I'm reading a great book at the moment, Dave Gorman's Googlewhack adventure. This book chronicles the epic procrastination techniques Dave Gorman used to avoid starting to write a novel. The novel he was going to write was about a guy who sees a brand new colour in a dream and travels the world searching for an example of it.
Sounds pretty far fetched, no? Well, quite by accident I came across this page in which a physicist Andrew Hamilton comes up with an entirely new colour.
In case you're put off by the maths on that page, here's a basic explanation. The human eye contains 3 types of colour receptors (call them red, green and blue for the sake of argument). When we see a particular colour, these receptors are triggered in some ratio - the ratio determines the hue of the colour you are looking at. The red receptors detect long wavelengths, the blue receptors detect short wavelengths and the green receptors detect wavelengths in between. However, even when looking at even the purest green light the red and/or blue receptors are triggered to some extent because their ranges of sensitivity all overlap.
So what would you see if you could trigger just the green receptors? It would be a colour that no-one had ever seen before, since light with this colour isn't physically possible. Hamilton hypothesizes that this colour (which he calls "psychedelic aquamarine") would be the colour you got if you took the colour of the water around the reefs of Heron Island in the Great Barrier Reef of Australia and subtracted white to make the colour even more saturated:
In this picture, psychedelic aquamarine would be to the bottom left image as the top right image is to the top left image. When I tried to apply the same transformation the bottom left image that I applied to the top left image to get the top right image, nothing happened (which is hardly surprising since obviously my computer is not capable of displaying psychedelic aquamarine).
According to Hamilton, people who are red/green colour-blind see psychadelic aquamarine all the time, whenever they look at something which us non-colour-blind people would see as red. Such a colour would stimulate the green receptors, but not the blue receptors (as it's too red) nor the red receptors (since these don't exist in such people). However, since we can only describe colours by mentioning things which are that colour, a colour-blind person's description of psychendelic aquamarine would probably sound very much like a colour-seeing person's description of the colour red.
I wonder if it would be possible to make a machine that would allow people to see this colour. It would have to work by scanning someone's retinas, identifying where the green receptors are and firing out photons in such directions that they only hit the green receptors. If someone built such a machine and stared into it, would they just see a very vivid (bluish) green or would they have the incredible experience of seeing a brand new colour that they would never have seen before?
[Update 8/8/2011] It turns out that it is possible to see psychedelic aquamarine with no special equipment, due to the fact that colour receptors get fatigued if they look at the same colour for a long time. The Eclipse of Mars illusion (about halfway down the page) exploits this fact to enable you to see psychedelic aquamarine. Try it - it's extremely impressive!
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