The Curious Case of John Dalton's Eyeballs

John Dalton, from 1766 to 1844, was a distinguished scientist who is known for, among many other things, his theories about atoms, his Law of Partial Pressures of Gases and Liquids published in 1803, and being one of the first scientists to study colorblindness. In fact, because of his work, for some time a common term for colorblindness was Daltonism, Colorblind himself, Dalton was only able to see two primary hues, a yellowish hue that encompassed a typical person's green, yellow and orange, and a second that was the equivalent of a non-colorblind person's perception of blue and violet. With regards to what most would call red, Dalton only saw, to quote, little more than a shade or defect of light. Since Dalton's brother shared this condition, Dalton believed that colorblindness was inherited. He also theorized, after much study, that its precise mechanism was that his eyes and his brother's were tinted blue in the vitreous humor, the gel in the middle of the eye in between the lens and the retina. Leaving instructions that his eyes be dissected after his death to confirm or disprove this theory, his assistant, Joseph Ransom, complied. On July 28, 1844, the day after Dalton's demise, Ransom excised Dalton's eyeballs and thoroughly dissected one, revealing a perfectly clear vitreous humor disproving part of Dalton's theory. Later scientists noted that Ransom shrewdly left the second eye almost intact, although that almost is significant here. Before he stored the remaining portions of Dalton's eyes, Ransom lopped off the posterior pole of the second eye, looked through Dalton's eyeball, and realized that when he did so, neither red nor green were distorted, but appeared of the proper color. Clearly, not only was the vitreous humor not the cause of Dalton's color blindness, but no other filter within the eye was responsible either. A contemporary of Dalton's, Thomas Young, who lived from 1773 to 1829 and was an English scientist and philosopher, among other things, first floated the theory that color vision was the result of sensitivity to a combination of three colors, red, blue, and green. He also hypothesized that color blindness was due to the absence or paralysis of those Now let's fast forward 150 years, because today we know that color perception is due to the presence of three types of photopigments in the cones of the retina. Each pigment is comprised of a specific type of protein bound together with retinal and different types of cone cells that are sensitive to different wavelengths. Short wave at 420 nanometers, middle wave at 530 nanometers, and long wave at 560 nanometers. Violet is at the short wavelength end of the visible spectrum, greens and yellows in the middle, and reds are at the long end. With these developments in the understanding of vision, many had theorized that since Dalton had difficulty with red, he was a protanope, or a person who could not see the colors at the long wave, LW, end of the visible spectrum. To verify or disprove this theory and finally solve the longstanding mystery of Dalton's vision, in 1995 a few intrepid researchers took samples of Dalton's eyeballs, preserved only in air, and subjected them to DNA analysis. They determined that Dalton actually had the required genes for the protein necessary for the photopigment for LW perception, but that he lacked the required middle wave gene. Proving that Dalton was partially right, that his color blindness was inherited, the researchers concluded that rather than being a protanope, he was actually a much more rare deuteronope, green blind. With this type of color blindness, rather than a mutation, the individual lacked medium wavelength cones altogether. Further, besides being unable to see several hues of color, deuteronopes do not see the vividness of colors at the higher range that others see, which in fully color-sighted people is due to the opposing signals between LW and MW photopigments.