In 1995, researchers from Cambridge University asked the Manchester Literary and Philosophical Society for a sample of an eyeball that had been sitting in a jar on a shelf since 1844. That eye had made some of the most important scientific observations in history. It鈥攁nd another just like it鈥攂elonged to John Dalton, the English schoolteacher who in the late years of the eighteenth century formulated the atomic theory.聽
Dalton had inferred from the way that elements combined with each other that these fundamental building blocks of matter were made of atoms, and that the atoms of any element were identical to each other but different in mass from the atoms of other elements. He also meticulously recorded his observations of weather patterns, the northern lights, and the behavior of gases. As well, Dalton discovered that he had made these observations with eyes that were different from others. He was color-blind! He already suspected that he had vision problems, because his fellow Quakers would occasionally object to the loud colours he wore; to his eye, the shade of his attire seemed quite sedate.
Then one night in 1792, Dalton noticed that a geranium plant that had appeared blue in the sunlight changed color by candlelight. Candlelight is composed of a different range of wavelengths, or colours, than sunlight. Newton had demonstrated this long before by passing light through a prism. Dalton questioned his friends about it, but they were puzzled because they witnessed no such color change. Something interesting was going on.
Dalton surmised that his eyes were somehow filtering out certain colors, and this prompted him to consider the possibility that the vitreous humor, the thick liquid inside his eyeballs, was a different color from that found in the eyeballs of others. He wasn鈥檛 keen on having his eyes taken out while he was still alive, but he requested that they be removed after his death and studied. His assistant, Joseph Ransome, complied. He squeezed out the liquid from the eye and found it to be perfectly normal. Then he made a hole in the back of one eyeball and looked through it. Not noticing any filtering effect, he concluded that color blindness did not stem from a physical change in the eyeball.
Ransome was wrong about that, but he wouldn鈥檛 have had the means at the time to determine the cause of colour blindness. Today, we can relate colour blindness to malfunctioning cells in the retina, the light-sensitive layer of tissue that lines the back and sides of the eyeball. There, color is perceived by cells called cones. We have three types: one type is sensitive to blue, another to green, and a third to yellows and reds. Color blindness is a malfunction in one or more of these cell types. Deuteropes, for example, cannot see the green part of the spectrum, protanopes are insensitive to red, and tritanopes are blind to blue.
Colour vision and the problems associated with it are encoded in our genes. That鈥檚 why the Cambridge researchers asked to investigate Dalton鈥檚 eyeballs. By 1995, the polymerase chain reaction (PCR) had been developed to the extent that a tiny sample of DNA could be reproduced and samples large enough for laboratory investigation generated. The researchers subjected the DNA they extracted from cells in a sample of Dalton鈥檚 retina to such a study and discovered that John Dalton was indeed a deuterope who saw the world differently from others. Dalton himself had presaged such genetic analysis by observing that while his friends saw no difference in the colour of the germanium by candlelight, his brother did.
Curiously, it was not the English but the French who commemorated Dalton鈥檚 observations about colour blindness in a significant way. The French term for colour blindness is daltonisme.
