Most people can see a million different colors in the world around them. But some (most of them male) have trouble distinguishing some reds from greens or blues from yellows. For centuries, diagnosing colorblindness was simple: A person was asked to name several colors, and the answers were compared with those of "normal" subjects. More reliable tests didn't come about until scientists understood more about how the human eye works.
In the late 1700s, English chemist and mathematician John Dalton came up with an explanation for his own, self-diagnosed colorblindness. The "liquid" within his eyeball, he surmised, must have been tinted blue, making it difficult for him to tell red from green.
On his death, Dalton requested that an autopsy confirm his hypothesis. Looking inside his eyeballs, examiners were unable to prove him correct. But in 1995, scientists at the University of London obtained DNA from Dalton's preserved eye tissue and determined that it was missing so-called green cones.
The human eye contains millions of light-sensing nerve cells, called rods and cones for their respective shapes. The rods are super-sensitive to light, but see only black and white. The cones contain pigment, and the gradations of colors the cones pick up enable humans (save the colorblind) to distinguish roughly a million different hues.
Just before Dalton's death, another Englishman, Thomas Young, had discovered pigmented cone cells in the eye. Young proposed (and was later proven right) that the human eye sees myriad colors employing cones in just three colors: red, blue and green.
He also accurately theorized that deficient cones led to deficient color vision. Dalton's lack of green cones, that is, accounted for his colorblindness. (Color vision defects range from mild to severe; while some people may see gray in place of green, others may see only certain shades of green.) Young's work paved the way for doctors and researchers to figure out how to diagnose colorblindness in the still-living.
In the 1870s, a Swedish doctor, Frithiof Holmgren, suspected that a disastrous local train accident may have been caused by the conductor's inability to distinguish between a green and red signal light. In setting out to test his hypothesis, Holmgren drew inspiration from Young's theory.
Holmgren devised a color vision test that required subjects to find, in a collection of more than 150 skeins of colored wool, those that most closely matched the red and green master skeins. To the dismay of Swedish officials, his test showed that nearly 5% of the railway workers had trouble telling the two colors apart. (In fact, about 8% of all men inherit an abnormal gene from their mothers imparting some degree of colorblindness.)
Variations on Holmgren's test (using colored beads or lights, for example) soon emerged. But even as these sorting tests were being developed, other eye doctors were experimenting with so-called pseudo-isochromatic tests. In these, an image is depicted in dots of different colors and sizes and appears differently (or not at all) depending on the color vision of the observer.
One of the most recognizable pseudoisochromatic tests was developed by a Japanese ophthalmologist, Shinobu Ishihara, during World War I. The Japanese military tasked Ishihara with developing a test to reliably identify the colorblind among its recruits.
Using watercolor paints, Ishihara hand-painted pictures of Japanese characters in various colors (orange characters on gray backgrounds, red on green, shades of blue and green on red), testing them on a colorblind colleague to make sure they worked. They did, and the Japanese Army began using them in 1916. Soon after, Ishihara swapped out the Japanese characters for Arabic numerals and began pitching his test to eye doctors around the world.
His self-promotion worked. More comprehensive color vision tests exist today, and researchers continue to develop exams to distinguish nuances of color vision deficiency that the Ishihara method can't. But the quick and easy Ishihara test is still among the most widely used.