The ancestors of modern humans developed color vision 30 million years ago. But it was not until the late 1700s that there are records of anyone seeing colors in an unusual way. English chemist John Dalton, who found that people thought he was joking when he asked whether a geranium flower was blue or pink, wrote a description in 1794 of what he saw for the Manchester Literary and Philosophical Society journal: His world was suffused by shades of blue and yellow, but contained none of the mysterious sensation known as red. “That part of the image which others call red,” he wrote, “appears to me little more than a shade or defect of light.” It was one of the first mentions of colorblindness in human history.
In the centuries since, we have discovered what it is that robs some people of such sensations. Those of us with standard vision, called trichromats, have three kinds of pigments, or cones, in our retinae, each sensitive to a certain range of light and spaced out across the visible spectrum so that they can together convey to the brain everything from red to violet. In the colorblind, a mutated ...
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