Color test plate by Shinibu Ishihara (1917). Color blind persons usually see the number 21.
The Arnolfi Marriage after a red-green color blindness simulation. The algorithm, which can be applied to any digital image file, can be found at the Vischeck site
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Eyck, Jan van (1434) The Arnolfini Marriage. Oil on wood, National Gallery, London.
Dalton reported:
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...
Dalton , John (1798) Extraordinary Facts Relating to the Vision of Colours, with Observations. Memoirs of the Literary and Philosophical Society of Manchester V:I, 28-45. London, Cadell and Davies.
The English chemist John Dalton (1766-1844) described his own colorblindness symptons in the first scientific paper on the subject in 1798. He reported that he saw a pink geranium as dull blue by day light and yellowish by candlelight. He proposed (incorrectly) that his condition was caused by a bluish tint in his vitreous humor, the jelly like substance that fills the eye. At his request one of his eyes was dissected after he died. It was perfectly normal. We know now that colorblindness is caused by defective genes on the X chromosome. It is more common in males (Dalton's Brother was also colorblind!) though females are "carriers." Colour-blind "sufferers" are defficient in one, two or all three of the Long, Medium or Short wavelength sensitive cones of the retina.
