Color Blindness—A Curious Defect
WHAT a shock for the sober-minded Quakers when they caught sight of John Dalton’s bright-red stockings! Usually clothed in somber colors—grays, browns, and black—they found John’s attire startling, to say the least. What had happened?
Dalton, born in 1766 in Eaglesfield, England, described blood as “bottlegreen” and considered a laurel leaf “a good match for [red] sealing wax.” Yes, Dalton, who became a famous chemist, suffered from color blindness, or more precisely, defective color vision.
To Dalton, red looked gray and very little different from green. No wonder his practical-joking friend could switch his stockings and cause such outrage! Interestingly, in some European countries, color blindness is known as Daltonism.
A Worldwide Problem
In 1980, Dr. Janet Voke of City University, London, estimated that more than two million people in Britain have defective color vision. In some isolated communities, comparatively few suffer with this problem. In Fiji, only 1 man in every 120 is color-blind, whereas in Canada, every 9th man, on the average, has less than normal color vision.
Color vision varies from person to person. According to a widely accepted theory, your vision is normal if you see white when three beams of light—one each of red, green, and blue—are combined in equal proportions. When the three beams are blended in varying proportions, other hues that are naturally discernible to you can be produced.
If, however, all the hues that are visible to you can be matched by mingling only two of these primary colors, and the addition of the third color does not make any noticeable difference, then your color vision is defective. You are what is called a dichromat. John Dalton was a red-blind dichromat.
The defects affecting those called monochromats are more severe. Those with this condition have no color discrimination. For monochromats, a color TV might just as well be a black-and-white model.
The majority of color-blind people are anomalous trichromats. The hues that are visible to these people are still a blending of all three primary colors, but anomalous trichromats differ in the proportions of the primary colors that they discern. If this is your problem, adjusting the color balance on your TV set may prompt cries of “That’s too red!” or “It’s too green!” from your color-normal companions.
What causes such defects? The New Encyclopædia Britannica identifies one culprit as your “wavelength discrimination apparatus.” Each of your eyes has about 130 million light receptors in the retina, but only 7 million of these give you color vision. These color receptors are called cones because of their cone shape.
People with normal color vision have three kinds of cones. Some cones respond best to the longer wavelengths of light (red). A second group senses the middle wavelengths (green), and the rest the shorter wavelengths (blue). If a group of cones is missing or fails to respond properly to its associated wavelength, you will have a problem. If you fail to sense red normally, for example, you will notice very little color change as tomatoes ripen from green through orange to red.
Damage to the optic nerve that affects the cones’ message to the brain can trigger color blindness. Even some forms of medication, such as certain antimalarial pills, have been known to disturb color perception. Some oral contraceptives reportedly can alter perception of blues, greens, and yellows. In the book Colour Vision Testing, Dr. Voke lists both tobacco and alcohol as responsible for some permanent red-green blindness.
Aging too takes its toll, particularly on a person’s sensitivity to blue light. Researcher R. Lakowski comments that color discrimination reaches a peak during adolescence, and it lasts to the age of 35. Then one’s color discrimination gradually deteriorates, especially after the age of 60.
Although defective color vision may be acquired during the course of one’s life, most of those who are color-blind are that way from birth. Why?
‘Like Grandfather, Like Grandson’
Normal human color vision is a special gift. When your cones function correctly and your optic nerves faithfully convey the encoded message to the brain, you then see in full color. “The trained human eye can discern as many as 150 hues,” reports the book How Animals See. “Many animals . . . probably do not see colors as we do. But the condition is normal in their eyes, not defective,” says The World Book Encyclopedia.
If your vision has always been defective, no doubt you inherited this trait. From whom? Health and Disease defines color blindness as a “sex-linked” genetic disorder “transmitted by females but usually turn[ing] up in alternate generations of males.” Thus, it is often, ‘like grandfather, like grandson.’
Color-Blind? How to Tell
Do you suspect that your children are color-blind? “If you notice that your 5- or 6-year-old child has difficulty identifying colors; if he wears mismatched socks; or if he cannot take the correct crayon from a box when asked to select one by color,” then, the book Childcraft says, “you should have his vision checked.” How?
One of the most popular ways of checking color vision is the Ishihara test. The examiner shows your child a series of cards covered with dots in many different hues. Arranged among the dots are patterns and numbers discernible to anyone who is color normal. Your child has to say what pattern or figure he sees. Looking at one chart, a red-blind child sees “6,” a green-blind child “9.” If your child sees “96,” he is color normal, according to that part of the test.
With the ever-increasing use of color in material that is used to instruct children, it is wise to know if your children’s color vision is defective. But since inherited color blindness is presently unalterable and incurable, can you do anything about it?
Precautions to Take
Hazel Rossotti, author of the book Colour, first recommends early diagnosis. Then the color-blind person can “be made aware of situations which are likely to cause confusion and taught to rely whenever possible on factors other than ambiguous colours.”
You can teach your color-blind child the meaning of the colored traffic signals on the roads. Though he may be able to distinguish a red stoplight from the green “go” light by its relative position, help him to take note of the different intensity or brightness of each light. Then, when he is on his own, he will read the signals correctly even in the dark.
If you are color-blind, it is good to avoid relying on color alone when making decisions. Since the brain can compensate for color-vision defects, supplement its fund of information by paying more attention to an object’s brightness, position, and shape. Do not hesitate to ask your color-normal friends and relatives for help.
In making important decisions, such as the type of work to do, you will be wise to take into account the disadvantages color-vision defects bring. In some occupations, color blindness can be a severe liability. It is, for example, a handicap for many chemists, pharmacists, color printers, and photographers. Good color vision is an asset to dentists when they match artificial teeth. Also, butchers and those in the catering industry can be more alert to changes in the condition of food when they have good color vision. Lack of color perception can make it more difficult for nurses and doctors to determine the state of health of their patients when examining them.
Everyone who sees clearly has a valuable asset. If your color vision is only partial, you should exercise particular care. For example, you should be aware that taking unnecessary drugs, drinking excessive amounts of alcohol, or using tobacco can cause a deterioration of color perception. If you have full-color vision, then you have a priceless gift to treasure.