called rhodopsin. Kuhne's experiments with rabbit retinas showed that light entering the eye changed the rhodopsin, and that light-changed rhodopsin formed images. Sir Isaac Newton had claimed light was made of tiny "particles" discharged by the sun or other shining objects. But this claim was disputed by Christiaan Huygens, who proposed light consisted not of solid particles but of energy "waves." Finally, Albert Einstein, a 20th Century scientist, led others to understand not only the function of the eye but the mysterious nature of light itself with its opposite forms. Einstein's theory of matter and energy were that they are different forms of the same thing, as steam and ice are different forms of water. Therefore, light behaves somewhat like matter, as in the particle theory, and somewhat like waves, as in the energy theory. This led to a clearer understanding of how light is received by the human eye. Negative afterimage information and descriptions:
"Another common type of visual experience is the after-image. This image typically takes place after looking at a bright object against a dark background, such as a bolt of lightning seen at night. If a person closes his eyes right after the flash, he will continue to see a light flash against a dark background. The original image lasts only a few seconds. It is followed by a negative after-image. If the original image is seen in black and white, the tones of the after-image are reversed. If the original image is in color, the negative after-image is seen in the complimentary colors. After-images cannot be scanned, that is, the image shifts as a person moves his eyes. Most people are not aware of after-images until they are told of their existence. Then, with practice, they are able to see them. People who spend prolonged periods staring at the same scene, for instance looking through a microscope, often experience a recurrent image. This image may occur immediately after looking at the original scene, or several hours later when they close their eyes to rest. People usually have little control over the appearance and disappearance of these images" (Samuels, Seeing With the Mind's Eye, P. 55).
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Contact info:
Mary J. Johnston
Web: www.visualexperiments.org
Email: mjohnston@visualexperiments.org