| The Human Eye
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| The eye is critical as our sense of sight. It is a complex sensory organ. Light enters through the cornea and then goes through the iris. After passing through the vitreous body, the light falls upon the retina, consisting of rods and cones. This creates a series of sensory messages through the optic fiber that then send the resulting messages to specific regions of the brain.
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| To allow light to pass through the lens without interference, blood does not flow through those cells. Instead, nutrients and oxygen are supplied in tears when we blink. The tear ducts help lubricate the eyes, keeping them smooth for vision and carrying away irritants and waste products, including dead eye cells. Blinking lubricates the eyes.
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| If the eye was more efficient, the nerves would exit through the back of the rods and cones. However, such is not the case. They instead flow from the inner surface of each cell to a specific spot where they exit the eye, a factor that actually supports the theory of evolution. Our field of visual has a blind spot that is therefore not perceived, and the brain compensates by creating a continuous image based on the neighboring data.
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| It is rather easy to test this yourself. On a piece of white paper, draw two black circles five or six inches away from one another: |
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| Close your left eye and hold the paper at your extended arm's length so that the left image is in front of your right eye. (It will also work if the right eye is closed and the right image is held before the left eye.) Slowly draw the paper inward so that the aligned dot moves toward the open eye. At some point, the other dot will disappear. When you move the paper closer, it will reappear! |
| Rods detect light, distinguishing brightness and differentiating black from white. Cones distinguish color. The primary colors of paints are red, yellow and blue. The primary colors of light, however, are red, green and blue. If all three varieties of cones receive equal activation, we perceive white. If they are uneven, we perceive colors. Which color we perceive depends on the proportion of nerve activation. |
| If we perceive the same color for an extended period, those cones become "tired" and this can play tricks on the brain. A simple experiment can show this. Take two white index cards. Select a marker of one color, other than yellow. Draw a circle in the middle of one card that is about the size of a quarter, coloring it in fully. Place that card on top of the second card. Stare it the circle for about 30 seconds and then rapidly remove it, continuing to stare at the remaining white card. The result should be surprising!
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| Color televisions operate in a similar manner. If you look closely at the screen of a console TV, you will see a set of evenly distributed red, blue and green electron beams that look like dots. If they are equal, you will perceive the color white. Uneven distribution of these primary colors create the broad spectrum we are able to observe. The average human eye can discern about a million different colors. We do not have enough names for each of these numerous shades.
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| Some people do not distinguish all these variants. Color blindness is when two colors are indistinguishable from one another. The most common case is red-green color blindness, which occurs in 8% of all men and 0.5% of all women. This trait is on the X gene of the chromosome that determines gender. Men have XY genes while women have XX genes. This means that women must have the genetic disorder on both chromosomes, which is why it is so less frequent. About 99% of color blindness is the red-green variety. A far rarer color blindness is blue-orange. It is the case for only 1% of those who are colorblind or 0.01% of all people. Appearing on a different gene, it is not sex-linked and therefore is of equal probability for each gender.
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| An interesting philosophical question is trying to come up with a way to describe a particular color to someone who has been blind since birth. We have all learned to use the same name for a color of a particular wavelength. However, we are unable to be certain if we are "seeing" the same color. All we are certain of is that we have learned to use the same name for identifying that color!
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| Just as the brain compensates for blind spots in interpreting what the eye perceives, it can also create the concept of movement. A movie or television show is actually a sequence of many individual pictures. The brain takes this series of images and interprets it as being equivalent to the actual "real world" motion we regularly see. Similarly, the image falling upon the retina has been inverted by the lens. The brain interprets the signals to perceive an image that matches the world before us!
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| Eye glasses compensate for visual errors. If the focal point of the image is either before or beyond the retina, the brain receives a distorted image. Whether one is near-sighted or far-sighted, it can be corrected with a pair of glasses. Likewise, glasses can compensate for a stigmatism or diplopia (double vision). The first glasses were invented in Italy in the 1280s. Prior to that, visual problems played a role in weeding out this negative factor. Since then, people who need glasses have been able to survive, thereby influencing the evolution of Homo sapiens.
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