Regular readers of this website will be aware that I am a big fan of Wikipedia, and I believe it comes in for a lot of unjust criticism. But even I have to admit that its section on John Dalton and colourblindness is very misleading.
Here's what Wikipedia has to say:
"Dalton suffered from a rare form of color blindness, known technically as deuteroanopia. He could see blue, but 'orange, yellow and green seem one colour which descends pretty uniformly from an intense to a rare yellow, making what I should call different shades of yellow.'"
Wikipedia's source is Now We Know Em (.com), which is the work of one Carl Leonard, a "historiographer" from Tulsa, Oklahoma. Here's what Mr. Leonard wrote:
"Examination of John Dalton's actual preserved eyeball in 1995 demonstrated that he actually had a less common kind of color blindness, deuteroanopia, in which medium wavelength sensitive cones are missing (rather than functioning with a mutated form of their pigment, as in the most common type of color blindness, deuteroanomaly).
"Besides the blue and purple of the spectrum, John was able to recognize only one color, yellow, or, as he said in a paper;
"'that part of the image which others call red appears to me little more than a shade or defect of light. After that the orange, yellow and green seem one colour which descends pretty uniformly from an intense to a rare yellow, making what I should call different shades of yellow.'"
The Wikipedia editor has not rendered Mr. Leonard's words quite faithfully, but Mr. Leonard himself is guilty of a couple of misdirections. For one thing, the condition is technically known as deuteranopia and not deuteroanopia. (This mistake is replicated on Wikipedia.) Mr. Leonard says that this is a "less common" form of colour blindness, but only later in the same sentence does he refer to the more common condition: deuteranomaly. On Wikipedia, this leads to deuteranopia being described as "rare".
The website Color-Blindness.com (which Mr. Leonard may well have used as a source) describes deutan color vision deficiencies (deuteranopia and deuteranomaly) as "by far the most common forms of color blindness". It says that approximately 1% of the male population, and 0.35% of females, suffer from deuteranopia, while deuteranomaly affects as many as 5% of males and 1% of females.
Colour–blindness (.com) itself does come across as a science–based website, but its English leaves a lot to be desired: for example, "you even might find some of your friends who's also suffering under this color vision deficiency", and "[in deuteranomaly] the medium wavelength sensitive cones (green) are missing at all."
Here's what I think Wikipedia should have said (with my words, some of which I've copied from Mr. Leonard's quotation, in italics):
"Dalton suffered from a form of color blindness known technically as deuteranopia, which is closely related to deuteranomaly (the most common form of all). He could see blue, but 'that part of the image which others call red appears to me little more than a shade or defect of light. After that the orange, yellow and green seem one colour which descends pretty uniformly from an intense to a rare yellow, making what I should call different shades of yellow.'"
Go on then, I hear you say: do the edit on Wikipedia yourself. I'm not going to do that right now, for two reasons: firstly, I'm not an expert and I may introduce errors of my own; and secondly, I want to wait and see if anyone else agrees with me that it's wrong.
© Haydn Thompson 2017