- Extra Reading
Evolving in a silent, dark world, organisms developed receptors that could detect and differentiate components of the electromagnetic spectrum from the sun. Computation of the proportions of different wavelengths emitted from objects is used to form the perception of colour by the visual system, enhancing the ability to differentiate objects from background. The beauty of colour, used by individuals, artists and commerce is important in all cultures from pre-history to the present.
This is a part of Professor Ayliffe's 2010/2011 series of lectures as Gresham Professor of Physic. The other lectures in this series include:
Fun with Visual Illusions
Correction of Optical Defects: From Spectacles to Lasers
Diabetes, Hypertension and Vasular Diseases of the Eye
Blindness in Children: The Global Perspective
Why we see what we do
30 March 2011
Professor William Ayliffe
Some of you in this audience will be aware that it is the 150th anniversary of the first colour photograph, which was projected at a lecture at the Royal Institute by James Clerk Maxwell. This is the photograph, showing a tartan ribbon, which was taken using the first SLR, invented by Maxwell’s friend. He took three pictures, using three different filters, and was then able to project this gorgeous image, showing three different colours for the first time ever.
Colour and Colour Vision
This lecture is concerned with the questions: “What is colour?” and “What is colour vision?” - not necessarily the same things. We are going to look at train crashes and colour blindness (which is quite gruesome); the antique use of colour in pigments – ancient red Welsh “Ladies”; the meaning of colour in medieval Europe; discovery of new pigments; talking about colour; language and colour; colour systems and the psychology of colour. So there is a fair amount of ground to cover here, which is appropriate because colour is probably one of the most complex issues that we deal with. The main purpose of this lecture is to give an overview of the whole field of colour, without going into depth with any aspects in particular.
Obviously, colour is a function of light because, without light, we cannot see colour. Light is that part of the electromagnetic spectrum that we can see, and that forms only a tiny portion. We cannot see x-rays and microwaves, and neither can we see radio waves or infrared.
The sun produces this electromagnetic radiation, and from it we are able to discern only a small portion because of the receptors that we have developed. Without these receptors, we would live in a dark universe. If we had not developed receptors that can feel heat from the infrared end of the spectrum, we would be living in a cold, dark universe, which is what it is. The bit that is not cold and the bit that is not dark is the bit we make up in our heads, which I have discussed, at some length, in previous lectures.
Photons of light are captured by the receptors to make an electrical signal, which goes to the brain and allows us to see. However, remember what Isaac Newton said: “For the Rays, to speak properly, have no Colour. In them there is nothing else than a certain power and disposition to stir up a sensation of this Colour or that.”
Let me explain what I mean. Colour is actually a noun for the spectral composition of light, and it derives from “color”, the Latin, combined with the Anglo-Norman “culur” - which provides conclusive proof, once and for all, that the Americans cannot spell and we can…
Colour vision is the capability of discriminating between light sources on the basis of their emitted or reflected wavelengths, and that is called their spectral content.
What do I actually mean by that? Well, if we only have one type of receptor and a mixed bag of radiation coming in - with short wavelengths and long wavelengths – then there is no way to discriminate between the two. What we need is at least two different types of receptors - one that is going to respond maximally to the short wave (often blue light), and one that is going to respond maximally to the longer wavelengths. This leaves us with the ability to differentiate the spectral composition of what is coming in – we can tell whether this light is more reddish or whether it is more bluish. That is not actually done in the retina. It turns out that animals do have different receptors, some responding to the long, some responding to the short, and some responding to those in between; and if you are a mantis shrimp, as I shall explain later, you respond to many wavelengths indeed, including ultraviolet.
Spectral sensitivity is how these receptors respond maximally. As you can see in this graph, at this wavelength we get a lot of response; at this wavelength, we get not very much response at all; and at this wavelength, there is no response at all. This particular receptor responds maximally at this wavelength. We could call it a shortwave length receptor or, if you like, a blue receptor, and this one could be a green receptor, but as you can see, it is not exactly green – it is sort of somewhere in between. This one is a long wave receptor, which is red. You will notice that the spectral composition of these two is very close, and that is due to a gene reduplication. They are very closely related genes. Bear in mind that, the receptor, when firing, is simply saying – “I am responding.” It is not saying, “Hey, I am responding and I am greenish.” All it is doing is responding maximally to a certain spectral composition. We reach “greenish” by comparing the responses to any particular gamut of light that comes in and adding them up, and that is done at a different layer.
This is a map of the brain. We have seen it in previous lectures, but not this colour wiring. We are looking at these things, called blobs, which are in between, and this is where the colour processing occurs. This is V1, the first visual area. The adjacent bit, surrounding it, is V2, which is the second visual area, and this is composed of thin stripes and thick stripes and things that are called interstripes. We are interested in the thin stripes, and there are clusters of cells that are concerned with the colour of the objects that are being seen. There are also luminance channels that tune this up or tune this down, comparing to the luminance of the stimulus that is being sent up from the blob area.
From here, they go to another layer, which is V4 and which was discovered by Semir Zeki at University College London. Here, form and colour come together, which means we can start to classify things in the visual scene as “coloured”.
Colour Blindness: Train Crashes and John Dalton
Colour blindness is something that vexes people when they first encounter it. Significant research was carried out into the condition by John Dalton, a Quaker from the Lake District who moved to teach at the New College in Manchester (in the eighteenth-century, Oxford and Cambridge excluded students outside of mainstream religions). In 1789, in the Philosophical and Literary Society of Manchester, he published “Extraordinary facts relating to the vision of colours”, which outlined his discovery that he was seeing colours different from other people. He worked out that he was missing something that other people had. This “something” was an “unknown unknown”, as Donald Rumsfeld would put it. It was a very difficult thing to work out and Dalton was a genius for doing so. He went on to pioneer research into atomic theory and discovered colour blindness, often named Daltonism after his theories.
What precisely is Daltonism? This slide shows a mother (coloured pink because we are not sexist) and a father (coloured blue because we are not sexist) who have four putative children and, being a democracy, they split the genes up properly and equally.
The first son (in blue) has normal colour vision – he has the blue, which is on another chromosome, and the red and green, and so he has three responses. He can now make, across the gamut of light, a pretty average and good response and have a full sensation of primate colour vision.
The first daughter has inherited both normal genes, so possesses normal colour vision.
The second daughter has inherited this defective gene, where the green is not quite as good because of the middle (M) receptor. However, because she can use the other chromosome (you only need one lot of genes, the other is spare) she makes normal colour, but she is a carrier of the defective gene. She will be like her mother, and has the potential for passing on colour deficiency to any male children.
The second son has a problem. As we know, boys are X-deficit females, they possess the different gene. His colour perception is shifted close to the red, so he is going to discriminate less colours than his siblings will. This is called deuteroanomaly, meaning an anomalous second pigment.
Dalton’s suffered from a different condition. He actually missed the pigment altogether – the gene was deleted or the effective product from the gene was not being made. We know this because he donated his eyes upon his death, knowing that one day someone would discover the cause of his condition. About fifteen years ago, when I was in Manchester, there was a conference on colour vision, and someone took the eyes out of the bottle and did PCR analysis and found the genetics, and proved that Dalton was right all along – he was missing something. He thought it was because he had coloured oil droplets between the eyes and the receptors which were affecting colour in one of the receptors, and interestingly enough, that is actually how some animals do colour vision – they have different coloured oil drops in their receptors to fine-tune vision.
Because it is the second pigment that’s missing, it is called deuteroanopia. He can still make colours because he has this pigment which is not X-linked, from a different chromosome, and he has the red pigment, which is fine, so he can see some colours, but his depth and range of colours, as he describes in the article and in his lecture, are not so good.
Another condition of colour blindness, protanopia, is a little more severe. The chromosomes are missing the long wave gene, so their total average of colours is less vision is biased towards that end of the spectrum.
Some of these conditions are quite common. An absent short wave gene is very rare – affecting only 0.005% of the population. Of course, because it is not X-linked, it occurs equally in males and females.
Deuteroanomaly does not really affect us, unless we go to the Board of Trade to have testing either to work as a pilot or, more importantly, in the chemical or textile industries. This is why there was so much interest in colours in Manchester, where there was a booming textiles industry – you have got to have absolutely perfect colour vision to work in that textile industry.
As a result, we sometimes need to test people’s colour vision, because a lot of people who are colour-deficit do not know that they are. In 1684, Dr Turberville met a thirty-two year old female with excellent vision, but she could not distinguish one colour from the next. That is unusual for a female. Most females do not suffer from this because they have normal X genes, so she must have been badly affected. She probably had a different disease, like a cone dystrophy that was affecting all the receptors, rather than a genetic defect of an individual colour. Nevertheless, he asked her to name colours in what became the first colour test. “What is that colour?” and she did not say red. “What is that colour?” and she did not say blue. She could not see colours. That was the test for colour vision.
Dr Turberville was renowned for his interesting treatments. He generally prescribed shaving the head and taking tobacco, which he had “…often known to do much good and never did any harm to the eyes.” He was famous because he cured the poor gratis. When removing a tumour from one man, without anaesthetic of course, the man remained entirely still without screaming in pain. He said, “To be sure, without doubt, this is a married man – otherwise, it would be impossible that he could be so patient.” The patient replied, “No, I am but a bachelor.”
About one hundred years later, Joseph Huddart writes a letter to Rev. Joseph Priestly. He had found a Cumberland shoemaker called Harris, who could only describe his shoes as light or dark, no matter what colour they were. He picked up a sock in the road one day and took it to the neighbour it belonged to, who said, “Thank you for returning my red sock.” To Harris’ mind, it looked just like all the other socks on the washing line. Mr Huddart realised that Harris was colour blind. Two of his brothers were also affected but ignored.
Seebeck, working in Berlin in 1847, created a colour test that moved beyond holding up coloured socks. He developed a whole test with hundreds of different coloured papers, which he identified to a sample and compared. He recognised, from this, that there were at least two types of colour defects, and he also recognised the anomalous, partial defects.
The Swedish scientist, Frithiof Holmgren, was responsible for the first successful attempt to standardise colour vision. Holmgren produced a series of test colours that had to be compared out of a box. The test takes about a minute to do, and he tested 2,220 soldiers; if confusion occurred between colours, the soldier was diagnosed as colour-blind.
When talking about Holmgren, we cannot avoid discussing a train crash that happened at Lagerlunda, Sweden, on the 1st November 1875. The crash occurred because of unclear signalling between the station master and the engine-driver. The train was signalled to go, the station master gave the all clear, and the train left the station, only to plough into another train that was coming down the same track line. Nine people were killed in the head-on collision, it created a scandal in Sweden and the station master was sacked and sentenced to six months in prison.
Interestingly, nothing about colour vision is mentioned in the court papers, but it did not stop Holmgren rushing in and testing the workers. Lo and behold, he found that thirteen out of two hundred and sixty-six railway workers were colour-blind, which is about 4.8% (just slightly lower than what we would expect in our population). He therefore concluded that colour vision was the cause of the mistaken signalling, which led to a law being passed that everybody who worked on the railways in Sweden had to have colour vision tests.
Actually, this was not the reason for the crash. A year earlier, exactly the same thing had happened at Thorpe in Norfolk. Here is a rather gruesome photo of the incident from The London Illustrated News, showing a body being thrown through the air. Victorians quite liked this. You never had to read between the lines. They were quite graphic actually. The accident engineer, Edward Tyler, invented a system, which was nothing to do with checking colour vision. He developed a special key which slotted into an electric device at the other end, and no train could come down that track until this was done, and that meant your train went past. A variance of this is still used on single track lines to this day.
There were developments in the colour vision test. Lord Rayleigh thought that it would be a good idea to match up lights, which led to the nagel anomaloscope, and versions of this are still used today.
In 1903, Dr Charles H. Williams developed the lantern test. As this Board of Trade lantern from 1912 shows – a very rare example that still survives – a little oil lamp projects various colours at six metres, allowing sailors, and subsequently airmen and various other people, to be tested for colour vision. I think there are only four of these still in existence, and this one can be found in the Optical Museum in Craven Street, London.
A number of other tests emerged. Pierce developed nitrocellulose chips (1934); Farnsworth-Munsell (1943) developed the 100 hue test – which we will come onto, because it is a very influential test; and then pseudoisochromatic charts.
Pseudoisochromatic charts work because we now know about colour confusions. This top-left example represents the normal, quaintly called “able-bodied” vision - you would not be allowed to get away with that these days. The “able-bodied” can distinguish between greens and blue-greens and pinks.
What happens if you are protanomic – that is, you are missing the third gene? The bottom left example shows that you cannot discriminate between target and background and you cannot see the shape. Tritonopes can, and deuteroanopes almost can, under the correct lighting conditions, but obviously it is going to be more difficult for them to do so.
Now, here comes the rub! This is really interesting. We noticed that the X-linked female could actually have an abnormal gene but still function normally. What happens if that X normal gene is also expressed in some of the cells? She has now got four colours, so it is quite possible that there are women out there who have what we call tetrachromacy: they can see, they have four receptors.
Does this exist in real life? It might well do, and Kimberly Jameson at the University of California is carrying out some research into this area. It is quite controversial, because you may have four colour receptors here, but can your brain interpret four signals? We know our brains can interpret three, and examples from nature show that brains can interpret two and one signals, but can brains ever interpret four?
It appears they might be able to. Mice are naturally dichromats, as most mammals (apart from primates) are, so they only have colour receptors. Mice seem to function perfectly well. They can tell the difference between cheese and mice poison, in my house anyway. For a mouse, that is good. For us, there may be a problem. Scientists transfected the human gene equivalent of what they were missing, and did some experiments on these mice, and the mice could see better colours than the non-transfected mice. Even though the brain is not hard-wired for extra channels, maybe it is soft-wired and able to adapt. In which case, why stop at four? You could build a human receptive up to eleven colours, which a mantis shrimp has.
In between, a lot of animals are tetrachromats. It enables animals like eagles to see the urine traces of mice in the grass when they are four hundred feet above the ground. Down comes the eagle, because it can see it, glowing in the dark, and hits the mouse, which thought it was doing perfectly well, hiding in the grass. Fish need to have good colour vision in order to navigate things like coral reefs, and to avoid things like the mantis shrimp. Spiders have great colour vision (though I am not sure why). Reptiles inherited good colour vision from our ancestors, though we have lost it. When we became mammals, at the same time the dinosaurs were evolving, we lived in burrows to avoid being eaten, and so we did not need all this stuff – it is a waste of genetic material, and we gradually lost it. We then had to re-acquire it as we came out of the burrows in the Cretaceous period. So, in fact, mammal colour vision is quite late, and evolved originally one, and then two, receptors – as I said, most mammals have two. In the case of humans, there is a duplication that occurred in one of our primate ancestors, which gives Old World monkeys and humans three colour receptors, but New World primates are still dichromats. There is, however, the occasional trichromat female, which is why you sometimes see female New World monkeys leading the troupe, because they are better at finding fruit.
We can now, through this colour mechanism, classify objects by colour. If we do not have colour, this picture is what we see. As soon as we have got colour, the thing of interest shoots out at us against a complex, textured background. If the object is edible and colour enables you to spot that straightaway, gives you a survival advantage.
So did colour receptors evolve just for finding food? I am quite intrigued, because I think more fruits in the Old World are red when they are ripe than all fruits are in the New World. I grew up in the New World and most of my fruits, I seem to remember, were yellow – things like pawpaws and bananas – and there was not really much red in my world. There were bits of it, some flowers were red, but that was nothing to do with me – that is to do with ultraviolet and attracting tetrachromats like bees and hummingbirds. Maybe fruits evolve along with the humans and animals in the New World – there is no point in being a red fruit there, you will go past your sell-by date. Conversely, there is a very big advantage in being a strawberry in England because you are going to be picked out by the trichromats.
The other thing trichromats can do as well is detect emotion. You can tell when someone is really angry with you because they go pink and purple in the face, and you know when to walk away before getting your nose thumped. It is also important for male monkeys detecting emotions in females, so they know when potential partners are ready to “say hello”.
So colour vision is obviously something that is really important for survival, so important that even anomalous colour vision is passed on and has not been wiped out by evolution.
Colour and Pigments in Art
In fact, colour has been used – often very beautifully - since pre-history. Here are some of the earliest images of use of painting and colour. This is carbon black. Prehistoric man also used some ochres, which are iron-containing earths, and yellow. In France, the cavemen travelled up to twenty-five miles for these earth pigments from the Lascaux Caves alone. They brought them back into the caves, where they ground them up and made them into pastes, using various gums and so on. We know that because we have found traces in natural hollows in the bottoms of caves, which were used as grinding bowls.
Ochre was also used in Wales. In this cave, they discovered the first prehistoric human fossil in 1823, named the Red Lady of Paviland. Unfortunately, it turned out not to be a boy rather than a lady, but you can see how the skeleton was dyed in red ochre. This dates from the Paleolithic era, the old Stone Age, before the Ice Age, and this says something about the micro-climate on this beach in Wales.
An even more ancient example of ochre has been found in South Africa, and may date to 75,000 years ago. That is amazing, considering that Stonehenge was built 3,000 years ago and most of the older fossils that have been found in Europe date only as far as 5,000 years ago. It is amazing to think that 75,000 years ago, our ancestors were producing art.
Yellow ochre is a beautiful colour, found in Lascaux, and produced from a slightly different chemical - iron oxyhydroxide – and so giving a different colour.
These beautiful images were made using carbon black. This type of art is quite interesting because they scrape the walls, which they did not do in the other places, to smooth it down before painting on it. They produced three-dimensional effects by carving into the wall too, so it was a little bit more than just painting, and one could call it the first example of bass relief I suppose.
Carbon blacks are made by heating wood in restricted air. Charcoal has been used by artists of all periods for sketching, even today. We also use them in modern photocopiers and laser printers.
Were the Ancient Greeks colour-blind? William Gladstone, a nineteenth-century Homeric scholar, thought they were, and the seemingly limited range of colours on ancient pebble mosaics provided evidence for this. What Gladstone failed to do, however, was to look at other artefacts from the archaeological record of the same period.
Here is a Greek encaustic painting, admittedly of a later period, where a limited palette is used intentionally. It is interesting that some of the commentators on Ancient Greek art were moaning about these newfangled colours that were coming in and how they were ruining the beauty of ancient art. Certainly, for faces, artists limited themselves to four colours and produced beautiful effects. I am grateful to the monks in Mount Sinai who allow me to show this image to you today; it is one of the oldest icons in existence.
Let us consider some other paintings, aside from encaustic portraits. Remember the story of the competition between the artists Parrhasius and Zeuxis. Zeuxis painted grapes so realistic that the birds flew down and pecked at the artwork; Zeuxis then turned to his rival and told him to pull back the curtains and reveal his painting, to which Parrhasius replied, “Actually, the curtains are the painting.” Well, they were using colours. Look at this beautiful Egyptian blue. Blues, pinks, gorgeous face painting, very naturalistic and beautiful colours. This is called the “Tomb of the Diver”. He probably was not a diver, but it was considered an allegory of existence between life and death, and this is the symposium to welcome him into his new life. Similar frescos are found in Etruscan tombs of the same age.
Ancient Egyptian art demonstrates, again, a limited palette. Egyptian artists used slightly more pigments than the four the Ancient Greeks were alleged to have (though the paintings I have shown you disprove these allegations). They used iron oxide, red ochre, yellow ochre, and amber as a basic palette. Interestingly enough, we have one of the palettes. It was found in Tutankhamen’s tomb between the paws of the guarding jackal, and was a gift to Tutankhamen from the Princess. It had a number of colours on it. This was the first artificial pigment made in the world, Egyptian blue, which is calcium copper silicate. They also had azurite and a number of other ones. They used vegetable dyes, principally as textiles, but also as inks, but not on the walls because they do not last. The Ancient Egyptians also learnt how to fix dyes into a transparent powder, a process known as lake making. Those of you who paint will know about madder lake and the various lake colours. This is how they were stabilised and enabled for use. Later on, and nowadays, they used chalk and alum.
Here are some gorgeous examples of early fresco painting to show where Gladstone was wrong. Minoan painting uses a lot of gorgeous colours, and there is a little sample of powdered Egyptian blue.
The Romans made great use of pigments. We know this because many of their frescos survive. This is a fragment from one of the original temples in the city – gorgeous colours, lovely blues, reds and yellows. The Ancient world was not as we imagine it. It was not stark, marble-coloured buildings. They were gaudy, awful, multicoloured to our taste. Our taste has evolved through what has remained above ground, so much so that, in some places, we have scraped off what was considered the stain, but was actually the original paint. So we probably would not have liked Ancient Greece and Rome. It probably would have seemed more like Butlins.
This floor is an example of how they might have looked. Of course, the oldest image of Christ known is not far from here. It is in the British Museum, in another pavement, found in Somerset. It is curious that the oldest image of Christ is in Somerset and not in the Mediterranean – this is probably because of the big iconoclasm that occurred in the Byzantine Empire, which did not extend to these shores in those days, due to the collapse of Roman influence.
We know a lot about what they did because of documents that survive. Probably one of the most important ones was the Libro dell’Arte. A prisoner of the Pope, wrote “ex stincharum, ecc” at the bottom of the manuscript. It translates as “I am in the stink”, and “the stink” was the prison in Florence. He wrote this on July 31st 1434, a time of the year when it was probably very stinky indeed. The prisoner was copying Cennini for the Pope, so many copies of this exist. He tells us how the paints are made and what they were used for. There are some earlier ones: Theophilus, the Mappae Clavicula and Heraclius. Again, substantial amounts of their writings remain, which tell us what they were up to.
Early medieval painting, particularly wall painting, used a number of colours very similar to the ones that we saw in the days of Ancient Greece and Rome. Natural chalk - an element that would play a big, beautiful part in the works of Michelangelo four centuries later – was being used by the artists for drawings. Some people actually regarded the drawings as a higher level of art than the painted frescos. They used a green earth for under-painting, and we shall come to that in a moment. Ultramarine, precious lapis was transported from Afghanistan via Persia, an amazingly expensive material that entered the markets, was ground down and made. The most expensive material made the best and longest lasting blue. They did have another material, called azurite, but azurite tends to change colour with time. It is not until the development of oil painting, where you can stop the chemical reactions going on, that you can start to use a lot of these pigments that they knew about.
Stained glass gives you the opportunity to use pigments in their full beauty. This is the oldest stained glass in situ, in St Paul’s Church in Jarrow. They learned how to do this by importing monks from France, who then taught glassmaking skills. They left and these skills were propagated locally.
The blue glass was subsequently used in other parts of the world as well. This is seventh century, and it is very rare and very precious because it is one of the only examples of Anglo-Saxon glass to still survive.
This is the time to talk about Pseudo-Dionysian light metaphysics, which we shall introduce by way of Abbot Suger, who was friend of Louis and Regent to his son. This was the famous church of St Denis Benedictine Priory, and this was where the French aristocracy and monarchy were buried. He decided to rebuild the church in the Romanesque style and to glaze the windows. Unfortunately, this is the only extant bit of glass within the cathedral, but there is lots of it outside and in the various museums in the area.
It is important to explain what this blue represented – that is, God’s light. God’s light was so dark that it could not really escape God, except to become lumen. Lux was therefore the dark light, the holy light. Now, ultramarine is dark and non-transparent. By melting the precious ultramarine into the soda of the glass, they made something that was opaque transparent, which allowed the lumen to come through, and provide a very godly experience. It was not meant to be bright. All of his windows were taken out and replaced in the 1300s because the church was so dark, people were tripping over each other.
Now, Pseudo-Dionysius (inadvertently conflated with St Denis) was a fifth century theologian, whose work was ascribed to the Athenian Convert of St Paul, the Areopagite. He comes to the West as a gift of the Byzantine Michael the Stammerer and is given to Louis the Pious at Compiegne. Charles the Bald subsequently commissions John Scotus to translate this Greek manuscript. It is quite interesting that the Irish and English monks were much better at Greek than the French and Italian monks, possibly because a lot of English or British soldiers were employed as guards to the Byzantine Emperors at various stages throughout history. After finishing the translation, John Scotus is sitting at the table with the King, and the King leans over to him and says, “Quid Distante inter Sottum et Scottum”. This translates as “What is the difference (or what is the distance) between an Irishman and a drunk?” John Scotus sits back and replies, “The width of this table, Your Highness.”
This is what it would have looked like. This is in Chartres cathedral, not far away. It is excessively dark. Each individual window is beautiful and bright and light, using lots of that Dionysian blue to transform the solid lux of God into this transparent lumen or light for us to see with. Chartres is well worth a visit if you want to see what St Denis used to look like.
We know a lot about this and how they are making it because we have got lots of images. Here they are making the glass, and they are making it in this furnace here; we know that they used to melt precious gems in here, under extreme heat, which they could generate with the bellows.
Later, this darkness of glass was replaced by a much lighter glass, and later medieval glass is much less colourful than the early medieval glass.
In the Renaissance and following the development of oil painting, they are able to introduce new colours. The oil stops the paints having chemical reactions with each other, which means that artists are able to use more colours.
Vermilion, which had been used in the Roman times, is brought back in. It is an extremely poisonous, mercuric mineral that is found naturally. The Cinnabar Mines in Spain were one of the closely guarded secrets of the Roman Empire, but then they found out how to make it artificially, and it became a very important pigment because of the alchemical way in which it was made – it formed one of the steps towards making the Philosopher’s Stone. It was also used as a lipstick, which is a rather nice way to dispose of your wife, I suppose.
Bone black could be made from two sources: either cheaply, from the chicken bones underneath the table, or from ivory. Following the fall of the Middle East, ivory became exquisitely expensive. No longer could you have diptyches sent out as gifts to people you wanted to influence because there was no ivory. In fact, many of these diptyches – you can see them in the V&A – are split into two, so they can re-carve something on the other bit that is not there. If you were an artist, you burnt the ivory to make ivory black, which is almost identical to bone black because ivory is a type of bone. It is not exactly identical, but it is to my eyes, and possibly to most people’s eyes here. What mattered what that the material it came from was expensive, which was just as important as the way that it was used and looked.
Crimson is a really important pigment, and can be seen in the coronation mantle of Roger II of Sicily (still available to view in the Sicilian Treasury). The pigment actually comes from the dried bodies of the kermes insect, and had been used for a long time. However, the problem was always collecting the insects, which can be difficult to spot, and this made crimson very expensive. Crimson was so important that it overtook imperial purple, which was made from whelks; this is why we refer to “the red carpet treatment”, because red was historically very precious and expensive. Albrecht Dürer used crimson for his painting, The Virgin and Child, which can be seen in the National Gallery.
Crimson was replaced by the Polish version, carmine, which was cheaper. It is a type of carminic acid, used for dying and inks, but difficult to use it for painting. It was very lucrative – one pound of this cost five Venetian pounds. In the 1500s, thirty metric tons of the stuff was sold in Poznań. Carmine comes from Polish cochineal insects, which pupate in the roots of herbs and look like small grains. Carmine superseded crimson because you need ten times fewer cochineals than you do kermes insects to make it.
The even cheaper cochineal then replaced the whole lot, after the Mexican conquests, and was used to dye the reds of the British Army and as a food dye. It went out of fashion with the invention of artificial dyes, but is now back in fashion following the discovery that artificial dyes cause cancer in rats.
Vermilion re-emerged later in this period. It can be seen in the paintings of Titian, one of the great colourists, and particularly in the Assumption of Mary (1516-1518). Titian’s final paintings are not examples of a man growing old and unable to appreciate colour; it may just be unfinished, because his other late paintings demonstrate magnificent colours.
Greens were used to under-paint, not because artists wanted people to have green faces. However, over the years, the pinks and the reds used for over-painting have faded, revealing the green earth underneath. The biggest and most famous deposit of this was near Verona, which was actually mined well up until the Second War. You can also get some of these pigments in the Mendip Hills of England.
Colour and Language
I briefly want to talk about the linguistics of colour. There was a conversation recorded by Aulus Gellius about Favorinus, who says: “The eye sees more colours than language can distinguish”, and that is very true. There may have been limited vocabularies for colours in the ancient world and in some ancient languages, but they may have had forty or fifty different words for the colour of horses in Kazakhstan. In fact, Marcus Fronto replied and said, “We have got seven colours for red.” Not all of them were in common use; some of them were just used for medicine.
The conclusion drawn from the colour-impoverished vocabularies of ancient languages was that the ancients had to be colour-blind, and that evolution had given the modern Europeans better colour vision. This theory was all the rage, of course, because Dalton had just discovered colour blindness and the theory of evolution was being circulated and talked about, and this theory provided a trendy and attractive mix of Daltonism and Darwinism. Furthermore, someone else had been studying a wide variety of other languages – the Old Testament, Ancient Indian Vedas, Icelandic sagas and the Koran – and had come to the same conclusion.
Distinguishing blue from green was one of the issues. We distinguish blue and green, but many languages do not; linguistics terms this “grue”. For example, Ancient Chinese uses qing to describe a bright day – qing tian, meaning a blue sky. Qing is also used to describe green vegetables – qing cai, green vegetables. Nowadays, they have two more words for blue, and so modern Chinese has lan and lu for green, such as lu tai, which means green tea.
What do I mean by a name, identifying colours? If I want terracotta, which shade do you give me? What do I mean by terracotta? It is quite important to understand this, particularly if you are painting the house for your loved one and you get the wrong shade of terracotta – you could be in serious trouble.
In a controversial 1969 study, Berlin and Kay identified several language stages: stage one, they describe white and black; stage two, red was added to white and black; red and green; red and yellow; and all languages seemed to evolve in this order. You added more colours until you arrived at the complete stage seven. These were labelled as “evolved languages”, mostly European but also Indian and other advanced languages. This study was subject to a rigorous critique and many procedural errors were exposed, but it is still a very interesting story to follow with some very fundamental things to say about how we see, speak and think.
In a previous talk, I mentioned theories of colour vision: originally the trichromatic and then the opponent theories from Ewald Hering. Thomas Young, the brilliant polymath, was the first to propose that there may be just three types of receptors that see colour. He reasoned that there are millions of colours out there, but we do not have millions of words for them, nor do we have millions of sensations for them. Instead, he postulated three types of receptors.
Hering argued that there are not three, but that the receptors are linked in pairs – making six. He said that you either see a red or a green, but you never see a greenish-red. In fact, greenish-red is a term rather like a southerly north-western breeze and it just does not make sense, and it does not make sense to us because of how we perceive it.
Colour scales have been in existence since Theophilus, who described black and white, and then put red in the middle. Francis Bacon put green in the middle. They were not exactly scales, but suggested that colours are ordered in perception. What would be the equal steps between these blues? How much white do I need to add to this? How much of this is white and how much of this is blue?
Tobias Mayer decided to address this problem using a diagram of triangles, and his ideas were taken further by Johann Heinrich Lambert. He thought in terms of a pyramid, with white at the top, and this system attempted to display all the colours there were in the world. Of course, this was impossible, because the colours apparent in insects or bird wing feathers occur not by pigment mixing but by interference patterns of light waves. As you can see, various people came up with different colour scales.
Goethe was a proponent of an opponent theory of colour, creating a symmetrically arranged colour wheel: "[F]or the colours diametrically opposed to each other in this diagram are those that reciprocally evoke each other in the eye. Thus, yellow demands violet; orange, blue; red, green; and vice versa: thus... all intermediate gradations reciprocally evoke each other.” Chevreul, who worked in the industry of dyes and pigments, developed Goethe’s colour wheel to show the simultaneous contrast of colours. Runge, a painter of some renown, took these colour circles and made them into solids. Albert Munsell developed this idea even further, defining three colour “dimensions”: the value (how much white or how much black is in a colour), the chroma (how many grams of pigment there are in the sample) and the hue. And colour scales, in turn, influenced the Bauhaus movement, particularly the art and theories of Johannes Itten and Josef Albers.
Subtractive colour mixing occurs when light is reflected off a surface or is filtered through a translucent object. Thus red pigment absorbs (subtracts) all of the light that is not red and only reflects red. As a consequence, mixing pigments and dyes gives a different result from combining coloured lights; any colour can be made from RGB lights,
The colour space is a comb and this can be mapped out. They got observers to tune in this with three colours until they matched that, and that is that number there, and then, change the colour, tune in again, it is here. From that, they were able to make a diagram where they could plot the colours. It looks a little bit similar to the tristimulus we mentioned before.
The Natural Colour System (NCS) is based on all of this, and many companies have adopted it because it is a little easier to use. For example, Virgin claim that this is their red, and you can go to the NCS and find “Virgin red” or perhaps “Ferrari red”. Each colour has a number attached to it based on where it is in the hue, how saturated or not saturated it is. Virgin red, for example, is S108Y90R. Why is that important? If you go along to the NCS, leaf through it, find the red you want, go and match the paint, then you can set up your own airline – as long as you do not call it Virgin and as long as Richard Branson has not patented that colour.
I would like to leave you with this concluding question: what are colours and what are not colours? We know what they are spectrally now. We also know that there are non-spectral colours, which we cannot make by mixing lights together. We also know, intuitively, that colours are different depending on the surface they come from, so reflectivity and texture affects it. It is also affected by the light that is shone on them.
Colour constancy is an example of subjective constancy, ensuring that the perceived colour of objects remains relatively constant under varying illumination conditions. A green apple, for instance, looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects. The physiological basis for colour constancy is thought to involve specialized neurons in the primary visual cortex that compute local ratios of cone activity. Colour constancy works only if the incident illumination contains a range of wavelengths. The different cone cells of the eye register different ranges of wavelengths of the light reflected by every object in the scene. From this information, the visual system attempts to determine the approximate composition of the illuminating light. This illumination is then discounted in order to obtain the object's "true colour" or reflectance: the wavelengths of light the object reflects. This reflectance then largely determines the perceived colour.
Ladies and gentlemen, thank you very much indeed.
©Professor William Ayliffe, Gresham College 2011
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