How do we know that the colours we see are the same as the colours that everyone else sees? Mostly, we can't -- but we are able to tell that some people do perceive colours differently, and have difficulty seeing the difference between some colours.
We call this condition colour blindness. Its other name is Daltonism, in honour of an English scientist named John Dalton, born on September 6, 1766, 250 years ago today.
Dalton made great contributions to the fields of atomic theory, chemistry and meteorology, but he was also the first person to formally describe a deficiency in colour perception.
He himself was colour blind. In 1794, he described his own colour blindness (he published in 1798), as well as that of his brother, correctly figuring out that the condition is hereditary. However, his theory as to the reason why was quickly discredited. Dalton, who mixed up scarlet with green and pink with blue, believed that the vitreous humour inside his eyeball was tinted blue, changing the way he perceived colour.
He left instructions after his death for his eyes to be examined to test this hypothesis, but the liquid inside his eyeballs contained no blue tint.
As we know now, colour blindness is usually caused by a lack in cone cells, which perceive colours inside the eyes. Normal vision is when all three types of cone cells are present and functioning correctly. Protanopia is when the red cones (long wavelength) are lacking, and deuteranopia is when the green cones (medium wavelength) are lacking. Tritanopia is when the blue cones (short wavelength) are lacking, and is very rare.
Some of Dalton's eye tissue was preserved, and DNA was extracted from this tissue and analysed in 1995. From this, we now know that Dalton had deuteranopia.
Each of these types of colour blindness can also be present in a milder form, where the cones are present but mutated. The most common of all forms of colour blindness is deuteranomaly, a milder form of deuteranopia. And men make up the vast majority of those affected by colour blindness, since the condition is linked to the X chromosome. Women have two, which means a backup if one has the colour blindness anomaly.
Click through the gallery, made using Color Oracle, for a glimpse of the world through a colour-blind lens.
You can also read more about colour blindness on the Colour Blind Awareness website -- and raise a glass to John Dalton, who knew he saw the world differently and set about proving it.